Journal of Economic Entomology Advance Access published December 4, 2015 Journal of Economic Entomology, 2015, 1–14 doi: 10.1093/jee/tov336 Research article
Arthropods in Relation to Plant Disease
Evaluation of Alternatives to Carbamate and Organophosphate Insecticides Against Thrips and Tomato Spotted Wilt Virus in Peanut Production K. Marasigan,1 M. Toews,1 R. Kemerait Jr,2 M. R. Abney,1 A. Culbreath,2 and R. Srinivasan1,3 1
Department of Entomology, University of Georgia, Tifton, GA 31793 ([email protected]; [email protected]; [email protected]; [email protected]), 2Department of Plant Pathology, University of Georgia, Tifton, GA 31793 ([email protected]; spotwilt@ uga.edu), and 3Corresponding author, e-mail: [email protected] Received 24 July 2015; Accepted 2 November 2015
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Abstract Thrips are important pests of peanut. They cause severe feeding injuries on peanut foliage in the early season. They also transmit Tomato spotted wilt virus (TSWV), which causes spotted wilt disease. At-plant insecticides and cultivars that exhibit field resistance to TSWV are often used to manage thrips and spotted wilt disease. Historically, peanut growers used the broad-spectrum insecticides aldicarb (IRAC class 1A; Temik) and phorate (IRAC class 1B; Thimet) for managing thrips and thereby reducing TSWV transmission. Aldicarb has not been produced since 2011 and its usage in peanut will be legally phased out in 2018; therefore, identification of alternative chemistries is critical for thrips and spotted wilt management. Here, eight alternative insecticides, with known thrips activity, were evaluated in field trials conducted from 2011 through 2013. In addition, different application methods of alternatives were also evaluated. Imidacloprid (Admire Pro), thiamethoxam (Actara), spinetoram (Radiant), and cyantraniliprole (Exirel) were as effective as aldicarb and phorate in suppressing thrips, but none of the insecticides significantly suppressed spotted wilt incidence. Nevertheless, greenhouse assays demonstrated that the same alternative insecticides were effective in suppressing thrips feeding and reducing TSWV transmission. Spotted wilt incidence in the greenhouse was more severe (80%) than in the field (5–25%). In general, field resistance to TSWV in cultivars only marginally influenced spotted wilt incidence. Results suggest that effective management of thrips using alternative insecticides and subsequent feeding reduction could improve yields under low to moderate virus pressure. Key words: tobacco thrips, spotted wilt, vector and disease management
Tobacco thrips, Frankliniella fusca (Hinds), and western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), are pests of peanut. The former is known to colonize and reproduce more efficiently on peanut foliage, especially in the early season, than F. occidentalis (Todd et al. 1995, 1996, 1997). Thrips feeding injuries in the early stages of peanut growth can suppress plant growth, delay crop maturity, and potentially cause yield losses. Besides causing direct feeding injuries, both thrips species also transmit Tomato spotted wilt virus (TSWV; German et al. 1992, Ullman et al. 1992, Jones 2005, Whitfield et al. 2005, Pappu et al. 2009, Riley et al. 2011). TSWV is in the family Bunyaviridae and genus Tospovirus (Adkins 2000, Culbreath et al. 2003, Whitfield et al. 2005). TSWV causes spotted wilt disease in peanut, which incidentally is one of the most destructive diseases affecting peanut (Arachis hypogaea L.) production in the United States (Culbreath et al. 1997, 2003; Bertrand 1998; Culbreath and Srinivasan 2011). TSWV has been causing serious yield losses from
the late 1980s. In one estimate, in 1997 alone, 12% of the peanut crop (>US$40 million) was lost due to spotted wilt in Georgia (Bertrand 1998, Culbreath and Srinivasan 2011). Though recent losses due to TSWV have declined compared with 1980s, they continue to be in millions of dollars annually. Of the two TSWV vector species, F. fusca is believed to be more important for spotted wilt epidemics than F. occidentalis, as it is an early and efficient peanut foliage colonizer (Todd et al. 1995, 1996, 1997). Peanut plants infected with TSWV at an early stage display severe symptoms compared with plants infected at a later stage (Todd et al. 1997, Culbreath et al. 2003, Shrestha et al. 2015). Early-season TSWV transmission by incoming viruliferous thrips are responsible for the primary spread of TSWV (Groves et al. 2001, Morsello et al 2008, Chappell et al. 2013). Primary spread of TSWV is more critical than secondary spread. Colonizing thrips acquire TSWV from infected plants and inoculate it to noninfected plants in the same field typically causing secondary spread. Also, secondary spread usually
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Digiuseppe 2010, GFB News 2010, Bayer CropScience 2011). Thus, it is critical to identify and integrate safe and effective alternatives to carbamate and organophosphate insecticides into peanut IPM programs. The objective of this research was to evaluate eight alternative insecticides under field conditions, and quantify their effect on the number of thrips, thrips feeding damage, and final incidence of TSWV on TSWV-resistant and TSWV-susceptible peanut cultivars. The alternatives were chosen based on their reported efficacy against thrips and reduction of Tospovirus transmission in other cropping systems (Todd et al. 1994a;b; Herbert et al. 2007; Srivastava et al. 2008; Jacobson and Kennedy 2013; Seal et al. 2014). Various application methods were evaluated including seed treatment, in-furrow treatment, and at-crack liquid spray. To validate field study findings, the alternative insecticides that performed effectively under field conditions were also evaluated under laboratory or greenhouse conditions.
Materials and Methods Field Experiments Land Preparation, Weeds and Fungal Pathogens Management Two weeks before planting, fertilizers were applied and soil beds were prepared with conventional tillage. Trials were conducted under conventional tillage and bedded in preparation for planting. The pre-emergence herbicides ethalfluralin (Sonalan, Dow Agrosciences, Indianapolis, IN; 2.34 liter/ha) and S-metolachlor (Dual Magnum, CDMS, Marysville, CA; 2.34 liter/ha) were broadcast two days prior to planting and incorporated with a tillavator (Kelley Manufacturing Company, Tifton, GA). One day after planting flumioxazin (Valor, Valent Biosciences, Libertyville, IL) was broadcast with a boom sprayer. NPK fertilizer 5-10-15 was applied at a rate of 448.15 kg/ha. Peanut seeds were planted with a two-row vacuum planter (Monosem, Edwardsville, KS) and gypsum (1,120.85 kg/ha) was broadcast over the plots 30 d after planting. Clethodim (Select Max, Valent Biosciences, Libertyville, IL; 0.56 kg/ha) herbicide was applied twice between 60 and 80 DAP to manage grassy weeds. Fungal diseases were managed using a fungicide spray program recommended for peanut by the University of Georgia Cooperative Extension (Beasley 2011, http://blog.extension.uga.edu/dooly/files/ 2014/06/Peanut-Rx-and-Peanut-Fungicide-Programs.pdf). Evaluation of Alternative Insecticides Trials were conducted in Tift County Georgia (31.5079 N, 83.5578 W) to evaluate eight alternative insecticides to aldicarb and phorate on two peanut cultivars, Georgia Green (TSWV-suscep tible; Branch 1996) and Georgia-06G (TSWV-resistant; Branch 2007) from 2011 to 2013. The insecticides and their method and rate of application are included in Table 1. A split-plot design was used. Peanut cultivars, the main plot factor were arranged in a randomized complete block design with four replications, and insecticide treatments were randomly assigned to subplots within each main plot. Each six-row plot measured 9.14 m long by 5.49 m wide. Peanut was planted on 20 June, 25 April, and 27 April in 2011, 2012, and 2013, respectively. In 2012 and 2013, thrips samples were collected weekly over five and six consecutive weeks, respectively beginning 3 wk after planting. In 2011, thrips were sampled for only four consecutive weeks due to delayed planting. From each subplot, on each sampling date, 10 quadrifoliate peanut terminals or blooms were collected in the first and last 3-wk periods, respectively. Each sample was placed in a glass vial containing 10 ml of 70% ethyl alcohol and transported to the laboratory. Thrips were
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occurs later in the growing season when plants are less susceptible to infection. Hence, primary spread causes more yield losses than secondary spread (Culbreath et al. 2003, Culbreath and Srinivasan 2011). Therefore, it is vital to manage thrips early in the season and reduce primary TSWV transmission by thrips. Thrips and spotted wilt management requires an integrated approach combining TSWV-resistant cultivars, insecticide applications to suppress thrips populations and reduce TSWV transmission, and cultural practices that manipulate thrips–crop–virus interactions. Peanut cultivars with field resistance to TSWV are the first line of defense to manage spotted wilt. Even though currently available cultivars are substantially more resistant to the virus than previous cultivars, they are not immune to TSWV (Culbreath et al. 2003, Culbreath and Srinivasan 2011, Shrestha et al. 2013). However, no current cultivars possess resistance to thrips, and yield loss can be apparent under substantial thrips and TSWV pressure. To prevent yield losses, growers often use TSWV-resistant cultivars in conjunction with insecticides and cultural tactics, such as planting date manipulation, alteration in tillage, variation in row pattern, and increase in seeding rate (Gorbet and Shokes 1994, Baldwin et al. 2001, Branch et al. 2003, Culbreath et al. 2003, Marois and Wright 2003, Cantonwine et al. 2006, Tillman et al. 2006, Culbreath et al. 2008, Culbreath and Srinivasan 2011). Cultivars resistant to TSWV are vital because there is no other management tactic that reliably disrupts thrips feeding behavior and resultant virus inoculation. Viruliferous thrips can transmit TSWV with a brief inoculation access period lasting only a few minutes (Wijkamp and Peters 1993); this may explain why insecticides that ultimately kill thrips provide little reduction in virus transmission. Despite the drawbacks mentioned above, insecticides are still used to manage early season thrips feeding damage. Thrips feeding alone can stunt and in severe cases can kill peanut seedlings. Under favorable growing conditions, peanut plants appear to recover quickly from thrips injury; however, it is not clear how injury to seedlings affects time to maturity and/or yield (Todd et al. 1997, Culbreath et al. 2003, Herbert et al. 2007, Tubbs et al. 2013). The insecticides aldicarb (Temik) and phorate (Thimet) are commonly applied in-furrow at planting (Culbreath et al. 2003, Culbreath et al. 2008). Numerous studies have evaluated the impact of these two insecticides on thrips population suppression, thrips feeding, and spotted wilt incidences (Baldwin et al. 2001, Herbert et al. 2007, Culbreath et al. 2008, Tubbs et al. 2013). Even though both insecticides suppressed thrips feeding and thrips populations, phorate applications reduced spotted wilt incidences more than aldicarb (Wright et al. 2000, Baldwin et al. 2001, Culbreath et al. 2003, Todd et al. 2005, Herbert et al. 2007, Culbreath et al. 2008, Hagan et al. 2009). Thrips suppression does not always directly correlate with spotted wilt reduction, and spotted wilt reduction following phorate application might not be due thrips suppression (Todd et al. 1994a,b, 1997; Todd and Culbreath 1995; Marois and Wright 2003; Herbert et al. 2007; Culbreath et al. 2008; Culbreath and Srinivasan 2011). A recent paper indicated that phorate application to peanut plants led to an upregulation of transcripts associated with defense signaling pathways and down regulation of transcripts associated with virus internalization and replication in plant cells (Jain et al. 2015). Though generally effective in suppressing thrips and reducing spotted wilt incidence, aldicarb and phorate possess high mammalian toxicity and cause undesirable nontarget effects (Digiuseppe 2010). Recently, the United States Environmental Protection Agency (US-EPA) and Bayer CropScience agreed to halt the production of aldicarb and phase out its usage by 2018 (AgroNews 2010,
Journal of Economic Entomology, 2015, Vol. 0, No. 0
4 Nicotinic acetylcholine receptor (nAChR) agonists
1
3 Sodium channel modulators
23 Inhibitors of acetyl CoA carboxylase/ Lipid biosynthesis inhibitor
5 Nicotinic acetylcholine receptor (nAChR) allosteric activators
1 Acetylcholinesterase (AChE) inhibitors
7
8
9
10
b
a
Mode of action (MoA): Unknown—a compound with an unknown mode of action (IRAC 2012). Based on manufacturers’ recommended rates.
1B Organophosphates
1A Carbamates
Spinosyns
Tetronic and tetramic acid derivatives
3A Pyrethroids
Phorate
Aldicarb
Spinetoram
Spirotetratmat
lambda-Cyhalothrin
Thimet 10G
Temik 15G
Radiant SC
Movento 2SC
Karate
Exirel
Azatin XL
Cruiser Maxx
Price (US$)
$150/kg $52.42/l $52.42/l $201.06/kg NA $199.74/l NA $84.86/l $240.78/l
$196.14/l
NA $7.32/kg
Rate per hectareb 0.20 kg 0.51 l 0.12 l 0.28 kg 0.28 kg 1.53 l 1.49 l 0.26 l 0.37 l
0.37 l
5.60 kg 5.60 kg
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28 Ryanodine receptor modulators
6
Cyantraniliprole
Azadirachtin
Unknown
5 Diamides
Thiamethoxam
4
Admire Pro
Imidacloprid Assail 30SG
Admire Pro
Actara
Trade name
Imidacloprid
Thiamethoxam
Active ingredient
Acetamiprid
Unknown
4A Neonicotinoids
Chemical name/ subgroup or exemplifying active ingredient
3
2
Main group and primary site of actiona
Treatment no.
Table 1. List of selected insecticides for field trials
In-furrow
In-furrow
At-crack
At-crack
At-crack
At-crack
At-crack
Seed treatment
At-crack
At-crack
In-furrow
At-crack
Type of application
Amvac
Bayer CropScience
Dow AgroSciences
Bayer CropScience
Syngenta
DuPont
OHP, Inc.
Syngenta
United Phosphorous, Inc.
Bayer CropScience
Bayer CropScience
Syngenta
Manufacturer
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Greenhouse and Laboratory Experiments Greenhouse and laboratory experiments were conducted at the Department of Entomology at UGA Tifton Campus. These experiments were conducted in 2013. The insecticides used in these experiments were chosen based on the results obtained in the field in 2011 and 2012. Noninfected Peanut Plants Two peanut cultivars, Georgia Green and Georgia-06G, were used for all experiments. Seeds were pregerminated on moistened paper towels and incubated in a growth chamber at 27 C for 1 wk. Germinated peanut seeds were transplanted into 10-cm-diameter plastic pots (Hummert International, St. Louis, MO) containing commercial potting mix (LT5 Sunshine mix, Sun Gro Horticulture Industries, Bellevue, WA). Peanut plants were placed in 47.5-cm3 thrips-proof cages with very fine 60 60 per square cm mesh count nylon netting (Megaview Science Co., Taichung, Taiwan) and maintained in a greenhouse at 25 to 30 C with 80–90% relative humidity (RH) and a photoperiod of 14:10 (L:D) h. TSWV-Infected Peanut Plants TSWV-infected peanut plants of the cultivar Georgia Green were initially collected from research plots located in Tift Co., Georgia, in 2009. Plants were maintained in a greenhouse under conditions described above by repeated inoculations using viruliferous thrips in thrips-proof cages (Shrestha et al. 2012). The infection status of inoculated plants was assessed using double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) as described in Shrestha et al (2012). Leaf tissue (0.1 g) was used for DAS-ELISA. The assay was performed in a 96-well microtiter plate (Maxisorp,
Nunc, Rochester, NY) with suitable positive and negative controls. Primary antibody (anti-TSWV IgG, monoclonal against nucleocapsid protein (N)) was used at a dilution ratio of 1:200 and the secondary antibody (anti-TSWV IgG conjugated with alkaline phosphatase) was also used at a 1:200 dilution ratio (Agdia, Elkhart, IN). Incubation and washing steps were followed as per the manufacturer’s instructions. Final absorbance values were measured at 405 nm in a microplate reader 1 h after substrate addition (Model Elx 800, Bio-Tek, Kocherwaldstr, Germany). An average absorbance value of negative control samples plus four standard deviations was used as threshold to identify TSWV positive samples. Maintenance of Nonviruliferous F. fusca A colony of nonviruliferous F. fusca was established from individuals collected from peanut blooms in Tift Co., Georgia, in 2009. Thrips were maintained in Munger cages (11.43 L by 8.89 W by 1.77 B cm3; Munger 1942) containing noninfected peanut leaflets dusted with pine pollen. The leaflets were obtained from peanut plants with no TSWV-specific symptoms and maintained in thripsproof cages, and hence they were assumed as noninfected leaflets. Occasionally, lack of TSWV infection in such plants was confirmed by subjecting subsamples to DAS-ELISA. The colony was maintained in a growth chamber (Thermo scientific, Dubuque, IA) at 27 C with a photoperiod of 14:10 (L:D) h. New foliage was added to Munger cages every two days. Evaluation of Alternative Insecticides on Thrips Feeding Two peanut cultivars and four insecticides were evaluated for their effect on thrips feeding. Insecticides were applied to 1-wk-old peanut plant as foliar sprays using 946.4-ml plastic spray bottle. Peanut plants were air dried prior to the release of adult thrips. For in-furrow application, insecticides were applied in the soil in proximity to seeds during planting. Details of the insecticides and their methods and rate of application are included in Table 2. Five 1-wk-old peanut plants of each cultivar were placed separately in thrips-proof cages. Approximately 0.05 g of pine pollen grains (Pinus taeda L.) were dusted on each plant. Ten nonviruliferous female adult F. fusca (up to 2 d old) from the research colony were collected in 0.6-ml microcentrifuge tubes using a paintbrush (2 Silver 5300 S Round, India) and then released at the base of each plant. Thrips feeding damage was assessed on each plant every two days for a month after initial thrips release. The feeding damage index (FDI) was calculated based on the formula deduced by Maris et al. (2003) with minor adjustments. FDI ¼
No: of leaflets with feeding scars severity of feeding injuries Total no:of leaflets in a plant
The following scale was used to represent the percent of leaf area covered with feeding scars (severity of feeding injuries) (1 ¼ 0–20%, 2 ¼ 21–40%, 3 ¼ 41–60%, 4 ¼ 61–80%, and 5 ¼ 81–100%). The experiment was repeated twice (N ¼ 15 plants for each treatment and for each cultivar). A completely randomized design (CRD) was used. Peanut cultivars and insecticides were considered fixed effects while replications were considered random effects. Statistical differences in feeding damage index at three time intervals, 10, 20, and 30 d after thrips release, were assessed. PROC GLIMMIX in SAS was used for the analysis. The same plant population was observed for the entire sampling period, hence the observations were considered as repeated measures. Least square means at a ¼ 0.05 significance level was used to compare differences between treatments.
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enumerated under a dissecting microscope (40x) (MEIJI TECHNO, Santa Clara, CA) and identified to species (Riley et al. 2011). In 2011, only negligible thrips damage was observed, and the plots were not rated for thrips feeding damage. Thrips feeding injury was assessed in 2012 and 2013 using a visual scale that measured from 0 to 10 (wherein 0 represented no feeding injury and 10 represented a dead plant; Lynch et al. 1984, Brandenburg et al. 1998). Feeding injury was assessed on peanut plants in the second and fifth rows of each plot at 5 wk after planting. Spotted wilt incidence was rated visually using a standard procedure (Culbreath et al. 1997). Plants exhibiting spotted wilt symptoms in the third and fourth rows of each plot were identified and rated (Culbreath et al. 1997). In every plot, TSWV-infected plants in rows were estimated using a 30-cm hit stick and converted to percentages. Plots were rated for spotted wilt 2 wk prior to harvest. At harvest, peanut plants in the third and fourth rows of each plot were dug, inverted, air-dried, picked, and weighed following standard protocols (Cantonwine et al. 2006). Thrips injury rating and counts, spotted wilt incidence, and yield data were subjected to linear mixed model analyses using PROC GLIMMIX in SAS (SAS Enterprise 4.2, SAS Institute, Cary, NC). Thrips counts across all treatments at each sampling date, as well as thrips counts for main plots (cultivars) and subplots (insecticides) across dates were analyzed. Peanut cultivars and insecticides were considered fixed effects while replications were considered random effects. Interactions between insecticides and peanut cultivars, if any, were also analyzed. Tukey–Kramer grouping, as an adjustment for multiple comparisons at a ¼ 0.05 significance level, was used to test the statistical significance of differences among treatments and between cultivars.
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Table 2. List of selected alternative insecticides for greenhouse experiment Treatment no.
Active ingredient
Trade name
Rate per hectare (Ha)a
Rate per peanut plantb
Type of application
Manufacturer
1 2 3 4 5 6
Thiamethoxam Imidacloprid Cyantraniliprole Spinetoram Aldicarb Phorate
Actara Admire Pro Exirel Radiant 1SC Temik 15G Thimet 10G
0.20 kg 0.12 l 1.49 l 0.37 l 5.60 kg 5.60 kg
0.0014 g 0.00087 ml 0.018 ml 0.0025 ml 0.039 g 0.039 g
Foliar Spray Foliar Spray Foliar Spray Foliar Spray In-furrow In-furrow
Syngenta Bayer CropScience DuPont Dow AgroSciences Bayer CropScience Amvac
a b
Based on manufacturers’ recommended rates. Insecticide doses were calculated on a per plant basis at 143,260 peanut plants per hectare.
Effect of Alternative Insecticides on Thrips-Mediated TSWV Transmission Two peanut cultivars were evaluated for TSWV susceptibility. The same four insecticides identified in the feeding assay were used for the transmission assay. Insecticides were applied as described previously, and details of insecticide application are included in Table 2. Five 1-wk-old peanut plants for each treatment were used in the experiment, and the whole experiment was repeated twice (N ¼ 15 plants for each treatment and for each cultivar). Each plant was individually enclosed in a cylindrical Mylar film (Grafix, Cleveland, PA) cage (pr2h ¼ 3.14 16 39-cm3) with a copper mesh top (mesh pore size—170 microns; TWP, Berkeley, CA). Each plant was dusted with 0.05 g of pine pollen. Ten potentially viruliferous thrips were placed in a 0.6-ml microcentrifuge tube (Fisher Scientific, Pittsburgh, PA) using a paintbrush and then released at the base of each peanut plant. TSWV infection status of plants was confirmed by DAS-ELISA as described previously (Shrestha et al. 2012). A completely randomized design was used. Peanut cultivars and insecticides were considered fixed effects while replication was considered a random effect. Incidence of TSWV infection was compared among treatments and between peanut cultivars. TSWV incidence was treated as a binomial response (positive or negative), and data were analyzed using PROC GENMOD in SAS. Pairwise contrasts were used to test the statistical significance of differences between treatment pairs.
Results Field Experiments Thrips Identification and Densities Thrips on peanut terminal foliage and blooms were counted and identified. In 2011, adult thrips were identified to species; however, in 2012 and 2013, adults were identified as vectors or nonvectors. In all years, all immature thrips counts were combined, as species cannot be identified. The percentage of immatures ranged from 31.70 6 0.76 to 72.25 6 0.31. The percentage of vector adults
ranged from 25.89 6 0.49 to 68.30 6 0.76. The nonvector adult percentage ranged from 1.49 6 0.18 to 4.68 6 0.20. Among vectors, F. fusca was the predominant species, the percentage of F. fusca adults sampled ranged from 95.37 6 0.74 to 98.72 6 0.08. Irrespective of thrips species, more thrips were found in 2013 than in 2011 and in 2012 (Fig. 1). Irrespective of treatments, mean thrips counts across each sampling week revealed that peak thrips populations were found towards the end of June in 2011, and towards the middle and later part of May in 2012 and 2013, respectively (Fig. 1). Thrips counts were not affected by cultivar in 2011 (df ¼ 1,6; F ¼ 0.00; P ¼ 0.9662), 2012 (df ¼ 1,6; F ¼ 3.78; P ¼ 0.0997), or 2013 (df ¼ 1,6; F ¼ 5.78; P ¼ 0.0531). No interactions were observed between insecticides and cultivars in 2011 (df ¼ 11,354; F ¼ 0.82; P ¼ 0.6231) or 2012 (df ¼ 11,426; F ¼ 0.73; P ¼ 0.7146), but a significant interaction was seen in 2013 (df ¼ 11,426; F ¼ 1.86; P < 0.0426). The interaction effects were minor and varied mostly within each variety. Interaction effects did not reveal that any insecticide performed better with one cultivar over the other. Hence, for the sake of simplicity, insecticide treatment differences were estimated across cultivars. In 2011, thrips populations were affected by insecticide treatments (df ¼ 11, 354; F ¼ 5.36; P < 0.0001). Thiamethoxam, imidacloprid (in-furrow), and cyantraniliprole applications were as effective as commercial standards in suppressing thrips (Fig. 2). However, thrips suppression following thiamethoxam, imidacloprid (in-furrow) applications was not different from that of nontreated control. In 2012, fewer thrips were collected from aldicarb-treated plots and cyantraniliproletreated plots compared with nontreated plots (df ¼ 11,426; F ¼ 5.16; P < 0.0001; Fig. 2). In 2013, fewer thrips were observed on cyantraniliprole-treated plots compared with thiamethoxam (seed treatment)-, lambda-cyhalothrin-, and spirotetramat-treated plots (df ¼ 11,426; F ¼ 11.83; P < 0.0001). However, thrips counts on cyantraniliprole-treated plots were not different from thrips counts on spinetoram- and phorate-treated plots (Fig. 2). Thrips Feeding Damage Thrips feeding damage varied with insecticide treatments in 2012 (df ¼ 11,66; F ¼ 16.80; P < 0.0001) and 2013 (df ¼ 11,66; F ¼ 50.48; P < 0.0001; Fig. 3). Interactions were observed between insecticides and cultivars in 2012 (df ¼ 11,66; F ¼ 2.56; P ¼ 0.0093) and 2013 (df ¼ 11,66; F ¼ 5.52; P < 0.0001). Significantly more feeding damage was observed on Georgia Green than on Georgia06G (df ¼ 1,6; F ¼ 18.40; P ¼ 0.0052) in 2013, but not in 2012 (df ¼ 1,6; F ¼ 1.53; P ¼ 0.2626). In 2012, irrespective of cultivar, acetamiprid, azadirachtin, imidacloprid, spinetoram, and cyantraniliprole were as effective as aldicarb and phorate in reducing thrips feeding damage (Fig. 3). However, feeding damage on plots treated with those alternative insecticides was not different from
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Maintenance of Potentially Viruliferous F. fusca A colony of potentially viruliferous thrips was initiated and maintained on TSWV-infected peanut foliage in Munger cages as described for nonviruliferous thrips. Infected foliage obtained either from the greenhouse or from the Tift County field site, was routinely added to each cage. Infection status of peanut foliage was confirmed by DAS-ELISA. Thrips reared for an entire generation (adult to adult) on TSWV-infected leaflets were considered potentially viruliferous.
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Fig. 1. Thrips seasonal dynamics over three years (2011–2013), counts presented were averaged across all treatments by date. Count data were subjected to linear mixed model analysis across treatments using PROC GLIMMIX in SAS.
the feeding damage observed on nontreated control plots (Fig. 3). In addition, spinetoram treatment reduced thrips feeding damage better on GA-06G than on GA-Green. In 2013, feeding damage on cyantraniliprole-, imidacloprid (in furrow)-, and spinetoram-treated plots was less than feeding damage on plots treated with other alternative insecticides, but was similar on aldicarb- and phorate-treated plots. Thrips feeding damage following imidacloprid (in furrow) treatment was less on GA-06G plots than on GA-Green plots.
Spotted Wilt Incidence No interactions were observed between insecticides and cultivars in 2011 (df ¼ 11,66; F ¼ 1.29; P ¼ 0.2519), 2012 (df ¼ 11,66; F ¼ 1.15; P ¼ 0.3389), or 2013 (df ¼ 11,66; F ¼ 1.20; P ¼ 0.3021). Irrespective of insecticide and cultivar evaluated, spotted wilt incidence in 2013 was higher than in 2011 and 2012 (Fig. 4). Spotted wilt incidence was not affected by insecticide treatment in 2011 (df ¼ 11,66; F ¼ 1.17; P ¼ 0.3244) or in 2012 (df ¼ 11,66; F ¼ 1.84; P ¼ 0.0652). Spotted wilt incidence did not vary between Georgia Green and
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Georgia-06G in 2011 (df ¼ 1,6; F ¼ 5.54; P ¼ 0.0568), 2012 (df ¼ 1,6; F ¼ 0.45; P ¼ 0.5294), or 2013 (df ¼ 1,6; F ¼ 0.26; P ¼ 0.6270). In 2013, spotted wilt incidence in imidacloprid (atcrack)- and cyantraniliprole-treated plots was significantly higher than phorate-treated plots (df ¼ 11,66; F ¼ 2.22; P ¼ 0.0235; Fig. 4). Spotted wilt incidence in plots treated with other insecticides was not different from spotted wilt incidence in nontreated plots. Yields Irrespective of insecticide treatment, yield in 2012 of Georgia-06G was greater than that of Georgia Green (df ¼ 1,6; F ¼ 10.53; P ¼ 0.0176), but not in 2011 (df ¼ 1,6; F ¼ 1.10; P ¼ 0.3340) or 2013(df ¼ 1,6; F ¼ 1.85; P ¼ 0.2226). Irrespective of the cultivar, yield was not influenced by insecticide treatment in 2011 (df ¼ 11,66; F ¼ 1.57; P ¼ 0.1286) or 2012 (df ¼ 11,66; F ¼ 1.25; P ¼ 0.2751). However, in 2013, yields varied with insecticide treatments (df ¼ 11,66; F ¼ 4.90; P < 0.0001). No interactions were observed between insecticides and cultivars in 2011 (df ¼ 11,66; F ¼ 0.85; P ¼ 0.5909), 2012 (df ¼ 11,66; F ¼ 0.42; P ¼ 0.9420), or 2013 (df ¼ 11,66; F ¼ 1.29; P ¼ 0.2518). In 2013, the lowest yield was obtained from nontreated plots. In contrast, the highest yield was obtained from phorate-treated plots (Fig. 5). Yields from azadirachtin-, lambda-cyhalothrin-, spirotetramat-, and thiamethoxam (at-crack)-treated plots were less than yields obtained from phoratetreated plots. Yields obtained from aldicarb, imidacloprid, spinetoram, cyantraniliprole, and thiamethoxam (seed treatment)
treatments were not different from yields obtained from phorate treatment (Fig. 5).
Greenhouse Evaluations Thrips Feeding Damage Feeding damage indices were calculated for each cultivar separately. Significant differences in thrips feeding damage indices at 10, 20, and 30 d post thrips release were observed among insecticide treatments on both cultivars (Table 3). Feeding damage in all insecticide treatments was lower than in the nontreated control for both cultivars on all assessment dates (Fig. 6). At 10 d post thrips release, all insecticides were as effective as aldicarb and phorate in reducing thrips feeding on Georgia Green. Whereas on Georgia-06G, feeding damage indices from imidacloprid, cyantraniliprole, spinetoram, and aldicarb treated plants were significantly less than thiamethoxam and phorate treated plants at 10 d post thrips release (Table 3). Similar patterns were observed at 20 and 30 d post thrips release (Table 3). Apart from minor differences among insecticide treatments, all insecticides suppressed thrips feeding. Thrips feeding damage indices at 20 and 30 d post release on plants receiving insecticide treatments were lower than the damage indices on nontreated control plants (Table 3). TSWV Transmission by Thrips The incidence of TSWV infection (%) was evaluated for each insecticide treatment in Georgia Green and Georgia-06G by DAS-ELISA
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Fig. 2. Mean (6SE) cumulative counts of thrips across 6 wk over four replications of each treatment in two peanut cultivars. Treatment means followed by the same letter indicate that the treatments are not significantly different from each other. Quadrifoliate peanut terminal leaves were collected on the first 3 wk while peanut blooms were collected on the last 3 wk. Thrips samples were collected in 70% ethyl alcohol and identified to species under a dissecting microscope. Mean separation letters A and E represent highest and lowest thrips counts.
8
Journal of Economic Entomology, 2015, Vol. 0, No. 0
at 4 wk post inoculation. TSWV incidence in nontreated plants was greater than in insecticide-treated Georgia Green and Georgia-06G plants (Table 4). Irrespective of cultivar, incidence of TSWV infection in nontreated plants (80%) was at least twice as high as treated plants (F
Arthropods in Relation to Plant Disease
Evaluation of Alternatives to Carbamate and Organophosphate Insecticides Against Thrips and Tomato Spotted Wilt Virus in Peanut Production K. Marasigan,1 M. Toews,1 R. Kemerait Jr,2 M. R. Abney,1 A. Culbreath,2 and R. Srinivasan1,3 1
Department of Entomology, University of Georgia, Tifton, GA 31793 ([email protected]; [email protected]; [email protected]; [email protected]), 2Department of Plant Pathology, University of Georgia, Tifton, GA 31793 ([email protected]; spotwilt@ uga.edu), and 3Corresponding author, e-mail: [email protected] Received 24 July 2015; Accepted 2 November 2015
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Abstract Thrips are important pests of peanut. They cause severe feeding injuries on peanut foliage in the early season. They also transmit Tomato spotted wilt virus (TSWV), which causes spotted wilt disease. At-plant insecticides and cultivars that exhibit field resistance to TSWV are often used to manage thrips and spotted wilt disease. Historically, peanut growers used the broad-spectrum insecticides aldicarb (IRAC class 1A; Temik) and phorate (IRAC class 1B; Thimet) for managing thrips and thereby reducing TSWV transmission. Aldicarb has not been produced since 2011 and its usage in peanut will be legally phased out in 2018; therefore, identification of alternative chemistries is critical for thrips and spotted wilt management. Here, eight alternative insecticides, with known thrips activity, were evaluated in field trials conducted from 2011 through 2013. In addition, different application methods of alternatives were also evaluated. Imidacloprid (Admire Pro), thiamethoxam (Actara), spinetoram (Radiant), and cyantraniliprole (Exirel) were as effective as aldicarb and phorate in suppressing thrips, but none of the insecticides significantly suppressed spotted wilt incidence. Nevertheless, greenhouse assays demonstrated that the same alternative insecticides were effective in suppressing thrips feeding and reducing TSWV transmission. Spotted wilt incidence in the greenhouse was more severe (80%) than in the field (5–25%). In general, field resistance to TSWV in cultivars only marginally influenced spotted wilt incidence. Results suggest that effective management of thrips using alternative insecticides and subsequent feeding reduction could improve yields under low to moderate virus pressure. Key words: tobacco thrips, spotted wilt, vector and disease management
Tobacco thrips, Frankliniella fusca (Hinds), and western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), are pests of peanut. The former is known to colonize and reproduce more efficiently on peanut foliage, especially in the early season, than F. occidentalis (Todd et al. 1995, 1996, 1997). Thrips feeding injuries in the early stages of peanut growth can suppress plant growth, delay crop maturity, and potentially cause yield losses. Besides causing direct feeding injuries, both thrips species also transmit Tomato spotted wilt virus (TSWV; German et al. 1992, Ullman et al. 1992, Jones 2005, Whitfield et al. 2005, Pappu et al. 2009, Riley et al. 2011). TSWV is in the family Bunyaviridae and genus Tospovirus (Adkins 2000, Culbreath et al. 2003, Whitfield et al. 2005). TSWV causes spotted wilt disease in peanut, which incidentally is one of the most destructive diseases affecting peanut (Arachis hypogaea L.) production in the United States (Culbreath et al. 1997, 2003; Bertrand 1998; Culbreath and Srinivasan 2011). TSWV has been causing serious yield losses from
the late 1980s. In one estimate, in 1997 alone, 12% of the peanut crop (>US$40 million) was lost due to spotted wilt in Georgia (Bertrand 1998, Culbreath and Srinivasan 2011). Though recent losses due to TSWV have declined compared with 1980s, they continue to be in millions of dollars annually. Of the two TSWV vector species, F. fusca is believed to be more important for spotted wilt epidemics than F. occidentalis, as it is an early and efficient peanut foliage colonizer (Todd et al. 1995, 1996, 1997). Peanut plants infected with TSWV at an early stage display severe symptoms compared with plants infected at a later stage (Todd et al. 1997, Culbreath et al. 2003, Shrestha et al. 2015). Early-season TSWV transmission by incoming viruliferous thrips are responsible for the primary spread of TSWV (Groves et al. 2001, Morsello et al 2008, Chappell et al. 2013). Primary spread of TSWV is more critical than secondary spread. Colonizing thrips acquire TSWV from infected plants and inoculate it to noninfected plants in the same field typically causing secondary spread. Also, secondary spread usually
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Digiuseppe 2010, GFB News 2010, Bayer CropScience 2011). Thus, it is critical to identify and integrate safe and effective alternatives to carbamate and organophosphate insecticides into peanut IPM programs. The objective of this research was to evaluate eight alternative insecticides under field conditions, and quantify their effect on the number of thrips, thrips feeding damage, and final incidence of TSWV on TSWV-resistant and TSWV-susceptible peanut cultivars. The alternatives were chosen based on their reported efficacy against thrips and reduction of Tospovirus transmission in other cropping systems (Todd et al. 1994a;b; Herbert et al. 2007; Srivastava et al. 2008; Jacobson and Kennedy 2013; Seal et al. 2014). Various application methods were evaluated including seed treatment, in-furrow treatment, and at-crack liquid spray. To validate field study findings, the alternative insecticides that performed effectively under field conditions were also evaluated under laboratory or greenhouse conditions.
Materials and Methods Field Experiments Land Preparation, Weeds and Fungal Pathogens Management Two weeks before planting, fertilizers were applied and soil beds were prepared with conventional tillage. Trials were conducted under conventional tillage and bedded in preparation for planting. The pre-emergence herbicides ethalfluralin (Sonalan, Dow Agrosciences, Indianapolis, IN; 2.34 liter/ha) and S-metolachlor (Dual Magnum, CDMS, Marysville, CA; 2.34 liter/ha) were broadcast two days prior to planting and incorporated with a tillavator (Kelley Manufacturing Company, Tifton, GA). One day after planting flumioxazin (Valor, Valent Biosciences, Libertyville, IL) was broadcast with a boom sprayer. NPK fertilizer 5-10-15 was applied at a rate of 448.15 kg/ha. Peanut seeds were planted with a two-row vacuum planter (Monosem, Edwardsville, KS) and gypsum (1,120.85 kg/ha) was broadcast over the plots 30 d after planting. Clethodim (Select Max, Valent Biosciences, Libertyville, IL; 0.56 kg/ha) herbicide was applied twice between 60 and 80 DAP to manage grassy weeds. Fungal diseases were managed using a fungicide spray program recommended for peanut by the University of Georgia Cooperative Extension (Beasley 2011, http://blog.extension.uga.edu/dooly/files/ 2014/06/Peanut-Rx-and-Peanut-Fungicide-Programs.pdf). Evaluation of Alternative Insecticides Trials were conducted in Tift County Georgia (31.5079 N, 83.5578 W) to evaluate eight alternative insecticides to aldicarb and phorate on two peanut cultivars, Georgia Green (TSWV-suscep tible; Branch 1996) and Georgia-06G (TSWV-resistant; Branch 2007) from 2011 to 2013. The insecticides and their method and rate of application are included in Table 1. A split-plot design was used. Peanut cultivars, the main plot factor were arranged in a randomized complete block design with four replications, and insecticide treatments were randomly assigned to subplots within each main plot. Each six-row plot measured 9.14 m long by 5.49 m wide. Peanut was planted on 20 June, 25 April, and 27 April in 2011, 2012, and 2013, respectively. In 2012 and 2013, thrips samples were collected weekly over five and six consecutive weeks, respectively beginning 3 wk after planting. In 2011, thrips were sampled for only four consecutive weeks due to delayed planting. From each subplot, on each sampling date, 10 quadrifoliate peanut terminals or blooms were collected in the first and last 3-wk periods, respectively. Each sample was placed in a glass vial containing 10 ml of 70% ethyl alcohol and transported to the laboratory. Thrips were
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occurs later in the growing season when plants are less susceptible to infection. Hence, primary spread causes more yield losses than secondary spread (Culbreath et al. 2003, Culbreath and Srinivasan 2011). Therefore, it is vital to manage thrips early in the season and reduce primary TSWV transmission by thrips. Thrips and spotted wilt management requires an integrated approach combining TSWV-resistant cultivars, insecticide applications to suppress thrips populations and reduce TSWV transmission, and cultural practices that manipulate thrips–crop–virus interactions. Peanut cultivars with field resistance to TSWV are the first line of defense to manage spotted wilt. Even though currently available cultivars are substantially more resistant to the virus than previous cultivars, they are not immune to TSWV (Culbreath et al. 2003, Culbreath and Srinivasan 2011, Shrestha et al. 2013). However, no current cultivars possess resistance to thrips, and yield loss can be apparent under substantial thrips and TSWV pressure. To prevent yield losses, growers often use TSWV-resistant cultivars in conjunction with insecticides and cultural tactics, such as planting date manipulation, alteration in tillage, variation in row pattern, and increase in seeding rate (Gorbet and Shokes 1994, Baldwin et al. 2001, Branch et al. 2003, Culbreath et al. 2003, Marois and Wright 2003, Cantonwine et al. 2006, Tillman et al. 2006, Culbreath et al. 2008, Culbreath and Srinivasan 2011). Cultivars resistant to TSWV are vital because there is no other management tactic that reliably disrupts thrips feeding behavior and resultant virus inoculation. Viruliferous thrips can transmit TSWV with a brief inoculation access period lasting only a few minutes (Wijkamp and Peters 1993); this may explain why insecticides that ultimately kill thrips provide little reduction in virus transmission. Despite the drawbacks mentioned above, insecticides are still used to manage early season thrips feeding damage. Thrips feeding alone can stunt and in severe cases can kill peanut seedlings. Under favorable growing conditions, peanut plants appear to recover quickly from thrips injury; however, it is not clear how injury to seedlings affects time to maturity and/or yield (Todd et al. 1997, Culbreath et al. 2003, Herbert et al. 2007, Tubbs et al. 2013). The insecticides aldicarb (Temik) and phorate (Thimet) are commonly applied in-furrow at planting (Culbreath et al. 2003, Culbreath et al. 2008). Numerous studies have evaluated the impact of these two insecticides on thrips population suppression, thrips feeding, and spotted wilt incidences (Baldwin et al. 2001, Herbert et al. 2007, Culbreath et al. 2008, Tubbs et al. 2013). Even though both insecticides suppressed thrips feeding and thrips populations, phorate applications reduced spotted wilt incidences more than aldicarb (Wright et al. 2000, Baldwin et al. 2001, Culbreath et al. 2003, Todd et al. 2005, Herbert et al. 2007, Culbreath et al. 2008, Hagan et al. 2009). Thrips suppression does not always directly correlate with spotted wilt reduction, and spotted wilt reduction following phorate application might not be due thrips suppression (Todd et al. 1994a,b, 1997; Todd and Culbreath 1995; Marois and Wright 2003; Herbert et al. 2007; Culbreath et al. 2008; Culbreath and Srinivasan 2011). A recent paper indicated that phorate application to peanut plants led to an upregulation of transcripts associated with defense signaling pathways and down regulation of transcripts associated with virus internalization and replication in plant cells (Jain et al. 2015). Though generally effective in suppressing thrips and reducing spotted wilt incidence, aldicarb and phorate possess high mammalian toxicity and cause undesirable nontarget effects (Digiuseppe 2010). Recently, the United States Environmental Protection Agency (US-EPA) and Bayer CropScience agreed to halt the production of aldicarb and phase out its usage by 2018 (AgroNews 2010,
Journal of Economic Entomology, 2015, Vol. 0, No. 0
4 Nicotinic acetylcholine receptor (nAChR) agonists
1
3 Sodium channel modulators
23 Inhibitors of acetyl CoA carboxylase/ Lipid biosynthesis inhibitor
5 Nicotinic acetylcholine receptor (nAChR) allosteric activators
1 Acetylcholinesterase (AChE) inhibitors
7
8
9
10
b
a
Mode of action (MoA): Unknown—a compound with an unknown mode of action (IRAC 2012). Based on manufacturers’ recommended rates.
1B Organophosphates
1A Carbamates
Spinosyns
Tetronic and tetramic acid derivatives
3A Pyrethroids
Phorate
Aldicarb
Spinetoram
Spirotetratmat
lambda-Cyhalothrin
Thimet 10G
Temik 15G
Radiant SC
Movento 2SC
Karate
Exirel
Azatin XL
Cruiser Maxx
Price (US$)
$150/kg $52.42/l $52.42/l $201.06/kg NA $199.74/l NA $84.86/l $240.78/l
$196.14/l
NA $7.32/kg
Rate per hectareb 0.20 kg 0.51 l 0.12 l 0.28 kg 0.28 kg 1.53 l 1.49 l 0.26 l 0.37 l
0.37 l
5.60 kg 5.60 kg
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11
28 Ryanodine receptor modulators
6
Cyantraniliprole
Azadirachtin
Unknown
5 Diamides
Thiamethoxam
4
Admire Pro
Imidacloprid Assail 30SG
Admire Pro
Actara
Trade name
Imidacloprid
Thiamethoxam
Active ingredient
Acetamiprid
Unknown
4A Neonicotinoids
Chemical name/ subgroup or exemplifying active ingredient
3
2
Main group and primary site of actiona
Treatment no.
Table 1. List of selected insecticides for field trials
In-furrow
In-furrow
At-crack
At-crack
At-crack
At-crack
At-crack
Seed treatment
At-crack
At-crack
In-furrow
At-crack
Type of application
Amvac
Bayer CropScience
Dow AgroSciences
Bayer CropScience
Syngenta
DuPont
OHP, Inc.
Syngenta
United Phosphorous, Inc.
Bayer CropScience
Bayer CropScience
Syngenta
Manufacturer
Journal of Economic Entomology, 2015, Vol. 0, No. 0 3
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Greenhouse and Laboratory Experiments Greenhouse and laboratory experiments were conducted at the Department of Entomology at UGA Tifton Campus. These experiments were conducted in 2013. The insecticides used in these experiments were chosen based on the results obtained in the field in 2011 and 2012. Noninfected Peanut Plants Two peanut cultivars, Georgia Green and Georgia-06G, were used for all experiments. Seeds were pregerminated on moistened paper towels and incubated in a growth chamber at 27 C for 1 wk. Germinated peanut seeds were transplanted into 10-cm-diameter plastic pots (Hummert International, St. Louis, MO) containing commercial potting mix (LT5 Sunshine mix, Sun Gro Horticulture Industries, Bellevue, WA). Peanut plants were placed in 47.5-cm3 thrips-proof cages with very fine 60 60 per square cm mesh count nylon netting (Megaview Science Co., Taichung, Taiwan) and maintained in a greenhouse at 25 to 30 C with 80–90% relative humidity (RH) and a photoperiod of 14:10 (L:D) h. TSWV-Infected Peanut Plants TSWV-infected peanut plants of the cultivar Georgia Green were initially collected from research plots located in Tift Co., Georgia, in 2009. Plants were maintained in a greenhouse under conditions described above by repeated inoculations using viruliferous thrips in thrips-proof cages (Shrestha et al. 2012). The infection status of inoculated plants was assessed using double antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) as described in Shrestha et al (2012). Leaf tissue (0.1 g) was used for DAS-ELISA. The assay was performed in a 96-well microtiter plate (Maxisorp,
Nunc, Rochester, NY) with suitable positive and negative controls. Primary antibody (anti-TSWV IgG, monoclonal against nucleocapsid protein (N)) was used at a dilution ratio of 1:200 and the secondary antibody (anti-TSWV IgG conjugated with alkaline phosphatase) was also used at a 1:200 dilution ratio (Agdia, Elkhart, IN). Incubation and washing steps were followed as per the manufacturer’s instructions. Final absorbance values were measured at 405 nm in a microplate reader 1 h after substrate addition (Model Elx 800, Bio-Tek, Kocherwaldstr, Germany). An average absorbance value of negative control samples plus four standard deviations was used as threshold to identify TSWV positive samples. Maintenance of Nonviruliferous F. fusca A colony of nonviruliferous F. fusca was established from individuals collected from peanut blooms in Tift Co., Georgia, in 2009. Thrips were maintained in Munger cages (11.43 L by 8.89 W by 1.77 B cm3; Munger 1942) containing noninfected peanut leaflets dusted with pine pollen. The leaflets were obtained from peanut plants with no TSWV-specific symptoms and maintained in thripsproof cages, and hence they were assumed as noninfected leaflets. Occasionally, lack of TSWV infection in such plants was confirmed by subjecting subsamples to DAS-ELISA. The colony was maintained in a growth chamber (Thermo scientific, Dubuque, IA) at 27 C with a photoperiod of 14:10 (L:D) h. New foliage was added to Munger cages every two days. Evaluation of Alternative Insecticides on Thrips Feeding Two peanut cultivars and four insecticides were evaluated for their effect on thrips feeding. Insecticides were applied to 1-wk-old peanut plant as foliar sprays using 946.4-ml plastic spray bottle. Peanut plants were air dried prior to the release of adult thrips. For in-furrow application, insecticides were applied in the soil in proximity to seeds during planting. Details of the insecticides and their methods and rate of application are included in Table 2. Five 1-wk-old peanut plants of each cultivar were placed separately in thrips-proof cages. Approximately 0.05 g of pine pollen grains (Pinus taeda L.) were dusted on each plant. Ten nonviruliferous female adult F. fusca (up to 2 d old) from the research colony were collected in 0.6-ml microcentrifuge tubes using a paintbrush (2 Silver 5300 S Round, India) and then released at the base of each plant. Thrips feeding damage was assessed on each plant every two days for a month after initial thrips release. The feeding damage index (FDI) was calculated based on the formula deduced by Maris et al. (2003) with minor adjustments. FDI ¼
No: of leaflets with feeding scars severity of feeding injuries Total no:of leaflets in a plant
The following scale was used to represent the percent of leaf area covered with feeding scars (severity of feeding injuries) (1 ¼ 0–20%, 2 ¼ 21–40%, 3 ¼ 41–60%, 4 ¼ 61–80%, and 5 ¼ 81–100%). The experiment was repeated twice (N ¼ 15 plants for each treatment and for each cultivar). A completely randomized design (CRD) was used. Peanut cultivars and insecticides were considered fixed effects while replications were considered random effects. Statistical differences in feeding damage index at three time intervals, 10, 20, and 30 d after thrips release, were assessed. PROC GLIMMIX in SAS was used for the analysis. The same plant population was observed for the entire sampling period, hence the observations were considered as repeated measures. Least square means at a ¼ 0.05 significance level was used to compare differences between treatments.
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enumerated under a dissecting microscope (40x) (MEIJI TECHNO, Santa Clara, CA) and identified to species (Riley et al. 2011). In 2011, only negligible thrips damage was observed, and the plots were not rated for thrips feeding damage. Thrips feeding injury was assessed in 2012 and 2013 using a visual scale that measured from 0 to 10 (wherein 0 represented no feeding injury and 10 represented a dead plant; Lynch et al. 1984, Brandenburg et al. 1998). Feeding injury was assessed on peanut plants in the second and fifth rows of each plot at 5 wk after planting. Spotted wilt incidence was rated visually using a standard procedure (Culbreath et al. 1997). Plants exhibiting spotted wilt symptoms in the third and fourth rows of each plot were identified and rated (Culbreath et al. 1997). In every plot, TSWV-infected plants in rows were estimated using a 30-cm hit stick and converted to percentages. Plots were rated for spotted wilt 2 wk prior to harvest. At harvest, peanut plants in the third and fourth rows of each plot were dug, inverted, air-dried, picked, and weighed following standard protocols (Cantonwine et al. 2006). Thrips injury rating and counts, spotted wilt incidence, and yield data were subjected to linear mixed model analyses using PROC GLIMMIX in SAS (SAS Enterprise 4.2, SAS Institute, Cary, NC). Thrips counts across all treatments at each sampling date, as well as thrips counts for main plots (cultivars) and subplots (insecticides) across dates were analyzed. Peanut cultivars and insecticides were considered fixed effects while replications were considered random effects. Interactions between insecticides and peanut cultivars, if any, were also analyzed. Tukey–Kramer grouping, as an adjustment for multiple comparisons at a ¼ 0.05 significance level, was used to test the statistical significance of differences among treatments and between cultivars.
Journal of Economic Entomology, 2015, Vol. 0, No. 0
Journal of Economic Entomology, 2015, Vol. 0, No. 0
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Table 2. List of selected alternative insecticides for greenhouse experiment Treatment no.
Active ingredient
Trade name
Rate per hectare (Ha)a
Rate per peanut plantb
Type of application
Manufacturer
1 2 3 4 5 6
Thiamethoxam Imidacloprid Cyantraniliprole Spinetoram Aldicarb Phorate
Actara Admire Pro Exirel Radiant 1SC Temik 15G Thimet 10G
0.20 kg 0.12 l 1.49 l 0.37 l 5.60 kg 5.60 kg
0.0014 g 0.00087 ml 0.018 ml 0.0025 ml 0.039 g 0.039 g
Foliar Spray Foliar Spray Foliar Spray Foliar Spray In-furrow In-furrow
Syngenta Bayer CropScience DuPont Dow AgroSciences Bayer CropScience Amvac
a b
Based on manufacturers’ recommended rates. Insecticide doses were calculated on a per plant basis at 143,260 peanut plants per hectare.
Effect of Alternative Insecticides on Thrips-Mediated TSWV Transmission Two peanut cultivars were evaluated for TSWV susceptibility. The same four insecticides identified in the feeding assay were used for the transmission assay. Insecticides were applied as described previously, and details of insecticide application are included in Table 2. Five 1-wk-old peanut plants for each treatment were used in the experiment, and the whole experiment was repeated twice (N ¼ 15 plants for each treatment and for each cultivar). Each plant was individually enclosed in a cylindrical Mylar film (Grafix, Cleveland, PA) cage (pr2h ¼ 3.14 16 39-cm3) with a copper mesh top (mesh pore size—170 microns; TWP, Berkeley, CA). Each plant was dusted with 0.05 g of pine pollen. Ten potentially viruliferous thrips were placed in a 0.6-ml microcentrifuge tube (Fisher Scientific, Pittsburgh, PA) using a paintbrush and then released at the base of each peanut plant. TSWV infection status of plants was confirmed by DAS-ELISA as described previously (Shrestha et al. 2012). A completely randomized design was used. Peanut cultivars and insecticides were considered fixed effects while replication was considered a random effect. Incidence of TSWV infection was compared among treatments and between peanut cultivars. TSWV incidence was treated as a binomial response (positive or negative), and data were analyzed using PROC GENMOD in SAS. Pairwise contrasts were used to test the statistical significance of differences between treatment pairs.
Results Field Experiments Thrips Identification and Densities Thrips on peanut terminal foliage and blooms were counted and identified. In 2011, adult thrips were identified to species; however, in 2012 and 2013, adults were identified as vectors or nonvectors. In all years, all immature thrips counts were combined, as species cannot be identified. The percentage of immatures ranged from 31.70 6 0.76 to 72.25 6 0.31. The percentage of vector adults
ranged from 25.89 6 0.49 to 68.30 6 0.76. The nonvector adult percentage ranged from 1.49 6 0.18 to 4.68 6 0.20. Among vectors, F. fusca was the predominant species, the percentage of F. fusca adults sampled ranged from 95.37 6 0.74 to 98.72 6 0.08. Irrespective of thrips species, more thrips were found in 2013 than in 2011 and in 2012 (Fig. 1). Irrespective of treatments, mean thrips counts across each sampling week revealed that peak thrips populations were found towards the end of June in 2011, and towards the middle and later part of May in 2012 and 2013, respectively (Fig. 1). Thrips counts were not affected by cultivar in 2011 (df ¼ 1,6; F ¼ 0.00; P ¼ 0.9662), 2012 (df ¼ 1,6; F ¼ 3.78; P ¼ 0.0997), or 2013 (df ¼ 1,6; F ¼ 5.78; P ¼ 0.0531). No interactions were observed between insecticides and cultivars in 2011 (df ¼ 11,354; F ¼ 0.82; P ¼ 0.6231) or 2012 (df ¼ 11,426; F ¼ 0.73; P ¼ 0.7146), but a significant interaction was seen in 2013 (df ¼ 11,426; F ¼ 1.86; P < 0.0426). The interaction effects were minor and varied mostly within each variety. Interaction effects did not reveal that any insecticide performed better with one cultivar over the other. Hence, for the sake of simplicity, insecticide treatment differences were estimated across cultivars. In 2011, thrips populations were affected by insecticide treatments (df ¼ 11, 354; F ¼ 5.36; P < 0.0001). Thiamethoxam, imidacloprid (in-furrow), and cyantraniliprole applications were as effective as commercial standards in suppressing thrips (Fig. 2). However, thrips suppression following thiamethoxam, imidacloprid (in-furrow) applications was not different from that of nontreated control. In 2012, fewer thrips were collected from aldicarb-treated plots and cyantraniliproletreated plots compared with nontreated plots (df ¼ 11,426; F ¼ 5.16; P < 0.0001; Fig. 2). In 2013, fewer thrips were observed on cyantraniliprole-treated plots compared with thiamethoxam (seed treatment)-, lambda-cyhalothrin-, and spirotetramat-treated plots (df ¼ 11,426; F ¼ 11.83; P < 0.0001). However, thrips counts on cyantraniliprole-treated plots were not different from thrips counts on spinetoram- and phorate-treated plots (Fig. 2). Thrips Feeding Damage Thrips feeding damage varied with insecticide treatments in 2012 (df ¼ 11,66; F ¼ 16.80; P < 0.0001) and 2013 (df ¼ 11,66; F ¼ 50.48; P < 0.0001; Fig. 3). Interactions were observed between insecticides and cultivars in 2012 (df ¼ 11,66; F ¼ 2.56; P ¼ 0.0093) and 2013 (df ¼ 11,66; F ¼ 5.52; P < 0.0001). Significantly more feeding damage was observed on Georgia Green than on Georgia06G (df ¼ 1,6; F ¼ 18.40; P ¼ 0.0052) in 2013, but not in 2012 (df ¼ 1,6; F ¼ 1.53; P ¼ 0.2626). In 2012, irrespective of cultivar, acetamiprid, azadirachtin, imidacloprid, spinetoram, and cyantraniliprole were as effective as aldicarb and phorate in reducing thrips feeding damage (Fig. 3). However, feeding damage on plots treated with those alternative insecticides was not different from
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Maintenance of Potentially Viruliferous F. fusca A colony of potentially viruliferous thrips was initiated and maintained on TSWV-infected peanut foliage in Munger cages as described for nonviruliferous thrips. Infected foliage obtained either from the greenhouse or from the Tift County field site, was routinely added to each cage. Infection status of peanut foliage was confirmed by DAS-ELISA. Thrips reared for an entire generation (adult to adult) on TSWV-infected leaflets were considered potentially viruliferous.
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Fig. 1. Thrips seasonal dynamics over three years (2011–2013), counts presented were averaged across all treatments by date. Count data were subjected to linear mixed model analysis across treatments using PROC GLIMMIX in SAS.
the feeding damage observed on nontreated control plots (Fig. 3). In addition, spinetoram treatment reduced thrips feeding damage better on GA-06G than on GA-Green. In 2013, feeding damage on cyantraniliprole-, imidacloprid (in furrow)-, and spinetoram-treated plots was less than feeding damage on plots treated with other alternative insecticides, but was similar on aldicarb- and phorate-treated plots. Thrips feeding damage following imidacloprid (in furrow) treatment was less on GA-06G plots than on GA-Green plots.
Spotted Wilt Incidence No interactions were observed between insecticides and cultivars in 2011 (df ¼ 11,66; F ¼ 1.29; P ¼ 0.2519), 2012 (df ¼ 11,66; F ¼ 1.15; P ¼ 0.3389), or 2013 (df ¼ 11,66; F ¼ 1.20; P ¼ 0.3021). Irrespective of insecticide and cultivar evaluated, spotted wilt incidence in 2013 was higher than in 2011 and 2012 (Fig. 4). Spotted wilt incidence was not affected by insecticide treatment in 2011 (df ¼ 11,66; F ¼ 1.17; P ¼ 0.3244) or in 2012 (df ¼ 11,66; F ¼ 1.84; P ¼ 0.0652). Spotted wilt incidence did not vary between Georgia Green and
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Georgia-06G in 2011 (df ¼ 1,6; F ¼ 5.54; P ¼ 0.0568), 2012 (df ¼ 1,6; F ¼ 0.45; P ¼ 0.5294), or 2013 (df ¼ 1,6; F ¼ 0.26; P ¼ 0.6270). In 2013, spotted wilt incidence in imidacloprid (atcrack)- and cyantraniliprole-treated plots was significantly higher than phorate-treated plots (df ¼ 11,66; F ¼ 2.22; P ¼ 0.0235; Fig. 4). Spotted wilt incidence in plots treated with other insecticides was not different from spotted wilt incidence in nontreated plots. Yields Irrespective of insecticide treatment, yield in 2012 of Georgia-06G was greater than that of Georgia Green (df ¼ 1,6; F ¼ 10.53; P ¼ 0.0176), but not in 2011 (df ¼ 1,6; F ¼ 1.10; P ¼ 0.3340) or 2013(df ¼ 1,6; F ¼ 1.85; P ¼ 0.2226). Irrespective of the cultivar, yield was not influenced by insecticide treatment in 2011 (df ¼ 11,66; F ¼ 1.57; P ¼ 0.1286) or 2012 (df ¼ 11,66; F ¼ 1.25; P ¼ 0.2751). However, in 2013, yields varied with insecticide treatments (df ¼ 11,66; F ¼ 4.90; P < 0.0001). No interactions were observed between insecticides and cultivars in 2011 (df ¼ 11,66; F ¼ 0.85; P ¼ 0.5909), 2012 (df ¼ 11,66; F ¼ 0.42; P ¼ 0.9420), or 2013 (df ¼ 11,66; F ¼ 1.29; P ¼ 0.2518). In 2013, the lowest yield was obtained from nontreated plots. In contrast, the highest yield was obtained from phorate-treated plots (Fig. 5). Yields from azadirachtin-, lambda-cyhalothrin-, spirotetramat-, and thiamethoxam (at-crack)-treated plots were less than yields obtained from phoratetreated plots. Yields obtained from aldicarb, imidacloprid, spinetoram, cyantraniliprole, and thiamethoxam (seed treatment)
treatments were not different from yields obtained from phorate treatment (Fig. 5).
Greenhouse Evaluations Thrips Feeding Damage Feeding damage indices were calculated for each cultivar separately. Significant differences in thrips feeding damage indices at 10, 20, and 30 d post thrips release were observed among insecticide treatments on both cultivars (Table 3). Feeding damage in all insecticide treatments was lower than in the nontreated control for both cultivars on all assessment dates (Fig. 6). At 10 d post thrips release, all insecticides were as effective as aldicarb and phorate in reducing thrips feeding on Georgia Green. Whereas on Georgia-06G, feeding damage indices from imidacloprid, cyantraniliprole, spinetoram, and aldicarb treated plants were significantly less than thiamethoxam and phorate treated plants at 10 d post thrips release (Table 3). Similar patterns were observed at 20 and 30 d post thrips release (Table 3). Apart from minor differences among insecticide treatments, all insecticides suppressed thrips feeding. Thrips feeding damage indices at 20 and 30 d post release on plants receiving insecticide treatments were lower than the damage indices on nontreated control plants (Table 3). TSWV Transmission by Thrips The incidence of TSWV infection (%) was evaluated for each insecticide treatment in Georgia Green and Georgia-06G by DAS-ELISA
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Fig. 2. Mean (6SE) cumulative counts of thrips across 6 wk over four replications of each treatment in two peanut cultivars. Treatment means followed by the same letter indicate that the treatments are not significantly different from each other. Quadrifoliate peanut terminal leaves were collected on the first 3 wk while peanut blooms were collected on the last 3 wk. Thrips samples were collected in 70% ethyl alcohol and identified to species under a dissecting microscope. Mean separation letters A and E represent highest and lowest thrips counts.
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at 4 wk post inoculation. TSWV incidence in nontreated plants was greater than in insecticide-treated Georgia Green and Georgia-06G plants (Table 4). Irrespective of cultivar, incidence of TSWV infection in nontreated plants (80%) was at least twice as high as treated plants (F
