Environmental concentrations and aquatic toxicity data on diflubenzuron (dimilin).

Critical Reviews in Toxicology, 22( 1):45-79 (1992)

Environmental Concentrations and Aquatic Toxicity Data on Diflubenzuron (Dimilin) Steven A. Fischer and Lenwood W. Hall, Jr.

Critical Reviews in Toxicology Downloaded from informahealthcare.com by CDL-UC San Diego on 02/02/15 For personal use only.

University of Maryland System, Maryland Agricultural Experiment Station, Wye Research and Education Center, P.O. Box 169, Queenstown, MD 21658

ABSTRACT: The insecticide diflubenzuron (DFB) is commonly used in various mid-Atlantic states for suppression of gypsy moths in hardwood forests. DFB is potentially toxic to nontarget biota because it can enter aquatic systems through aerial application or runoff after precipitation events. Based on this concern, the objectives of this study were to: (1) compile, review, and synthesize literature on the fate, persistence, and environmental concentrations of DFB in both freshwater and saltwater environments; (2) compile, review, and synthesize acute and chronic aquatic toxicity data on DFB effects on freshwater and saltwater organisms; (3) assess possible risk to aquatic biota associated with the use of this insecticide in one specific area (Maryland); and (4) recommend future research based on the data gaps identified from this study. DFB has low solubility in water and exists as a technical grade (TG) and wettable powder (W)formulation. The toxicity of both formulations is similar at concentrations 50,000pg/l. Fish were also reported to accumulate DFB rapidly during acute exposures but were capable of eliminating this insecticide within 7 days. Most of the DFB aquatic toxicity studies with saltwater organisms were conducted with invertebrates. The most acutely sensitive species tested was the premolt stage of grass shrimp (96-h LC,, = 1 . 1 1 pg/l). The mummichog, Fundulus hereroclitus, the most resistant species tested, had a 96-h LC,, of 32.99 mg/l. The lowest reported chronic effect concentration for saltwater organisms exposed to DFB was 0.075 pg/l. This concentration was reported to significantly reduce reproduction in the mysid shrimp, Mysidopsis bahia. Data from the State of Maryland were used as an example for predicting the potential environmental effects of DFB on aquatic biota in Maryland waters. A case can be made for possible environmental effects given the worst case conditions of the most sensitive species (premolt stage of grass shrimp with a 96-h LC,, of 1.11 pg/l) exposed to the highest reported environmental concentration (1.5 pg/l DFB in water). However, in most cases, the present data base would suggest that environmental effects are not likely.

KEY WORDS: Dimilin, diflubenzuron, aquatic toxicity data, environmental concentrations, insecticide

1040-8444/92/$.50 0 1992 by CRC Press, Inc.

45


1. INTRODUCTION Diflubenzuron (DFB) is the common name of the insect growth regulator sold commercially as Dimilin. The U . S . Environmental Protection Agency (EPA) approved the use of DFB as a restricted-use pesticide for gypsy moth (Lymantra dispar) control in 1976. In subsequent years, the EPA has approved the use of DFB for the control of numerous agncultural (e.g., cotton boll weevil, Anthonomus grandis; cotton leafperforator, Spodoptera sp. ; soybean velvetbean caterpillar, Anticarsia gemmatalis) and forest insect pests (e.g., Nantucket pinetree moth, Rhyacionia frustrana; Douglas-fir tussock moth, Orgyia pseudotsugutu; forest tent caterpillar, Malacosoma spp.). Direct application of DFB in water or wetlands is prohibited due to the extreme toxicity to aquatic invertebrates (nontarget organisms); the exception to this regulation occurs in California, where DFB is used for dipteran (mosquito larvae) control. Acceptable DFB application rates and procedures for gypsy moth suppression are listed in Table l * . I DFB is a benzamide chitin inhibitor and, therefore, larval and immature stages of chitinproducing aquatic organisms are most susceptible to DFB due to the frequency of molting cycles. Leighton et a1.’ indicated that DFB did not directly inhibit chitin synthetase but rather inhibited serine protease; this prevents the conversion of chitin synthetase zymogen into the active enzyme. Christiansen et al.3 reported that 10 kg/1 DFB significantly reduced the incorporation of both glucose and N-acetylyglucosamine (NAGA) during the secretion of the exocuticle (premolt stage) in Rhithropanopens harrisii larvae. The incorporation of glucose was also greatly inhibited during secretion of the endocuticle (postmolt stage), while the incorporation of NAGA at this step was reduced to a lesser degree. Mulder and Gijswijt. reported that the new cuticle could not endure muscular contractions and the increased turgor pressure associated with the molting process. They reported that death occurred either when the new cuticle ruptured or by starvation. The use of DFB for gypsy moth suppression in various mid-Atlantic states results in the load-

*

All tables will follow the text.

46

ing of this insecticide into aquatic systems. The state of Maryland is used as an example in this review to assess the potential risk to the environment. The single source of DFB contamination to the waters in Maryland results from spraying programs for the suppression of gypsy moths in hardwood forests. Spraying programs were initiated in 1983; between 30,880 and 46,540 ha of forested land were treated with DFB annually from 1984 through 1986 in Maryland.’ Since 1985, spray crews for the State of Maryland have been prohibited from spraying DFB (WP-25) within 1000 ft of any water body; however, private contractors may spray DFB up to the water’s edge. Drift from aerial spraying, runoff from treated areas, and ingestion of DFB-treated leaf litter may result in the exposure of nontarget aquatic invertebrates to lethal and sublethal concentrations of DFB (see the appropriate following sections). Presently, the use of DFB for suppression of gypsy moth infestations in the state of Maryland is viewed with mixed emotions because the potential effects on Chesapeake Bay resources are unknown. For example, the potential effects of DFB on blue crabs has been postulated as a cause for concern because the blue crab fishery is an important resource in the state. The present study was initiated to address the potential effects of DFB on important natural resources in mid-Atlantic states, specifically in the State of Maryland. The objectives of this study were to: (1) compile, review, and synthesize literature on the environmental concentrations, fate, and persistence of DFB in both freshwater and saltwater environments; (2) compile, review and synthesize acute and chronic aquatic toxicity data on DFB effects on freshwater and saltwater organisms; (3) synthesize previous data to predict potential effects of DFB on aquatic biota associated with DFB application in Maryland; and (4) recommend future research based on the data gaps identified from this study. II. CHEMISTRY This review of DFB chemistry has been divided into five categories: background technical data, comparative toxicity, fate and persistence, analytical techniques, and degradation products.

Each component of DFB chemistry is subsequently discussed.

A. Background Technical Data


Listed here are the identifying features and chemical characteristics of DFB (EPA6).

Common name: diflubenzuron Chemical Abstracts name: N-[ [(4-chlorophenyl)amino]carbony1]-2,6-difluorobenzamide Chemical name (IUPC): 1-(4-chlorophenyl)-3(2,6-difluorobenzoyl) urea Trade names: Dimilin, Vigilante, Difluron, Largon, DU, DU 112307, OMS 1804, PDD 6040-1, PH 60-40, TH 6040, and 1-(4-chlorophenyl)-3-(2,6-difluorobenzoyl)urea Producers: Duphar B .V., The Netherlands; Uniroyal Chemical Company, Middlebury, CT. Identifying features: 0 0 0

0

0 0

Empirical formula - C,,H,ClF,N,O, Molecular weight - 310.7 Chemical characteristics - white crystalline solid; no odor; melting point = 210 to 230°C; solubility in water at 20 to 25°C = 0.1 mg/l Pesticide type - benzamide chitin inhibitor ENT Registry No. - 29 054 Chemical Abstracts No. - 35367-38-5 EPA Shaughnessy Code - 108201

Mammalian toxicology: the allowable daily intake of DFB is 0.02 mg/kg/day. Low acute toxicity levels of DFB to mammals through oral (rat: >4640 mg/kg), dermal (rabbit: >4000 mg/kg), and inhalation routes (rat: >2.88 mg/l) have been reported (EPA'). In addition, the EPA' has determined that DFB is not an oncogen, teratogen, mutagen, or neurotoxin.

B. Comparative Toxicity Results Technical grade (TG) material is a white crystalline solid that has a low solubility in water

(0.2 mg/l at 20°C), but is moderately soluble in organic solvents (e.g., acetone). The wettable powder (WP-25) formulation contains additives that increase the solubility of DFB in water. A difference in the mean particle size of DFB also exists between TG (10 p.m) and WP-25 (2 pm).* Mulder and Gijswijt" and Maas et al.9 reported that the particle size of the active ingredient significantly influenced the biological activity of DFB formulations. Mulder and Gijswijt. indicated that an inverse relationship between particle size and DFB toxicity existed at concentrations of 300 pg/1 DFB (Table 2); however, at 10 pg/l DFB,no such relationship between particle size and DFB toxicity was observed. Similarly, Wilson and Costlow lo reported no significant difference in the toxicity of DFB formulations to Palaemonetes pugio when concentrations ranged from 0.1 to 5.0 pg/1 DFB (Table 2). These investigators concluded that no difference in toxicity between formulations would occur as long as DFB concentrations were 17 days. The researchers believed that abiotic factors were responsible for the degradation of DFB because a similar degradation rate was detected under sterile conditions. Conversely, Schimmel et al.25reported a half-life of only 4 days under similar conditions as in the previous study (Table 3). The differences in persistence between the two studies were most likely the result of variable particle sizes produced by the two organic solvents; Pritchard et al.24used ethanol, while acetone was used by Schimmel et al.25 Maas et al.9 had previously reported that the addition of TG to an organic solvent may produce variable-sized particles resulting in variable degradation rates (see Section 1I.B). Results from laboratory and field studies have demonstrated the importance of substrate to the fate and persistence of DFB in estuarine systems. Christiansen and Costlow22exposed crab larvae (R. harishii) in a saltwater-only system (20 ppt salinity) to a single treatment of 10 pg/l TG DFB. Total mortality occurred when the larvae were


exposed to DFB solutions 1- to 42-days old; mortality decreased from 86 to 5% between days 50 to 59 (Table 3). Cunningham and treated a saltmarsh three times with 45 g/ha WP-25 at 2-week intervals. Concentrations of DFB remained >0.4 pg/1 for 1 week post-treatment, while DFB concentrations in sediment samples remained at 100 pg/l (Table 3). Cunningham et al.27examined the persistence of WP-25 in estuarine water-only and estuarine watedsediment microcosms. The half-life estimate for DFB in the sedimentlwater system (5.3 days) was significantly less than in the water only system (1 7.8 ~~ that DFB days; Table 3). C ~ n n i n g h a mindicated would likely be less persistent in estuarine systems because the high organic matter concentrations would likely adsorb DFB from the water column.

2.5 cm of soil, and that DFB mobility was lower in clay than in sandy loam soil (Table 3). The authors concluded it was unlikely that WP-25 would leach into groundwater at the application rates tested. Carringer et a1.I3 reported that the nonionic , hydrophobic properties of DFB were responsible for the rapid adsorption to organic matter. Nimmo et al.33reported similar adsorption properties for DFB degradation products.

4. Vegetation

A few studies have reported on DFB residues in vegetation. Schaefer and Dupras12 treated a field four times with 0.3 oz A.I./A at 2-week intervals. Their data indicated that successive treatments of DFB at 2-week intervals had an additive effect on DFB residues in vegetation (Table 3). Martinat et al.34 reported a rapid de3. Sediment crease in DFB residues on oak leaves during the first 10 days following DFB application at a rate Previous studies have reported that the fate of 70.9 g/ha; however, residues then stabilized and persistence of DFB in sediment was depenbetween 100 and 180 pg/1 through 21 days (Table dent on factors such as DFB particle size, soil 3). Nigg et al .3’ compared the persistence of DFB temperature, and soil composition. Several studon orange leaves between cool, dry and hot, wet ies concluded that biological factors were reseasons. They reported that DFB degradation was sponsible for the degradation of DFB.8*29*32 virtually nonexistent during the cool, dry season; Nimmo et al.32also reported that anaerobic conhowever, a half-life estimate of 28 days was calditions prolonged the persistence of DFB threeculated for DFB during the hot, wet season. Nigg fold at 24°C (Table 3). Data from several studies et al.3’ also reported that the persistence of 4indicated that the half-life of WP-25 and TG DFB chlorophenyl urea was longer in cool, dry seasons in soil was 72 mg/l for several fish species (Table 4). Similar LC,, values were reported for 2,6-difluorobenzoic acid, the second degradation product of DFB (Table 4). The researchers also stated that 4-chloroaniline was the most toxic degradation product to aquatic organisms; 96-h LC,, values ranged from 2.4 to 23 mg/l (Table 4). Schaefer et a1.@ examined the response of bluegill (Lepomis rnacrochirus) to another degradation product of DFB, p-chlorophenyl urea. The investigators concluded that the compound was too polar to be absorbed through the gill membrane (Table 4).

A. Maryland and Chesapeake Bay Environmental monitoring for DFB residues in water, soil, and vegetation has been limited within the State of Maryland. Five studies were conducted to examine the fate and persistence of DFB within Maryland. SmuckeP analyzed water, soil and foliage samples collected from Plum Creek (Cecil County) for DFB residues. Plum Creek was considered a worst-case scenario in terms of DFB persistence because the water was naturally acidic (refer to Section 1I.C). Concentrations of DFB ranged from 0.1 to 0.7 pg/l in water samples collected 1 to 3 weeks post-application (Table 5); the sample site was approximately 1.6 km from the nearest spray path. Dimilin residues were highly variable from soil and foliage samples; however, DFB was detected from oak leaves treated 10 months prior. The author concluded that tidal flushing was most likely responsible for the observed levels of DFB in water and sediment samples at the mouth of Plum Creek, and that leaf drop could continuously contribute to DFB concentrations in water and soil. SmuckeP conducted a similar DFB residue survey at Brown's Manor Cove (Cecil County); the sample site was approximately 0.5 km west of the spray path. Dimilin concentrations d 1.5 pg/l were detected in water samples collected after a rainfall 3 weeks post-treatment. The investigator also detected 0.3 to 0.6 pg/l DFB in


water samples collected 6 months post-application. Most surprising was the discovery of a water sample containing 0.6 pg/1 DFB collected 10.5 months post-treatment. Swift and Cummins4’ reported that DFB residues averaged 230 Fg/1 from leaves collected in a stream in Allegany County (Table 5 ) ; the watershed had been treated with DFB 5 months earlier. Swift et al.’ reported that, although DFB leached from the leaves rather quickly in submerged flowing water, DFB residues were still detected on leaves for at least 4 months (Table 5). L a ~ r i detected e ~ ~ DFB concentrations ranging between 0 and 460 kg/l in suspended sediment samples collected from the Patuxent River (Baltimore-Washington Parkway downriver to Chalk Point) 6 weeks following spring DFB spraying (Table 5). The author concluded that, although concentrations of DFB collected in suspended sediment samples were significant enough to induce toxic responses, exposure time was limited and thus population impacts would be minimal. Swift et al.5 compared the process rate for control and DFB-treated leaf-packs in an untreated stream (Piney Run, Garrett County). Surprisingly, the data indicated that DFB-treated leafpacks were processed faster than untreated leafpacks. The authors concluded that invertebrate drift was assumed to be responsible for the recolonization of the leaf-packs. They also cautioned that this natural phenomenon could potentially obscure differences in processing rates between DFB and control leaf-packs, resulting from DFB-induced mortality. The researchers also suggested that DFB may act as an attractant for aquatic invertebrates. Contrary to Swift et a1.,5 Swift and Cummins4’ reported that processing rates were slower in leaf-packs placed in four DFB-treated streams in Allegany County than in control streams (Table 5). Swift and Cummins4’ stated that DFB-induced mortality of leaflitterfeeding shredders significantly affected nutrient dynamics in forested streams by reducing the turnover rate of leaflitter organic matter. Bioassay experiments conducted by Swift et al.5 indicated that survival for Tipula and Platycentropus larvae was significantly less when fed DFBtreated leaves than when fed untreated leaves (Table 5 ) . In addition, growth was significantly less

for Platycentropus larvae fed DFB-treated vs. untreated leaves.

B. Other States Willcox and C ~ f f e conducted y~~ a review of the toxicology and environmental fate of DFB for the U.S. Forest Service in Pennsylvania. The researchers concluded that a single aerial application at varying concentrations of DFB (68 to 326 g/ha) was environmentally safe to organisms at all trophic levels. Huber and Collins5’ and Huber and Mancheste?’ examined various aspects of DFB and Dipel (a Bacillus thuringiensis product) applications in the Tusquitee Ranger District of the Nantahala National Forest in North Carolina; Dipel was applied in 300-ft buffer strips between streams and DFB spray blocks. During 1987, 2 days after the second application, a 0.61-in. rainfall produced an average concentration of 2.6 pg/1 DFB in through-fall precipitation samples (Table 5 ) . These results were similar to those reported by Jones and KochenderfeP in West Virginia. Also during this storm event, a peak concentration of 0.41 Fg/l DFB was detected during water sample analysis. Examination of aquatic invertebrate populations during both studies indicated that, although decreases in the more intolerant species (Plecopterans) were observed, species taxa richness indices remained high. Due to the short residence time of DFB in the stream, the researchers concluded that no adverse impact on water quality and aquatic invertebrates would occur with the gypsy moth eradication plan. Aquatic invertebrate drift samples collected during the 1988 study were inconclusive as to whether or not DFB resulted in “catastrophic drift”. Jones and KochenderfeF2 examined the persistence of DFB in a headwater stream and on leaflitter in Fernow Experimental Forest in West Virginia. The data showed that -0.6 in. of rain 1 h post-application resulted in a peak of 2.1 Fg/l DFB from a stream water sample (Table 5 ) . Also, no detectable concentration of DFB was recorded following a 1-in. rainfall 1 week posttreatment. Analysis of through-fall precipitation revealed that approximately 10 times more DFB

51


reached stream waters during rain events than during aerial application; DFB was detected in through-fall samples for 16 days post-treatment (Table 5). Sediment samples collected 1 h postspray contained S10 pg/l DFB, but these concentrations had declined below detection limit within 1 week. Analysis of leaflitter indicated that concentrations G3600 pg/l DFB were persistent after 5 months. The authors concluded that, although toxic concentrations of DFB reached streams during aerial application and as a result of wash-off from foliage, the persistence of DFB in high-gradient headwater streams was short. D o ~ n e y analyzed ,~ water samples collected immediately after aerial application and rainfall events in the George Washington National Forest in Virginia. Data showed that trace quantities of DFB (25,000 times more susceptible to DFB than fishes during acute exposures. The most sensitive freshwater species tested in acute tests was the Amphipod, Hyallela azteca (96-h LC50 = 1.84 pg/l). This result is not surprising because the mechanism of DFB toxicity is the disruption of chitin formation, which is found with this taxon. Life stage also influenced the susceptibility of organisms to DFB during acute and chronic exposures. Larval and immature stages of aquatic organisms were most susceptible to DFB due to the frequency of molt-

52

ing cycles (see Section I). For example, the 96h LC,, for a mature Plecopteran, Skwala sp., was >100,000 pg/l. Results from chronic field studies indicate that the susceptibility to DFB is also dependent on the timing of DFB application in relation to an organism's molt cycle. Several studies were conducted on the responses of various zooplankton species to wholelake treatments with DFB (Table 6). Results from field'9*53-55 and laboratory indicated that Caldoceran and Copepod populations may be totally eliminated for extended periods following exposure to DFB concentrations greater than =7 pg/l (Table 6). Numerous studies have investigated the survival of various aquatic invertebrate species exposed to DFB (Table 6). Rodrigues and KaushW9 reported that H. azteca (Amphipoda) was more susceptible to DFB at 25 than 15°C and when vegetation remained during the study. The authors concluded that temperature and substrate vegetation may significantly increase mortality of aquatic invertebrates (Table 6). Hansen and G a ~ t o nexamined ~~ the survival of Ephemeropterans (mayflies) and Plecopterans (stoneflies) to DFB in a simulated stream system. Their data indicated that continuous exposure to 2 1 pg/l DFB would completely eliminate virtually all species of both insect orders within 1 month (Table 6). Hansen and G ~ t o and n ~Ali ~ and M ~ l l a , ~ . ~ ~ reported that populations of Oligochaeta and Coleopteran species were not significantly reduced following exposure to 2 4 and G50 pg/l DFB, respectively; however, Ali and Lord5, did report that a larval Coleopteran species was completely eliminated within 24 days following exposure to 2 6 pg/l DFB (Table 6). Three studies concluded that snail species (Gastropoda) were not affected by DFB concentrations s50 pg/1 (Table 6). Sublethal responses of aquatic invertebrates exposed to DFB were also observed. Moshen and Mulla60reported that various Dipteran larvae exhibited incomplete molting after a 15-min exposure to 100 pg/l DFB (Table 6). The effects of DFB on fish species were examined in several studies. Results indicated that the 96-h LC,, concentrations were generally >50,000 pg/l DFB (Table 6). Colwell and SchaefeP investigated the bioconcentration of DFB in brown bullhead (Ictalurus nebulosus) and


black crappie (Pomoxis nigromaculatus). Diflubenzuron residues ranged from 291 to 466 ng/g of tissue in the two species 1 day post-treatment (Table 6); however, DFB residue levels were undetectable within 7 days post-treatment. These researchers also reported that, although significant dietary changes were detected for 1 month post-treatment, individual fish growth, condition, and stomach fullness were not significantly reduced following treatment. Apperson et reported similar data for bluegill (L. macrochirus) from a lake treated with DFB (Table 6). Apperson et al. 2o and Schaefer et a1 reported that DFB residues in white crappie (P. annularis) and bluegill tissue were more than 50 times greater than water concentrations within 4 days posttreatment (Table 6); both studies also reported rapid elimination of DFB from body tissues.

B. Estuarine Data were compiled for 15 estuarine and marine species exposed to TG and WP-25 formulations of DFB (Table 7). Most of these studies were conducted with invertebrates. The premolt stage of grass shrimp was the most acutely sensitive species tested (96-h LC50 = 1.1 pg/I). Since grass shrimp produce chitin during their development, it is not surprising that this species is sensitive to a chitin-inhibitor such as DFB. The mummichog Fundulus heteroclitus was the most resistant species tested (96-h LC,, = 32.99 mg/l). Farlow et a1.68 examined the impact of repeated DFB-treatments (28 g/ha) for mosquito control on aquatic invertebrates in an oligohaline (10 pg/l. The response of Mysidopsis bahia to continuous and intermittent exposure of TG DFB was evaluated in three studies. Nimmo et a1.75-77refor ported similar 96-h LC,, values ( ~ 2 . p@) 0 juveniles exposed to continuous and intermittent DFB conditions (Table 7). Their data also indicated that continuous exposure to DFB concentrations as low as 0.075 pg/l significantly reduced the reproductive potential in females; the average number of young per female ranged between 2.4 and 13.5 during exposure vs. 21.4 in controls (Table 7). The effects of both TG and WP-25 formulations of DFB on various life stages of grass shrimp, Palaemonetes pugio, were evaluated in several studies. Wilson and C o ~ t l o wcalculated ~~ 96-h LC50 values (WP-25) for P. pugio larvae (1.44 pg/l), postlarvae (1.62 pg/l), and adult (>200 pg/l) (Table 7). Results from studies conducted by EG&G78 and Wilson and Costlowlo indicated that total mortality occurred when the larvae were exposed to TG and WP-25 DFB concentrations 2 2 . 5 pg/l (Table 7). Similarly, Hester et reported that juvenile grass shrimp suffered 60% mortality after a tidal pond was treated with 45 g/ha DFB (Table 7). Wilson et

53


a1.80.82 reported that exposure to 0.3 to 1.0 pg/1 DFB caused reversal in the phototactic response of larval grass shrimp (Table 7). Weis and PerlmutteP examined the avoidance and burrowing behavior responses of the fiddler crab, Uca pugilator, exposed to WP-25 DFB (Table 7). Their results indicated that burrowing activities were significantly reduced when the crabs were exposed to 20.5 pg/l DFB for 1 week or more. They also reported that U . pugifator exhibited no avoidance to sand contaminated with 1000 pg/l DFB. Hester et a1.81reported that 40.5% of the juvenile fiddler crabs suffered mortality following treatment of a tidal pond with 45 g/ha DFB (Table 7). Cunningham and Myersz3reported no-effect concentrations for molting (20 pg/l) and escape ability (0.2 pg/l) iii juvaiiie fiddier crabs (Tabie 7 ~ . Forward and Costlow88 examined the responses of R . harrisii larvae to sublethal concentrations of DFB (Table 7). The lowest effect concentrations to significantly increase swimming speeds in larvae were between 0.3 and 0.5 pg/l DFB. The investigators also reported that only stage IV larvae exhibited significant changes in phototactic behavior; the lowest effect concentration was 0.1 pg/l DFB. Chnstiansen et reported that survival was significantly reduced when crab larvae were exposed to 1 pg/l DFB and greater (Table 7). Several other investigators reported that survival, growth, and molting of various crab species were significantly affected by DFB concentrations S5 pg/1. Costlowgo reported that stone crab (Menippe rnercenaria) larvae suffered total mortality when exposed to 20.5 pg/l TG DFB (Table 7). Results from Christiansen et al.87indicate that survival of Sesarma reticulutum larvae was significantly reduced during exposure to nominal DFB concentrations 2 3 pg/1 (Table 7). Survival of larval and juvenile blue crabs, Cuflinectes supidus, exposed to DFB was determined in two studies. Results from a study conducted by C o ~ t l o windicated ~~ that survival was 100,000 pg/l). In chronic tests, a 30-day LC,, of 0.1 pg/l was reported for the Tricopteran, Clistorinia magnifcu. DFB concentrations of 1 pg/l or greater were also reported to eliminate Plecopterans (stoneflies) and Ephemeropterans (mayflies) populations after 1 month of exposure. Freshwater fish were resistant to acute exposures of DFB as 96-h LC,,s were generally >50,000 kg/l. Fish were also reported to accumulate DFB rapidly during acute exposures, but were capable of eliminating this insecticide within 7 days. Toxicity data were available for 15 saltwater species exposed to DFB. Most of the DFB aquatic toxicity studies with saltwater organisms were conducted with invertebrates. The most acutely sensitive species tested was the premolt stage of grass shrimp (96-h LC,, = 1.11 bg/l). The mummichog, the most resistant species tested, had a 96-h LC,, of 32.99 mg/l. The lowest reported chronic effect concentration for saltwater organ-


isms exposed to DFB was 0.075 pg/l. This concentration was reported to reduce significantly reproduction in the mysid shrimp, Mysidopsis bahia. Limited data were available for predicting potential environmental effects of DFB on aquatic biota in Maryland waters. To make this prediction accurately, data on both concentration and exposure times are needed to determine the possible risk to aquatic biota. A case can be made for possible environmental effects given the worstcase conditions of the most-sensitive species (premolt stage of grass shrimp with a 96-h LC,, of 1 . 1 1 pg/1) exposed to the highest reported environmental concentration (1.5 kg/l DFB in water) for a 96-h period. Lower concentrations of DFB (0.075 pg/l) were also shown to cause significant effects on nontarget organisms if exposure to a stressful concentration occurred for an extended period of time. However, based on the short half-life of DFB in water, it appears that concentrations decrease rapidly over time. One other potential environmental problem that may occur from DFB spraying is the persistence of this insecticide in leaflitter ( 5 to 10 months). Nutrient dynamics in streams may be greatly changed by this source of DFB. Although all of the above scenarios are possible for DFB effects on aquatic biota, the limited data base would suggest that adverse environmental effects are minimal.

with DFB. Monitoring during and after rain events is suggested to determine the worstcase environmental concentrations in the water column. Toxicity studies should be conducted to determine the effects of DFB on the premolt and “soft crab” stage of the blue crab. These studies should be conducted using environmental concentrations of DFB reported from monitoring studies proposed previously. Future research should examine the responses of nontarget organisms (i.e., invertebrates) to sublethal concentrations of DFB. Endpoints of these studies should be susceptibility to predation, reduced or impaired feeding ability, altered reproductive behavior, and disruption of the molting cycle (cuticle growth). Specific studies to examine the long-term effects of DFB residues on aquatic insects, particularly shredders and grazers, are recommended. These studies should be designed to evaluate the impacts of DFB-induced insect mortality on stream nutrient dynamics. Buffer strips between DFB spray blocks and water bodies should be utilized in an attempt to eliminate, or minimize DFB introduction into aquatic systems via application drift, runoff, or through-fall precipitation. Studies are needed to determine adequate sizes for buffer strips.

VI. RECOMMENDATIONS

ACKNOWLEDGMENTS The following recommendations are proposed based on the information in this paper: 0

Environmental monitoring studies are recommended to determine the spatial and temporal distribution of DFB residue concentrations in water, sediment, and leaflitter in aquatic habitats that border areas sprayed

We would like to acknowledge the Maryland Department of Agriculture for sponsoring this study. Special consideration is extended to Mr. Robert Tichenor for his constructive comments on the manuscript. The University of Maryland’s Agricultural Experiment Station is also acknowledged for providing support for the study.

55

TABLE 1 Recommended Diflubenzuron (DFB) Spraying Concentrations and Frequency of Treatment for Gypsy Moth Larvae' Tolerance, use, and limitations

Dosage and formulation

Ornamental and/or shade trees Use limited to quarantine programs involving the movement of nursery stock 0.25-0.5 oZIA (WP-25) from infested to noninfested areas Foliar application: apply two times at 7- to 14-day intervals.


Forest trees and lands 0.25-1 .O oZIA (WP-25)

Foliar application: apply prior to full leaf expansion when larvae are in the first through third instar; apply in 0.5-2.5 gal of water per acre by aircraft or in 1.510 gal of water per acre by ground equipment (mist blower); do not make more than one application per year Use limited to U.S. Dept. of Agriculture, Animal Plant Health and Inspection Service, Plant Protection and Quarantine personnel, and state cooperators involved in quarantine programs Foliar application: apply for use in eradication of isolated infestations; make two applications at 7- to 14-day intervals Use limited to quarantine programs involving the movement of nursery stock from infested to noninfested areas; make two applications at 7- to 14-day intervals

0.25-0.5 oZIA (WP-25)

0.25-0.5 O AZ I

Note: 0.25 oz1A

=

0.017 kglha.

TABLE 2 Results of Diflubenzuron (DFB) Comparative Toxicity Studies between Technical Grade (TG) Material and Wettable Powder (WP) Formulations

Species Pieris brassicae

Temp. ("C)

Salinity (PPt)

-

-

Results

Ref.

Percent mortality at various DFB concentrations:

4

pg/I active ingredient

Particle size (pm) 3000 10-20 4-10 2.5 Fg/l during a 15-day exposure No significant difference in the acute or chronic toxicity of WP and TG to larvae was detected; the authors suggested that no significant difference between the two formulations occurred because the test concentrations were below water solubility; it was concluded that valid comparisons could be made between TG and WP data as long as the concentrations were < 10 pg/l DFB

10

TABLE 3 Fate and Persistence of Diflubenzuron (DFB) Compounds Collected in Water Column and Sediment Samples from Freshwater and Saltwater Environments and Vegetation


DFB compound/ analytical method

Temp. (“C)

Freshwater TG/TG/-

-

TG/GLC

36

WP-25/HPLC

10-38

-

Media

Freshwater Freshwater (lab) Freshwater Freshwater (lab)

Freshwater (lab and field)

Freshwater (lab)

WP-25/HPLC

12-27

Pond

WP-25/HPLC

20-25

Water column

WP-25/HPLC

10-38

Freshwater (lab and field)

Fate and persistence

Ref.

DFB half-life was 100 mg/l > 100 mgll

43 43 43 43

Static

22

40

7.2

48-h EC,

=

43 mg/l

43

Static Static Static Static

12 22 22 22

40 40 40 40

7.2 7.2 7.2 7.2

96-h LC, 96-h LC, 96-h LC, 96-h LC,

= 14 mg/l = 12 mg/l

43 43 43 43

Static

20

-

-

NA

NA

Field

= 72 mg/l

=

= =

23 mg/l 2.4 mg/l

Exposure of adults to 10 Fg/I p-chlorophenyl urea for 24 h resulted in no uptake from the treated water; the authors concluded that the hydrolytic metabolite was too polar to be absorbed through the gills Cultured in 100 Fg/I DFB; 80% of the DFB degraded to p-chloropheylureaand p-chloroaniline

44

17

Note: All results are nominal concentrations. NA = not applicable.

61

TABLE 5 Environmental Concentrations of Diflubenzuron (DFB) Measured at Various Locations Throughout the United States Sample location


Maryland Garrett County

Allegany County

Analytical method

HPLC

HPLC

5 Poplar leaves were treated with WP-25 to obtain a final concentration of 10 mg active ingredient per square meter; treated and control leaf-packs were then placed in an untreated stream (Piney Run) Leaf-pack results DFB-treated leaf-packs were processed at a faster rate than controls; DFB may act as an attractant for aquatic invertebrates; the data also suggested that DFB leached relatively quickly from the leaves, however, residues were detected from treated leaves submerged in flowing water for at least 4 months Bioassay results: Tipula - held at 17°C for 430 degree-days; survival was significantly lower for larvae fed DFB-treated leaves (4%) than controls (96%) Platycentropus - held at 10°C for 330 degree-days; the DFB concentration was measured at 6400 pg/l; survival was significantly lower for larvae fed DFB-treated leaves (-22%) than controls (>80%);growth of larvae fed DFB-treated leaves was also significantly less than that of the control larvae Red maple leaves were collected from a DFB-treated area; average 47 DFB residue was 230 pg/I Field studies: There was a significant reduction in the rate of leaf processing for an untreated leaf-pack in a treated watershed vs. an untreated watershed

DFB-treated sites Hill Run Charleston Run McMullen Run Brice Hollow Run Control sites Evitts Creek North Br. Little Bear Creek South Br. Little Bear Creek Piney Run Plum Creek Cecil County

62

HPLC

Ref.

Data

% Losddegree-day

0.1 1 0.12 0.12 0.12

0.1 5 0.15 0.15 0.17

Residues of DFB were analyzed in water, soil, and foliage samples 45 collected 3 weeks post-treatment Water: DFB concentrations ranged between 0.1 and 0.7 pg/l in samples collected 1 to 3 weeks post-application; the sample site was approximately 1.6 km from the nearest spray path Soil and Foliage: DFB residues were variable; DFB was detected in oak leaves retained on a tree overwinter (10 months posttreatment)

TABLE 5 (continued) Environmental Concentrations of Diflubenzuron (DFB) Measured at Various Locations Throughout the United States


Sample location

Analytical method

Brown’s Manor Cove Cecil County

HPLC

Patuxent River

HPLC

Data

Ref.

46 DFB residues were analyzed from water samples collected for 10 months post-treatment;the sample site was approximately 0.5 km from the spray path; DFB concentrations 4 . 5 pg/I were detected in water samples collected after a rainfall 3 weeks post-treatment; DFB concentrations ranging from 0.3 to 0.6 pg/l were detected in water samples 6 months post-treatment; 0.6 pg/l DFB was detected in water 10.5 months post-application 48 Concentrations of DFB detected in suspended sediment 6 weeks post-application ranged from 0-460 kg/I; microcosm tests indicated that DFB was significantly metabolized within 48 h in estuarine sediments and completely metabolized within 24 h in freshwater sediments; although concentrations of DFB reaching the river were enough to produce toxic responses, exposure time was limited and thus impacts on entire populations wou\d be minimal

Virginia George Washington National Forest

HPLC

49 Application concentrations similar to that used in North Carolina; a 0.1 -in. rainfall 60 h post-treatment produced a maximum concentration of 0.36 pg/I DFB; a 0.75-in. rainfall 83 h post-treatment produced peak concentrations

Influence of pH-dependent aquatic toxicity of ionizable pharmaceuticals on risk assessments over environmental pH ranges.

Sources, concentrations, and exposure effects of environmental gestagens on fish and other aquatic wildlife, with an emphasis on reproduction.

Amines and amine-related compounds in surface waters: a review of sources, concentrations and aquatic toxicity.

Aquatic Environmental Health and Toxicology.

Spatiotemporal variations in species diversity to assess environmental impacts on aquatic communities: The use of biomass data versus density data.

Ecotoxicogenomic approaches for understanding molecular mechanisms of environmental chemical toxicity using aquatic invertebrate, Daphnia model organism.

Environmental Effects of BPA: Focus on Aquatic Species.

Toxicity of Altosid and Dimilin to juvenile rainbow trout and coho salmon.

Reduced movement of precocious male Atlantic salmon parr into sublethal Dimilin-G1 and carrier concentrations.

Effect of diflubenzuron on an estuarine crustacean.

Modeling nanomaterial environmental fate in aquatic systems.

Acute aquatic toxicity of alkyl phenol ethoxylates.

Toxicity of alkyldinitrophenols to some aquatic organisms.

Aquatic insects and trace metals: bioavailability, bioaccumulation, and toxicity.

Toxicity of Environmental Contaminants.

Identifying and designing chemicals with minimal acute aquatic toxicity.

Graphene in the aquatic environment: adsorption, dispersion, toxicity and transformation.

Toxicity of anthelmintic drugs (fenbendazole and flubendazole) to aquatic organisms.

Fate of diflubenzuron in cattle and sheep.

Influences of sediment geochemistry on metal accumulation rates and toxicity in the aquatic oligochaete Tubifex tubifex.

Impact of environmentally based chemical hardness on uranium speciation and toxicity in six aquatic species.

Toxicity of fluoride to aquatic species and evaluation of toxicity modifying factors.

Framework for enhancing the statistical design of aquatic environmental studies.

Comparison of embryo toxicity using two classes of aquatic vertebrates.