PARATHION PERSISTENCE ON SOUTH AFRICAN CITRUS L. P. VAN DYK Plant Protection Research Institute Private Bag X134, Pretoria, South Africa, 0001

The fate of parathion applied to citrus was affected by rain, sun and wind, but not much by fruit variety, while the application method had an important effect. The formulation influenced the persistence of parathion on leaves and glass in the laboratory, but had no influence on the persistence on leaves or fruit in the field. The deposits of the emulsifiable concentrate and of the wettable powder and oil mixture were higher than that of the wettable powder alone, but since the rates of disappearance were the same, higher harvest-time parathion residues on and in the fruit resulted when the first two formulations were used. The time needed to reach a tolerance value increased for applications made later in the growing season. South African citrus is grown under greatly varying climatic conditions. The different areas all have their specific pest problems and although biological control of citrus pests has been implemented, the need for corrective and preventative control by chemicals still remains. Parathion (O,O-diethyl-O-p-nitrophenyl phosphorothioate) is used to control red scale, Aonidiella aurantii (Mask.) and thrips, Scirtothrips aurantii. Both the emulsifiable concentrate and wettable powder formulations are approved for use in South Africa and a parathion wettable powder spray is also often applied with 0.5% (v/v) of oil added for increased effectiveness and to reduce the toxicity of parathion to bees (Bot et al 1973). A monitoring program to determine the parathion residues on export citrus was started in 1968 by the South African Department of Agriculture. An unacceptable percentage of the citrus samples analyzed contained residues that were higher than the 0.5 ppm tolerance set by some of South Africa's principal European export countries which have been recently enforcing tolerances generally much lower than those of the United States Food and Drug Administration. Thus the South African Department of Agriculture was obliged to extend the interval between application of a pesticide and harvesting. The extended safety period was estimated by extrapolating available data, usually from the initial registration studies (Wiese and Bot 1971) but this proved to be unsatisfactory. Research was therefore undertaken to obtain data to more precisely determine the necessary safety period, including studies to compare the persistence of parathion on citrus of different types and in different localities. The rate of disappearance of parathion from glass surfaces, leaves, and fruit was studied. The glass surface study should give an indication of the influence of most Archives of Environmental Contamination and Toxicology, Vol. 4, 289-311 (1976) 1976 by Springer-Verlag New York Inc.

289

290

L . P . van Dyk

factors on the disappearance rate, exclusive of those particular to a living plant. The leaves should give an indication of the influence of climatic conditions; leaf growth was eliminated as a dilution factor because only mature leaves were sampled, The fruit study is important because it concerns the edible plant part and the growth dilution factor is important for fruit residues. Another factor specific tO fruit is the oily wax of the citrus rind which retains some pesticides longer than expected.

Experimental Materials. The wettable powder formulation of parathion, 25% w/w and the 50% w/w emulsifiable concentrate were products of Cyanamid and Agricura Chemicals Ltd. respectively. The technical parathion was a product of Bayer. The oil used in some spray mixtures was an emulsifiable hydrocarbon oil 80% v/v of KynochCapex. Parathion was sprayed as 0.075% w/v emulsion of the active ingredient in water in all experiments. Oil was added as 0.5% in water in some experiments. Application Methods. A continuous-belt sprayer was used to apply parathion to glass surfaces. The sprayer consisted of a pressure operated jet nozzle above a moving belt, on which the glass plates were placed. Optimum conditions for spraying the glass slides were experimentally determined to be 176.5 kPa and10.24 rn/min, respectively. Some leaves and fruit were sprayed to the point of run-off in the laboratory with a back-mounted hand sprayer equipped with two nozzles producing fine spray droplets. A stirrer kept wettable powders in suspension. In some experiments the fruit was dipped individually into emulsions made up to the usual spray concentration. The fruit was immersed, then removed immediately, and placed on drying racks. The field experimental plots consisted of fifteen fully grown trees in a row near the center of a grove. Plots of trees for the different varieties were selected as near as possible to one another. The 15 trees were each sprayed from all sides, although only the nine middle trees (three replicates of three trees each) were used for experimental sampling. The trees in the experimental orchard were sprayed with the equipment available at the specific farm. On the Nelspruit, Citrusdal and Rustenburg farms, tractormounted sprayers with 450-L tanks and dual outlets, each fitted with two cone-type nozzles, were used. The operating pressure of the sprayer was 2750 kPa; delivering 20 L/rain from each outlet. The sprayer on the Malelene farm was a special tractor-operated spray rig of 1800-L capacity, also with dual outlets but each with three cone-type nozzles which delivered 15 L/min at 2750 kPa. The trees were sprayed to the point of run-off (about 25 L/tree). The actual applications were made by the experienced spray operators employed on each farm thus introducing another variable.

Parathion Persistence on Citrus

291

Optimum sample size determination. Glass plates (6.25 c m 2) w e r e sprayed with an emulsion of the parathion emulsifiable concentrate. Samples consisting of 1, 3, 5 and 10 plates each were analyzed. A young tree, less than two m high, was sprayed with parathion emulsifiable concentrate. As soon as the leaves were dry samples consisting of three replicates of 10, 20, 30, 100, and 150 leaves each were picked from the outside perimeter of the tree and later analyzed. A large bearing Valencia orange tree was also sprayed with parathion emulsifiable concentrate at the Nelspruit farm. Immature fruits, about ten mm in diameter, were picked when they were dry. Triplicate samples were collected from near the outside perimeter of the tree and consisted of 5, 15, 25, 30, and 60 fruits each. The experiment was repeated on two later occasions when the fruits had grown to about 5 mm and 75 mm in diameter. The maximum variation from the average residue for each sample size was determined and plotted against the number of glass plates, leaves, or fruit taken per sample (Figure 1). This variation for glass plates fell rapidly up to a sample size of three. Larger samples did not increase the precision; thus three plates per sample was considered the optimum sample size. The variation for leaves fell rapidly up to 20 leaves in a sample. The increase in precision from 20 to 150 leaves was slight; thus a sample size of 20 to 30 leaves was optimum. The optimum sample size for fruit depended on the size of the fruit collected; a sample of fruit measuring up to ten mm in diameter should contain at least 30 fruit, but with larger fruit a sample of about 15 gave consistent results. Glass plates were randomly sampled. Leaves and fruit were selected randomly from any location within a band one to two m high around the perimeter of the trees. The leaf and fruit samples were collected in paper bags in the field and each bag was analyzed along with its contents.

Storage procedure. It was not possible to process all samples from the field immediately; therefore samples were arbitrarily stored at 4 ~ or - 2 0 ~ as space permitted, and the effect of storage losses of parathion residues was concurrently determined. Individual fruits in some treated samples were cut in half, one half was stored at either 4~ or 20~ and the other half was analyzed immediately. The stored halves of these samples were analyzed four weeks later and compared to corresponding fresh samples. All samples were weighed before and after storage to determine the inevitable weight loss for the necessary correction in the calculation of results. The loss of parathion residues was neglegible at both storage temperatures during the four-week storage period. The average residue immediately after sampling was 2.69 ppm and was 2.73 ppm after 28 days at 4~ and 2.78 ppm after 28 days at -20~ Extraction. A mixture of 25% chloroform in hexane (v/v) was used for extraction of all substrates.

292

L. P. van Dyk

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Parathion Persistence on Citrus

293

Glass plates. The glass plates were handled with forceps and washed with the solvent mixture into a 100-ml glass beaker. The solvent was transferred to a Kuderna-Danish evaporator (Gunther and Blinn 1955) and concentrated on a stream bath to less than ten ml, then adjusted to ten ml in a volumetric flask. Leaves. A subsample of 1.00 g from each leaf sample was weighed into a glass crucible; the subsample was placed in a drying oven overnight at 130~ and the water loss was determined. The loss in weight due to normal dehydration was appreciable, therefore the concentration of parathion found in leaves was expressed on a dry weight basis. The leaf sample was weighed and the leaves were cut into small pieces with a food chopper. The solvent mixture was added to the sample (200 ml/sam~ple) in a blending jar and blended for two to three min with a high speed Ultra Turrax mechanical blender. The sample was left at room temperature overnight (about 16 hr) in a sealed jar and then filtered through a fluted Whatman 1PS filter paper into a graduated cylinder for volume notation. The solution was evaporated to less than ten ml in a conical flask fitted with a three ball Snyder column on a steambath, then transferred to a ten ml volumetric flask and made up to volume with the solvent mixture. Citrus fruit. In the experiments where the rate of disappearance of parathion was studied, no differentiation between rind or pulp and juice was made. If the total sample weight was less than I00 g, the whole sample was cut into pieces of about one cm 3, the weight was determined and 200 ml of the solvent mixture was added. When the total sample weight was more than 100 g, the sample was similarly cut into pieces and a 100-g subsample was used. Samples taken at harvest time, or market samples, were weighed, the fruits were each cut in half and dejuiced on a citrus juicer, the rind was weighed again, chopped, a 100 g subsample was taken, and 200 ml of the solvent mixture was added. The whole fruit or rind subsamples to which 200 ml of the solvent mixture had been added in a blender container were then extracted and prepared for analysis by the same procedures as for the leaves, following blending. The mixture of pulp and juice obtained from some samples was macerated in a blender and 400 ml was then extracted for three min in a separatory funnel with 200 ml of the solvent mixttare. After phase separation the solvent phase was collected and the juice was shaken for another three min with 100 ml of the solvent mixture. The solvent phase was again collected and the combined extracts were concentrated and made up to exactly ten ml.

Paper bags. The paper bags in which the samples were collected were cut up and extracted twice with I00 ml of the solvent mixture. The two extracts were combined, concentrated and made up to ten ml. This was analyzed separately and the parathion content was added to the respective total sample parathion residue or an appropriate fraction was added when only a part of the total sample was analyzed.

294

L . P . van Dyk

Analytical method. The separation, detection and quantitative determination of parathion was done by gas chromatography using a flame photometric detector (Brody and Chaney 1966) with a 526-nm filter. No cleanup was required for any of the samples. A glass column (0.8 m m i d x 2m), packed with 3% w/w SE-30 on Chromosorb W (80/100 mesh), was used. The operating temperature o f the column and inlet was 210%C. Nitrogen was used as carrier gas at a flow rate of 100 ml/min. Under these conditions the retention time for parathion was 94 sec. The gas flow rates for operation of the flame detector were 150, 40 and 40 ml/min for hydrogen, oxygen and air, respectively. Standardization was obtained by daily injection of several parathion standard solutions; peak areas were determined by an electronic integrator. For parathion, the response was linear between 0.6 and 400 ng//zl injected in five /~1 of solvent mixture. Residue half-life determination. The practical disappearance of a pesticide can be represented graphically as the plot of the logarithm of the concentration against the time elapsed after application. (Gunther 1969). Residue data plotted on this basis yield principally two lines, the first called the degradation curve and the second the persistence curve. From the persistence curve Gunther (1969) determines the halflife, i.e., the time for half of the residue to lose its analytical identity. The IUPAC Commission on Terminal Residues does not accept the half-life concept as a valid measure (Hill 1971), but conceded that "semilogarithmic plots are useful for visualization of residue data" Various workers have found that half-life of a pesticide determined from the persistence curve is not dependent on weathering or formulation of the pesticide (Ebeling 1963, Gunther 1969). The degradation curve, however, is influenced by weathering. The effect of weathering on persistence can thus only be studied from a combination of the degradation and persistence curves. Although the slope of the persistence curve may not be influenced by weathering, the line will be displaced and the time to reach a certain tolerance level is thus influenced by degradation factors. It is clear that when a safety period must be established, the influence of the degradation factors on the analytical data presented should be known. The numerical plot of the pesticide concentation against time shows a reciprocal power function relationship. The solution to this type of equation can be done by the logarithmic transformation of both the concentration and the time factor which then gives a straight line. This line represents both degradation and persistence and the half-life calculated from this is a function of weathering, plant factors, and of the pesticide. A linear regression equation of the following form can be calculated for the disappearance of a pesticide: Y=aX+B Y = log (y + 1); y = ppm X = log (x + 1); x = time in days after application

Parathion Persistence on Citrus a = slope of the B = intercept of deposit plus b = antilog B =

295

straight line the straight line and represents the logarithm of the effective one (hereafter referred to as log effective deposit) effective deposit

The half-life was chosen as a measure of the rate of disappearance of the pesticide. For this work the half-life will be defined as the time it will take the pesticide to decrease from b to b/2. The term b/2 is equal to B - log 2 thus when (B - log 2) is substituted for Y in the regression equation, it becomes: B -

log 2 = a X + B or X = - l o g 2/a

The slope will always be negative, thus the expression may be simplified to: X = log 2/a = 0.3010/a. This is the expression for the logarithm of the half-life plus one (hereafter referred to as log half-life).

Determination of the influence of temperature and humidity Laboratory conditions. Unlit controlled condition rooms were used to determine the influence of temperature and humidity on the disappearance of parathion from glass plates, from leaves, and from fruit. Constant temperature rooms of 4 ~ 15 ~ 25 ~ and 350C were used for the leaf experiments. One 25~ room was kept at 80% relative humidity (RH) and another 25~ room and all other rooms were at ambient RH (about 50%) which was recorded on a hygrograph. Temperatures of about 20 ~ 26 ~ and 30~ were used for the fruit experiments, with an average RH of 60%. An experiment at 200C and 85% RH was also included. Glass plates were sprayed with technical, wettable powder, and emulsifiable concentrate parathion and placed in rooms with the temperature controlled between 4 ~ and 35~ and the RH between 50 and 70%. Samples were taken at 1, 3, 7, 14 and 28 days after placement in the controlled conditions and the amount of parathion left on each plate was determined. There were three replicates. Some plates were placed in closed glass containers saturated with parathion vapor at four temperatures ranging from 4 ~ to 35~ and were sampled at the same time intervals. The leaves of young potted Valencia orange and lemon trees were sprayed with wettable powder and emulsifiable concentrate parathion and allowed to dry; the trees were then placed in a controlled temperature and humidity room. A sample of five leaves per tree was taken from three Valencia orange or lemon trees for each of three replications at intervals from one to 28 days, for parathion residue determinations. The rate of disappearance was expressed as the log half-life calculated from the regression equation. The results are presented as the general factorial analysis of variance of the log half-life.

296

L . P . van Dyk

Ripe fruit of Valencia and navel oranges, lemons, tangerines, and grapefruit were placed in three groups of 30 fruits for each variety; each group represented a replication. The fruits were sprayed with parathion wettable powder and emulsifiable concentrate and samples of five fruits each were taken at intervals from one to 42 days after spraying. Compensation for weight loss due to dehydration was provided by determining the weight loss from untreated fruit kept under similar conditions. The evaluation of the results was similar to that for leaves. Field conditions. The average temperature and RH over the period of a field experiment were calculated from the readings taken every two hours by a thermohygrograph. The maximum temperature was read from a maximum-minimum thermometer daily and the maximum over the experimental time was determined. Correlation of the temperature and RH with the log half-lives was done to evaluate the effect of average temperature and RH and maximum temperature on the rate of disappearance of parathion from fruit and from leaves from trees in a citrus grove.

Determination of the influence of sunlight. The ultraviolet region of sunlight has the greatest influence on pesticide breakdown (Archer 1971, Mitchell et al. 1968). The study of the effect of sunlight should also include the photodecomposition products formed, but this was regarded as outside the scope of the present study where the aim was to determine the relative importance of sunlight in the disappearance of parathion from citrus fruits. Controlled conditions. Picked fruit was exposed to sunlight on bright winter days on which there was practically no wind. The fruits were sprayed with the emulsifiable concentrate and wettable powder fomulations to run-off. A yellow citrus fruit, grapefruit, and an orange citrus fruit, Valencia orange, were used. The fruits were sprayed with parathion and exposed to sunlight one and three days after treatment; control fruits were kept in the shade. Triplicate samples of control and exposed fruits were taken after one to six hr exposure to sunlight and the parathion residue concentrations on the exposed and unexposed samples were compared. Field experiments. The rate of disappearance of parathion expressed as the log half-life of the field leaf and fruit samples was correlated with the average number of hr of sunlight per day over the experimental period and the correlation coefficient was determined.

Determination of the influence of rain. The fomulation of a pesticide determines its spreading ability and the wetting of plant surfaces; conversely the formulation also has an influence on its removal by rain (Ebeling 1963). The effect of rain thus depends partly on the type of formulation and the wetting agents used but, because commercial formulations contain different wetting agents, only the two specified commercial formulations of parathion were studied. Controlled conditions. The leaves of young potted Valencia orange and lemon trees were sprayed and left to dry. The trees were then placed under an irrigation

Parathion Persistence on Citrus

297

sprayer which delivered a constant simulated rain of ten mm every 18 min. Triplicate samples of 15 leaves each from three threes were taken immediately before the " r a i n " started and after five to 30 mm of rain had been delivered. Lemons and grapefruit were used in the experiment to compare the rough textured lemon rind to the smooth textured grapefruit rind. The fruits were dipped in the specified spray concentration of either the wettable powder with or without oil, or of the emulsifiable concentrate and left to dry for about two hr, they placed under the irrigation sprayer used for the leaves. Triplicate samples of five fruits each were taken before and after 5 to 50 mm of rain had been delivered.

Field experiments. The amount of rain measured during a field trial was correlated with the log half-lives of parathion to determine the relative importance of rain on the rate of parathion disappearance. Determination of the influence of growth. Growth dilution may be the most important contributing factor in apparent pesticide disappearance (Hill 1971). This effect was determined by spraying at different times during the growing season. The first spray was applied when the fruit was very small, about one week after 90% blossom petal fall, the second was nine weeks after petal fall, when the fruit was about ten cm in diameter, and the third was 18 weeks after petal fall, when the fruit was about full grown. Triplicate samples were collected from one to 42 days after application of the parathion spray. Regression equations were calculated from the analytical result from each replicate and the log half-lives were determined from these equations. The factorial analysis of variance of these log half-lives then reveals the effect of growth. Determination of the effect of locality The difference in the rate of parathion disappearance at different localities is mainly the result of climatic influences. The experimental sites (Nelspruit, Malelene, Citrusdal and Rustenburg) were chosen to represent the different South African citrus areas. The climates of these four areas vary and their weather data were recorded. It was thought that leaf samples would give a more significant difference between localities, but both fruit and leaf samples were analyzed and the log half-lives were calculated from the regression equations. Then factorial analysis of variance was used to compare the half-lives so that the effect of locality could be assessed. Determination of the influence of variety. Navel and Valencia oranges and grapefruit were used in all localities .except Rustenburg where no navels were available. Lemons and tangerines were used in field experiments at Rustenburg only. In the field trials fully grown trees were sprayed with replicates, spraying methods, and sampling as previously described. The rate of disappearance of parathion from each variety was expressed as the log half-live and compared statistically by factorial analysis of variance.

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L . P . van Dyk

Laboratory experiments. The influence of two types of citrus Ieaves, those from lemon and Valencia orange trees and the influence of navel and Valencia oranges, lemons, tangerines and grapefruit were studied with the calculated log half-lives under constant temperature and RH as described. The influence of two types of leaves from Valencia oranges and lemon trees and two types .of fruit,-lemon and grapefruit, were studied with the log half-lives as affected by rain. The interactions of growth, locality and formulation with citrus variety were studied with the results from the field experiments. Determination of the influence of formulation. The exact composition of the formulation of a commercial pesticide is a trade secret; therefore the chemical company whose product was used was named. In field trials, parathion was applied as a wettable powder with or without 0.5% (v/v) oil added. In laboratory trials the rate of penetration of the pesticide into the fruit was studied; rapid penetration may be a function of the formulation. Emulsions of the wettable powder with or without oil and the emulsifiable concentrate were prepared and fully grown navel and Valencia oranges, and grapefruit, were dipped into the agitated emulsion and placed on racks to dry. Two hr after treatment the fruit was sampled, one fruit per sample, replicated three times. The samples were washed with 0.01% v/v Teepol solution and the whole fruit and the respective wash solutions were extracted and analyzed. The amounts of parathion residue contained in the fruit and in the wash solution were determined and the amount penetrated was calculated as a percentage of the total residue. Samples were taken 2, 4, 6 and 24 hr after treatment. The results were compared statistically to determine the influence of formulation and variety on the rate of penetration. The effects of formulation as a function of sunlight and rain were also studied as described previously.

Determination of the influence of washing. The effect of variety on washing was evaluated by using coarse-rind lemons and smooth-rind grapefruit. The influence of formulation on washing was tested by the use of a wettable powder and an emulsifiable concentrate. Emulsions containing 0.075% parathion were prepared and the fruit were dipped in them, placed on racks and allowed to dry. One day after treatment duplicate samples of the fruit were either rinsed with water, dipped in water, dipped in 0.01% (v/v) Teepol, or stripped with 50% (v/v) ethanol or hexane. The aqueous solutions and the fruits were extracted and analyzed. The hexane strip solution was concentrated and analyzed. The amount of parathion left in the fruit after washing was expressed as a percentage of the total and the relative effectiveness of the methods compared as functions of the variety and formulation.

Statistical analysis. The log half-lives and the log effective deposit were compared statistically with a factorial analysis of variance (Guenther 1964). Usually this was the analysis of variance of a split-plot design. In a few instances either the actual values or the percent remaining of the original deposit were compared. The

Parathion Persistence on Citrus

299

degrees of freedom (dO, and F-ratio are presented. The average values used in the calculations are presented and compared in the results and discussion section. In most cases the average values themselves were calculated from the regression equations, which represent the relationship between the parathion concentration (ppm) and the time elapsed after spraying (days). The calculated values were compared in their logarithmic form to ensure statistical uniformity. The climatic factors and the log half-lives were compared with simple correlation analysis and the significant correlations are presented.

Results and Discussion Parathion was found in all of the mature harvest-time samples collected from field plots. The highest residue was 4.87 ppm and the average was 1.34 ppm for 66 samples (Table I). Residues were marginally higher on fruit which received a wettable powder-oil spray. These relatively high residues indicate a probable need to change application methods to decrease the parathion residues at harvest. Varietal effect. Fruit. The log half-lives differed highly significantly among varieties sprayed at Rustenburg (F = 4.64, df = 3 and 35, p < 0.01); compare the averages: lemons

Valencias

grapefruit

tangerines

0.8614

0.6935

0.6504

0.8873 SMDr 1 = 0.2115

The rate of disappearance from tangerines was faster than from Valencia oranges and lemons but not faster than from grapefruit. In experiments at other localities no differences among the varieties were observed. With fruit in the laboratory the rate of disappearance of parathion was influenced by the variety (F = 18.2, df = 4, and 92, p < 0.01); compare the average log half-lives: navels 1.8466 SMDr = 0.3722

grapefruit

Valencias

lemons

tangerines

1.7583

1.5483

1.4131

0.8271

Thus the rate of disappearance of parathion from tangerines was the fastest, while the disappearance rates from grapefruit, Valencia oranges and lemons were the same, however parathion disappeared faster from lemons than from navel oranges. This disappearance was found to be humidity-dependent, because there was a highly significant interaction between the variety and the controlled temperature and RH of the laboratory (F = 9.4, df 12 and 92, p < 0.01); compare the average log half-lives: tSMD.r = smallest significant difference according to the method of Tukey at p = 0.05.

300

L. P. van Dyk grapefruit

navels

Valencias

lemons

tangerines

30~

60% RH

0.704

1.0464

0.6602

1.0145

1.0170

18~

60% RH

1.2230

1.9083

1.3441

1.6694

1.1876

20~

85% RH

2.9307

3.31 I0

2.6294

2.144l

0.6420

25~

85% RH

2.1720

1.1206

1.5593

0.8244

0.4619

SMDT = 0.9762 Generally a low RH gives a faster rate of disappearance, but this is not true for all varieties. The rate of disappearance of parathion from tangerines was not influenced by RH or temperature. There was no difference among varieties at 60% RH. The effect of low RH in the field would be to obscure the difference in disappearance rates among the varieties.

Table I. Parathion residues on and in field fruit samples taken two

weeks prior to sample harvesting Residues, ppm a Wettable powder

Dates Locality Citrusdal

Variety

Wettable powder-oil

Plot A

B

Plot

Sprayed

Sampled

C

A

B

C

22 Mar

22 June

1.03 1.33 1.33 0.83 1.45 0.58

22Mar

19May

3.15 2.46 3.71 3.13 3.92 3.59

26 Jan

22 Apr

1.45 1.60 2.33 3.04 2.82 3.39

26 Jan

11 May

2.20 1.57 2.52 4.78 3.01 4.87

25Jan

22Apr

0.53 0.51 0.57 1.10 0.55 0.57

25Jan

11 May

1.18 0.28 0.43 0.59 0.82 0.93

25 Jan

11 May

0.46 0.32 0.29 0.71 0.58 0.55

28 Jan

10June

0.48 0.30 0.29 0.49 0.20 0.22

28Jan

19May

0.74 0.54 1.11 1.38 1.13 1.47

28Jan

19 May

0.48 0.67 0.46 0.51 0.32 0.67

28 Jan

I0 June

0.92 1.03 0.80 1.40 0.59 0.92

grapefruit navels

Malelane

grapefruit Valencia

Nelspruit

grapefruit navels Valencia

Rustenburg

grapefruit lemon tangerine Valencia

aAverage of three replicates/plot

Parathion Persistence on Citrus

301

There is a significant interaction between variety and sunlight exposure (F = 5.7, df = 1 and 45, p < 0.01); compare the average parathion residues in ppm: Valencias

grapefruit

sunlight

3.5

2.25

no sunlight

4.12

3.90

S M D r = 0.70 The average residue on exposed grapefruit was smaller than on exposed Valencias, but no difference was observed between unexposed fruit. This relationship between variety and sunlight was observed with three-day-old residues but not with oneday-old residues. Rain would remove more parathion from grapefruit than from lemons if it occurred three hr after spraying (Figure 2).

10(

+~

== "0

Lemons

to~ i. 0

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0

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20

30

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L . P . van Dyk

The citrus variety influences the parathion penetration (F = 141, df 2 and 48, p < 0.01); compare the average percentages penetrated after a certain time:

SMDT =

Valencia

grapefruit

navel

62.4

71.9

84.3

6.5

The speed of penetration decreased significantly from navel oranges, to grapefruit, to Valencia oranges. The difference is dependent on the parathion formulation used (F = 14.0, df 4 and 48, p < 0.01); examine the highly significant interaction between variety and formulation by comparing the average percentages penetrated:

Wettable powder wettable powder and oil emulsifiable concentrate SMDT = 7.4

grapefruit

navels

Valencias

68.0 86.6 62.1

79.3 93.2 80.3

48.9 82.8 55.4

With the parathion wettable powder-oil spray there was no difference between Valencia oranges and grapefruit or between grapefruit and navels, and with parathion emulsifiable concentrate the rates of penetration of parathion into Valencias and grapefruit did not differ.

Deposit. The effective deposit of parathion on citrus fruit was influenced by variety (F = 11.5, df = 2 and 136, p < 0.05); compare the average log effective deposits: navels 1.1954 SMDT = 0.0881

grapefruit

Valencias

1.2091

1.3549

The effective deposit was higher on Valencia oranges than on grapefruit and navels. In laboratory experiments the parathion deposits on Valencia oranges and grapefruit differed highly significantly (F = 13.0, df = 1 and 45, P < 0.01); as shown by the average residues of 3.74 ppm for Valencias and 3.08 ppm for grapefruit. It is concluded that the parathion deposits on Valencia oranges will generally be higher than those on grapefruit or navel oranges. This may be solely due to the weight to volume relationship at the time of spraying. The persistence of parathion on Valencia oranges, grapefruit and navel oranges is usually the same but a higher deposit is usually found on Valencia oranges and this may present a residue problem. This may be overcome in practice by the longer growth period o f Valencia oranges; when they are sprayed at the same date as other varieties the eventual harvest-time residues will not be any higher (Table I).

Parathion Persistence on Citrus

303

Leaves. There was no difference in the persistence of parathi.on on grapefruit, navel or Valencia leaves. In laboratory experiments no differences were found in the disappearance rates from Valencia orange and lemon leaves.

Locality and climatic effect. The locality had no influence on the persistence of parathion on fruit, but the persistence on leaves was influenced by locality (F = 13.2, df 2 and 136, p < 0.01); compare the average log half-lives: Nelspruit

Malelane

Citrusdal

0.2490

0.2514

0.3571

SMDT = 0.0574 Parathion was more persistent on leaves at Citrusdal than at Nelspruit and Malelane. This constrasting behavior on fruit and leaves illustrates the interaction between the plant part to which a pesticide is applied and the removal of that pesticide by climatic factors. The growth dilution of fruit overshadows the climatic effect. Growth dilution was eliminated as a factor with leaves, therefore the effect of climate can be observed. The main climatic difference between Citrusdal, Nelspruit, and Malelane was the rainfall. Ten times less rain fell at Citrusdal over the experimental period than at the other localities (Table II), however, in the correlation analysis the effect of rain was not significant (Table III). The persistence of parathion on leaves was influenced by the temperature and RH in laboratory experiments (F = 10.5, df 4 and 44, p < 0.01); compare the average log half-lives: 4~

50 RH

25~

1.0746

50 RH

0.9883

15~

50 RH

25~

0.6726 SMDr = 0.3319

85 RH

0.6632

350C, 50 RH 0.4205

Parathion appeared to be more persistent at 4~ but its persistence at 4~ did not differ significantly from that at 25~ Parathion was least persistent at 35~ but again this did not differ significantly from the persistence at 15~ It thus appears that a higher temperature gives a higher rate of disappearance of parathion, but this increase is not very great. High or low RH did not influence the persistence of parathion on leaves at 25~ The temperature and RH influence the rate of disappearance of parathion from fully grown fruit in the laboratory (F = 53.4, df 3 and 92, p < 0.01), compare the average log half-lives: 20~

85 RH

2.3315

18~

60 RH

1.4665

260C, 85 RH

300C, 60 RH

1.2276

0.8891

SMDr = 0.3135

304

L.P. T a b l e II.

van Dyk

Climatic conditions for the four localities during the experiment Max temp (~

Max temp (~

Wind km/day

Sunshine hr/day

Total rain (ram)

Locality

Season

Mean RH

Citrusdal

spring 70 s u m m e r 70-71 l a t e - s u m m e r 70-71

59 50 54

27.3 32.7 26.8

20.0 24.0 19.4

t42.1 142.8 107.4

10.4 10.8 7.9

1.8 7.1 5.8

Malelane

spring 69 summer 69-70 late-summer 69-70 spring 70 s u m m e r 70-71 l a t e - s u m m e r 70-71

72 71 70 74 76 80

38.0 35.8 35.6 29.1 30.8 29.7

23.1 25.0 25.4 28.0 24.0 23.0

148.7 130.5 113.9

5.3 6.5 7,7 6.9 6.5 7,5

71.3 119.1 38.3 72.6 129.8 104.1

Nelspruit

spring 69 summer 69-70 late-summer 69-70 spring 70 s u m m e r 70-71 l a t e - s u m m e r 70-71

75 77 73 65 68 71

37.2 34.8 35.5 27.2 29.7 28.5

18.9 20.9 22.7 19.3 22.0 21.7

672.2 583.0 480.0

5.5 6.8 7.3 6.7 6.0 7.5

119.7 243.9 59.9 106.6 63.7 163.7

Rustenburg

spring 69 summer 69-70 late s u m m e r 6 9 - 7 0 spring 70 s u m m e r 70-71 l a t e - s u m m e r 70-71

68 72 70 51 64 71

36.0 33.3 38.4 27.6 28.3 28.7

19.6 21.2 21.8 19.0 21.7 22.0

180.3 123.4 85.1

8.4 9.2 9.1 9.1 8.5 9.6

66.7 73.1 29.6 133.7 150.6 131.7

T a b l e III.

Significant correlations between the log half-lives of parathion on and in fruit and leaves and the climatic conditions Significant correlation coefficient

df

wettable powder

- 0.4859

18

sunshine

wettable powder

- 0.5298

21

navels

sunshine

wettable powder

- 0.4142

19

fruit

grapefruit

sunshine

wettable powder

- 0.4965

18

fruit

valencia

sunshine

wettable powder

- 0.4163

21

Plant Variety

Climatic factor

Spray mixture

leaves

grapefruit

sunshine

leaves

valencias

leaves

part

Parathion Persistence on Citrus

305

High RH increased the persistence of parathion on fruit kept at the same temperature. An increase in the temperature increased the rate of disappearance of parathion except in the case of tangerines (see varietal effect). The influence of RH on parathion persistence on fruit but not on leaves indicates a complex relationship between RH and parathion persistence. On glass surfaces a higher RH seemed to increase the loss of parathion, but this effect varied with the temperature and the formulation (Figure 3). The loss of the technical parathion from glass is very rapid compared to the formulated product (Figure 3). The evaporation of parathion is very important, because in closed containers saturated with parathion vapor the loss of parathion only become apparent at relatively high temperatures (Figure 3). Sunlight influenced the persistence of parathion. In field experiments the log half-lives of parathion applied as a wettable powder spray on leaves and fruit correlated negatively with the average number of hours of sunlight/day. Thus sunlight increased the rate of parathiofl disappearance (Table III), however, this correlation was not noticeable with the wettable powder-oil spray mixture. In laboratory experiments sunlight influenced the rate of disappearance of parathion one day after application to fruit (F = 28.3, df 1 and 45, p < 0.01); parathion was applied as either the emulsifiable concentrate or the wettable powder; compare the averages of the residues (2.14 ppm in sunlight and 2.89 ppm without sunlight). Sunlight also increased the loss of parathion from fruit exposed three days after parathion application (F = 43.8, df 1 and 45, p < 0.01); compare the average residues (2.80 ppm in sunlight and 4.01 ppm without sunlight). With samples exposed one day after treatment the duration of exposure was important (F = 7.5, df 3 and 45, p < 0.01); compare the average residues in ppm:

sunlight no sunlight

1 hr

2 hr

4 hr

6 hr

2.55 2.97

2.71 3.14

1.91 2.48

1.40 2.96

SMDr = 0.91 One and two hr of sunlight exposure caused no significant differences between exposed and unexposed fruit, this difference only became apparent after four hr. The duration of exposure had no influence on samples exposed to sunlight three days after treatment. The differences noted between formulations is not due to sunlight, because the interaction between sunlight and formulation was not significant (F = 0.02, df 1 and 45, p > 0.05). Locality and application effect. The deposits of parathion on fruit differed among localities (F = 59.7, df 2 and 136, p < 0.01); compare the average log effective deposits:

306

L . P . van Dyk Citrusdal 1.0478

Nelspruit

Malelane

1.2611

1.4505

SMDT = 0.0881 The effective deposits were fount to be higher at Malelane than at Nelspruit and higher at both these places than at Citrusdal. The same spray concentration was applied at these localities, but the spraying e q u i p m e n t and techniques were not standardized. The inconsistent application techniques thus resulted in higher re-

100

/

90

80

70 "0 e-

.o

60. U

'q

~

9~

50

E

40.

._

o

== o

30

"

20 I

10

0

X

x

m

I 10

i

I 20

n

I 30

Temperature ~C

Fig. 3. Loss of parathion from glass after 14 days at the plotted temperature. 9 = technical, 9 = wettable powder 70% RH, II = wettable powder 50% RH, 9 = emulsifiable concentrate 50% RH, ~1' = emulsifiable concentrate 70% RH, x = emulsifiable concentrate in closed containers.

Parathion Persistence on Citrus

307

sidues at some localities. This could give rise to serious problems, because in areas where a higher effective deposit was laid down, e.g. Malelane, where the growth season is shorter, the result will be higher residues at market time as the rate of disappearance of parathion is not dependent on the locality. G r o w t h effect. The rate of disappearance of parathion on fruit is influenced by the spray season (F = 99.9, df 2 and 136, p < 0.01); compare the average log half-lives of the field experiments done at Nelspruit, Malelane, and Citrusdal: late-summer spray 0.7840

summer spray

spring spray

0.6626 SMDT = 0.0718

0.3716

Parathion appeared to be more persistent on fruit when it was applied later in the growing season but the spray season had no effect on the persistence of parathion on leaves. It is concluded that growth dilution of fruit causes a great apparent loss of parathion early in the growing season. The size of the fruit at spray time influences the parathion deposition (F = 136, df = 2 and 136, p < 0.01); compare the ~iverage log effective deposits: spring spray 1.5798

summer spray

late-summer spray

1.1438

1.0357

SMDT = 0.0833 The log effective deposit of parathion on fruit decreased with increased fruit size. This is dependent on the spray mixture because there is a significant interaction between spray mixture and spray season (F = 4.2, df 2 and 136, p < 0.05); compare the average log effective deposits:

wettable powder wettable powder-oil

spring spray

summer spray

1.5197 1.6399

1.0251 1.2625

late-summer spray 0.8752 1.1961

SMDT = 0.1439 The addition of oil to the spray mixture obscured the difference between the summer and late-summer sprays. The difference between the deposits of the parathion spray mixture with or without oil could not be'observed after the spring spray. This may be due to the small size of the fruit at the time of spraying. The spray season effect on deposition was also influenced by the locality (F = 10.3, df = 4, and 136, p < 0.01); compare the average log effective deposits:

L. P. van Dyk

308 spring spray

summer spray

late-summer spray

1.6588 1.8613 1.2194

1.1006 1.3778 0.9530

1.0238 1.1123 0.9710

Nelspruit Malelane Citrusdale SMDT = 0.1925

There were no significant differences between the log effective deposits after the summer and late-summer sprays at both Nelspruit and Malelane.

Formulation effect. Parathion disappeared at the same rate from fruit, whether it was applied as a wettable powder or a wettable powder-oil spray mixture, however, the parathion effective deposits were influenced by the spray mixture (F = 63.2, df = 1 and 136, p < 0.01): The log effective deposits were 1.3662 for wettable powder-oil and 1.1401 for wettable powder. The addition of oil to the spray mixture increased the parathion effective deposit. This was influenced by the spray season as discussed previously and also by the locality (F = 3.6, df = 2 and 136, p = 0.05); compare the average log effective deposits:

wettable powder wettable powder-oil SMDr = 0.1439

Nelspruit

Malelane

Citrusdal

1.1302 1.3919

1.3024 1.5986

0.9876 1.1081

The parathion wettable powder-oil mixture did not give a higher effective deposit at Citrusdal. Generally the addition of oil gave a higher effective deposit of parathion on fruit and this may be the explanation for the higher residues found on some fruit treated with the spray mixture containing oil. In laboratory experiments the persistence of parathion on fruit was not influenced by the formulation, but it had an effect on the persistence of parathion on leaves (F = 7.7, df = 1 and 44, p < 0.05) as shown by the average log half-lives of 0.8659 for emulsifiable concentrate and 0.6619 for wettable powder. Parathion is more persistent on leaves when it is applied as an emulsifiable concentrate. Technical parathion was very sensitive to temperature on glass surfaces; it vaporized even at 4~ (Figure 3). It disappeared from glass surfaces more rapidly when applied as the wettable powder formulation than when applied as the emulsifiable concentrate (Figure 3). The formulation of parathion did not influence the effect of sunlight, because the interaction between sunlight treatment and formulation proved to be insignificant. Parathion removal by rain was furthermore not influenced by the formulation (Table IV) because the regression coefficients of the wettable powder and the emulsifiable

Parathion Persistence on Citrus

309

Table IV. Regression equations for the relationship between simulated

rain and the parathion residues on and in citrus fruit and leaves Parathion formulation

Plant part

Equation

F-ratio

df

wettable powder

fruit (grapefruit) fruit(lemon) leaves(Valencia) leaves (lemon)

Y = - 0 . 3 4 2 X + 1.96 Y = - O . 3 8 1 X + 2.13 Y =-108X +425 Y = - 5 9 X + 250

19.50 18.94 125.65 45.57

12 12 12 12

wettable powder-oil

fruit (grapefruit)

Y = - 0 . 4 0 9 X + 2.02

24.77

12

emulsifiable concentrate

fruit (grapefruit) fruit (lemon) leaves (Valencia) leaves (lemon)

Y= Y= Y= Y=

34.22 27.61 27.61 22.87

12 12 12 12

- 0 . 5 0 1 X + 2.99 - 0 . 4 1 2 X + 2.85 - 0 . 1 4 8 X + 616 - 2 3 X + 125

Y = ppm parathion X -- logarithm (ram rain + 1 )

2.4

Valencias

E Q,

CL

c~ t..

1.6

2

I

i

1 Oct.

1 Nov.

I I Dec.

i

I

1 Jan.

1 Feb.

Application date ,.Q

E e-

2.6

Navels

"6 E i. O .d

1.6

,

I

I

I

I

I

1 Oct.

1 Nov.

1 Dec.

1 Jan.

1 Feb.

Application date

Fig. 4.

Relationship of parathion application date with the time required to reach 0.5 ppm.

310

L . P . van Dyk

concentrate did not differ significantly on fruit (t = 0.56 for grapefruit), nor on leaves (t = 2.14 for Valencias and t = 4.90 for lemons). Thus the formulations tested were equally affected by sun, wind and rain. The parathion formulation influenced the penetration into fruit (F = 189, df = 2 and 48, p < 0.01). The penetration of parathion is more rapid when it is applied as a wettable-powder':oil spray mixture, as shown by the average percentages penetrated: wettable powder

emulsifiable concentrate

wettable powder-oil

65.4

65.6

87.5

SMDr = 6.5 The removal of parathion from fruit by washing was influenced by the formulation in which it was applied (F = 15.7, df = 1 and 24, p < 0.01); the average percentages left after washing were 75.7 for wettable powder and 81.5 for emulsifiable concentrate. Parathion applied as the wettable powder was more easily washed from fruit than that applied as the emulsifiable concentrate.

Effect of the year of spraying. The year or growing season in which parathion was applied influenced the rate of its disappearance (F = 5.3, df = 1 and 88, p < 0.05); the average log half-lives were 0.8182 for the first year and 0.6969 for the second. Parathion was more persistent during the first growing season at Nelspruit and Malelane, possibly due to less sunshine during the spring spraying season or of less rain during the late-summer season of the first year, or a combination of factors (Table II).

General discussion A simple explanation of the influence of each of the factors which may play a role in the persistence of parathion is not possible. The complexity arises from a characteristic that is inherent in this type of problem, namely the interaction that occurs between factors. In the statistical analyses the readily explainable secondary interactions were discussed and were seen to have a qualifying effect on most of the main factors. Third, fourth and higher interactions were not even considered, but incorporated into the error of each analysis so that it becomes clear that a simplistic approach to explain field behaviour is impractical. The general effects of the factors were studied, however, and general but not absolute conclusions were drawn. The time needed to reach a tolerance value of 0.5 ppm increased with parathion sprays applied later in the growing season (Figure 4). From the graphs in Figure 4 it is clear that parathion sprays applied later than early December in South Africa will not give acceptable residues at harvest time for navel or Valencia oranges. It also appears that it will generally take longer for parathion applied to navel oranges to reach the tolerance value. As a result of this study the safety period for parathion of

Parathion Persistence on Citrus

31[

42 days between last application and harvesting has been abolished. South African citrus growers are now advised not to apply parathion later than eight weeks after flower petal fall (Bot et al. 1973).

Acknowledgments The assistance of A. Badenhorst, M. Krause and P. van Niekerk in the laboratory and field work and of D. E. Ott in the preparation of the manuscript during the author's visit to the University of California, Riverside, is gratefully acknowledged.

References Archer, T. E.: Malathion residues on ladino clover seen screenings exposed to ultraviolet irradiation. Bull. Environ. Contamin. Toxicol. 6, 142 (1971). Bot, J., N. deL. Genis, and N. Hollings: A guide to the use of pesticides and fungicides in South Africa. Pretoria: Department of Agricultural Technical Services (1973). Brody, S. S., and J. E. Chaney: Flame photometric detector: The application of a specific detector for phosphorous and sulfur compounds, sensitive to subnanogram quantities. J. Gas Chromatogr. 4, 42 (1966). Ebeling, W.: Analysis of the basic processes involved in the deposition, degradation, persistence, and effectiveness of pesticides. Residue Reviews 3, 35 (1963). Guenther, W. C.: Analysis of variance. New York: Prentice Hall (1964). Gunther, F. A.: Insecticide residues in citrus fruits. Residue Reviews 28, 1 (1969). Gunther, F. A., and R. C. Blinn: Analysis of insecticides and acaricides. New York and London: Interscience (1955). Hill, K. R." Report of half-life working party. J. Ass. Offic. Anal. Chemists, 54, 1316 (1971). Mitchell, T. H., and J. H. Ruzicka, J. Thomson, and B. B. Wheals: The chromatographic determination of organophophorous pesticides. III The effect of irradiation of the parent compounds. J. Chromatogr. 32, 17 (1968). Wiese, I. H., and J. Bot: Pesticide regulation in South Africa. Residue Reviews 35, 49 (1971).