Pestic. Sci. 1985, 16, 143-146

Dimethoate Spray Residues in Strawberries

Stephen Goodwin, Nazir Ahmad" and Graeme Newellb

Horticultural Research Station, PO Box 720, Gosford, New South Wales 2250, "Biological and Chemical Research Institute, Rydalmere, New South Wales 2116 and bHuwkesbury College of Agriculture, Richmond, New South Wales 2753, Australia

(Revised manuscript received 16 August 1984)

____

Strawberries were sprayed with solutions of 200, 300 and 500 mg dimethoate litre-' and analysed for dimethoate residues up to 21 days after the final spraying. The residues were found to be below the National Health and Medical Research Council limit of 2 mg kg-', 1, 2 and 4 days after spraying. Consequently, the withholding period of 7 days could be reduced to 3 days to allow uninterrupted picking of the ripe strawberry crop.

1. Introduction

For nearly 30 years, dimethoate (0,O-dimethyl S-methylcarbamoylmethyl phosphorodithioate) has been used as a broad spectrum insecticide, generally with a 7-day withholding period between the last day of application of the spray and the harvesting of the crop. While this has been acceptable in most crops, this period has restricted the use of this insecticide for strawberries grown in New South Wales, and has prevented the uninterrupted picking of ripe fruit during spring and summer.

Although dimethoate is a low hazard insecticide and passes into the aqueous tissues of the plant where it rapidly undergoes oxidation and hydrolysis,'12 there has been no comprehensive analysis of dimethoate residues in strawberries in Australia. The aim of this present investigation was to determine the dimethoate residues in strawberries that had been subjected to a spraying schedule, and to assess the results in respect of the maximum residue limit, set by the National Health and Medical Research Council of Au~t ra l i a ,~ of 2 mg kg-' for strawberries.

2. Experimental methods 2.1. Trial design Virus free runners of the strawberry cultivars Torrey, Tioga and Naratoga were planted in a caged area (0.1 ha) at the Horticultural Research Station, Gosford in May 1980. The soil, previously fumigated with 1,2-dibromoethane (ethylene dibromide) and chloropicrin, and fertilised with fowl manure (100 kg ha-'), superphosphate (62 kg ha-') and dolomite (40 kg ha-'), was prepared into beds in the normal manner with black plastic (0.05 mmx90 cm), giving beds 70 cm wide and 45 cm apart. The plants were grown in double rows down each bed, with rows 0.3 m apart and 0.2 m between plants. Biwall trickle irrigation was used.

The trial was arranged in a randomised block design with three replicates of each variety, each replicate consisting of 265 plants. Dimethoate 30% emulsifiable concentrate (Roche-Maag) spray (200, 300 and 500 mg litre-') was applied to run-off by a knapsack sprayer on 1, 8, 15 and 22 September 1980. Fruit samples were randomly collected from each plot soon after the fourth spray and then at 1, 2, 3, 4, 7 , 14 and 21 days. Each sample was placed in a plastic bag, immediately cooled in ice boxes in the field, and then kept in a freezer (-15°C) in the laboratory until analysed.

143

144 S. Goodwin et al.

2.2. Analytical procedure Fruit samples were prepared for analysis by removal of the inedible parts, and then chopped and thoroughly mixed. A sample (25 g) was blended with acetonitrile (100 ml) in an Omnimixer for 3 min and then filtered through a Whatman No. 1 filter paper into a round-bottom flask. The cup and residues on the filter paper were washed twice with acetonitrile (2x25 ml). The filtrate was mixed with water (40 ml), and the acetonitrile was removed by a rotary vacuum evaporator. The aqueous layer was extracted with chloroform (3x25 ml). The chloroform extract was evaporated in a Kuderna-Danish evaporator, and the chloroform was replaced by hexane by repeated addition of hexane and e~apora t ion .~

The gas-liquid chromatographic analyses were carried out on a Varian model 2700 gas chromatograph equipped with a specific thermionic detector; the injector temperature was 225"C, column temperature 220"C, detector temperature 275"C, nitrogen flow rate 30 (L0.2) ml min-', and air flow rate 175 (+5) ml min-'. The chromatography was carried out on a stainless steel column (2 m long and 3.2 mm i.d.) packed with 5.2% OV-225 on Chromosorb W (80-100 mesh).

The efficiency of the procedure was determined using a control sample fortified with dimethoate (0.5, 1, 1.5 and 2 mg kg-') and omethoate, prior to the addition of acetonitrile. The dimethoate in hexane was added to a crushed sample of frozen strawberries (100 g) uncontaminated with pesticide, and stirred with a glass rod. The treated sample was allowed to stand for 2 h at room temperature and the control sample (25 g) was then taken. The rest of the analytical procedure was the same. The average recovery of dimethoate and omethoate was 93 (+2)%.

3. Results and discussion

The mean dimethoate residues for the three strawberry varieties sprayed with 200, 300 and 500 mg litre-', 0, 1, 2, 3, 4, 7, 14 and 21 days after the fourth spray, are given in Tables 1-3, respectively. At each dimethoate spray concentration, the residues on the three strawberry varieties were not significantly different (f50.05).

The National Health and Medical Research Council (NHMRC) of Australia limit of 2 mg kg-' for dimethoate in fruits, was reached for the 200, 300 and 500 mg litre-' sprays 1, 2 and 4 days, respectively, after the final spray for each of the strawberry varieties. No omethoate was detected in any sample (detection limit 0.05 mg kg-'); for confirmation of this, some of the samples with high dimethoate residues were chromatographed on a 2% PEG column with electron-capture de t e~ t ion .~ Similarly, thin-layer chromatography failed to show any omethoate in the samples tested.

Previous studies on the metabolism of dimethoate in plants have concentrated on establish- ing the relationship between the oxygen analogue omethoate (0,O-dimethyl S-methyl-

Table 1. Mean dimethoate residues in three varieties of strawberry fruit following four spray applications at 200 mg litre-'

Dimethoate residues (ks .e . ) (mg kg-I)

Strawberry variety - Number of days

after fourth spray Torrey Tioga Naratoga Mean

0 1 2 3 4 7

14 21

2.69 (k0.17) 1.98 (k0.46)

0.99 (k0.03) 0.54 (20.10) 0.36 (C0.08) 0.46 (k0.23) 0.34 (C0.08)

1.51 (kO.lO)

3.33 (k0.21) 2.17 (k0.48) 1.37 (i0.24) 1.03 (k0.24) 0.81 (f0.33) 0.75 (kO.18)

0.06 (kO.O1) 0.11 (k0.02)

2.40 (k0.20) 1.78 (C0.20) 1.83 (k0.26) 1.28 (k0.20) 0.95 (i0.05) 0.82 (f0.05) 0.33 (fO.lO) 0.14 (k0.01)

2.81 (k0.27) 1.98 ( k O . l l ) 1.57 (i0.14)

0.77 (i0.12) 0.64 (It0.14) 0.30 (fO.lO)

1.10 (k0.09)

0.18 (f0.08)

Dimethoate spray residues in strawberries 145

Table 2. Mean dimethoate residues in three varieties of strawberry fruit following four spray applications at 300 mg litre-'

Dimethoate residues (ks.e.) (mg kg-')

Number of days after fourth spray

0 1

__-__-

3 4 7

14 21

Strawberry variety

Torrey

3.23 (k0.30) 2.18 (t0.44) 1.45 (k0.24) 0.93 (10.13) 0.54 (k0.12) 0.37 (k0.49)

0.29 (k0.02)

~-

0.15 (k0.02)

Tioga ~-

3.91 (3Z0.09) 3.25 (k0.20) 2.09 (f0.19) 1.42 (k0.12) 1.35 (k0.22) 0.82 (f0.07) 0.30 (kO.lO) 0.29 (f0.18)

Naratoga

3.80 (k0.81)

2.17 (k0.07) 1.78 (k0.12) 1.31 (k0.21) 0.88 (kO.ll) 0.30 (kO.lO) 0.31 (50.03)

2.25 (k0.12)

Mean

3.65 (k0.21) 2.56 (f0.34) 1.90 (f0.23) 1.38 (k0.25) 1.07 (50.26) 0.69 (k0.16) 0.25 (k0.05) 0.30 (kO.01)

Table 3. Mean dimethoate residues in three varieties of strawberry fruit following four spray applications at 500 mg litre-'

Dimethoate residues ( h e . ) (mg kg-I)

Strawberry variety Number of days -

after fourth spray Torrey Tioga Naratoga

0 7.25 (k0.45) 6.59 (k0.41) 5.06 (20.48) 1 5.95 (k0.39) 4.91 (k0.39) 4.67 (t0.43) 2 3.25 (k0.67) 3.75 (i0.74) 3.74 (k0.80) 3 1.95 (k0.12) 2.73 (k0.21) 2.39 (3~0.27) 4 1.68 (k0.17) 1.45 (k0.33) 1.99 (k0.19) 7 1.18 (k0.08) 0.78 (t0.09) 1.00 (k0.21)

14 0.57 (k0.14) 0.57 (f0.06) 0.44 (k0.11) 21 0.47 (k0.10) 0.33 (t0.14) 0.33 (10.02)

Mean

6.30 (k0.65) 5.18 (k0.39) 3.58 (k0.16) 2.36 (k0.23) 1.71 (k0.16) 0.99 (k0.12) 0.53 (i0.04) 0.38 (k0.05)

Table 4. Exponential equations for the decrease of dimethoate residues in strawber- ries over time

Equation for decrease of dimethoate residues in each straw- berry variety Dimethoate

spray concentration (mg litre-') Torrey Tioga Naratoga ______

500 y=4.14e4' 13' y=4,16e-" 14' y24.08e-O 14'

300 y=2.50e-0 121 y=2.82e-O 15' y=2,51e-O I*'

200 y= 1.34e-O 13' y=2,16e-" 14' y=2,03e-" IJ'

carbamoylmethyl phosphorothioate) and the parent compound. L,ba Other workers have re- ported conflicting results on the oxidation of dimethoate to o m e t h ~ a t e . ~ , ~ In foliar applications, the dimethoate molecule has to penetrate the leaf and fruit, and this appears to result in very low oxidation rates. From this study, decrease of the dimethoate residues over time, for each of the strawberry varieties at each dimethoate spray concentration, was expressed by the equations given in Table 4. For each strawberry variety, there was no significant difference (Z50.05) between the rate constants obtained at each of the dimethoate spray concentrations. Logarithms of the dimethoate residues over time were found to have a linear relationship, indicating a first-order kinetic reaction. Like most organophosphorus compounds, the residues decreased most rapidly within the first few days after spraying.

The recommended dimethoate spray concentration of 300 mg litre-' for strawberries in New South Wales resulted in mean residues below the NHMRC maximum residue limit of 2 mg kg-', 2 days after the final spray. On the basis of the variation in the residue levels for each of the varieties of strawberries, the minimum withholding period between final spraying and harvesting

146 S . Goodwin ef al.

could be safely reduced to 3 days. This reduction in the withholding period from the usual 7 days would permit the use of dimethoate on strawberries during spring-summer for the control of strawberry aphid Chaetosiphon fragaefolii (Cockerell), without interrupting the normal harvest- ing of the crop.

References 1. 2. 3.

4. 5. 6. 7. 8. 9.

Alessandrini, M. E. Residue Rev. 1962, 1, 92-111. Santi, R.; Giacomelli, R . J . J. Agric. Food Chem. 1962, 10, 257-261. Withholding Periods, Muximum Residue Limits and Poisons Schedules for Agricultural and Veterinary Chemicals, Pesticide Section. Dept. of Primary Industry, Canberra, A.C.T., Document PB376, 1980, 3rd edn. Voss, G . I . ; Baunok, I . : Geissbuhler, H. Residue Rev. 1971, 37, 101-132. Storherr, R. W.; Watts, R. R. J. Assoc. Off. Anal. Chem. 1969, 47, 511-513. Belal, M . H.; Gomaa, E. A. A. Bull. Environ. Contam. Toxicol. 1979, 22,726730. Dauterman, W. C.; Viado, G. B.; Casida, Y . E.; Obrien, R. D. J. Agric. Food Chem. 1960, 8, 115-119. Rowlands, D . .I. J . Sci. Food Agric. 1966, 7 , 9@93. Lucier, G. W.; Menzer, R. E. J. Agric. Food Chem. 1968, 16, 936945.