Stability of organophosphate and pyrethroid pesticides on
wheat in storage
Irshad A.K. Afridi
a, Zahida Parveen
b,*, S. Zafar Masud
b aDepartment of Chemistry, University of Karachi, Karachi 75270, Pakistan
b
Pesticide Research Institute, Tropical Agricultural Research Centre, Pakistan Agricultural Research Council, Karachi University Campus, Old Block No. 09 and 10, Karachi 75270, Pakistan
Accepted 7 April 2000
Abstract
The pesticides chlorpyriphos-methyl, pirimiphos-methyl and permethrin were applied to wheat and stored for 52 weeks at 25, 30, 35 and 408C, and at 10 and 13% m.c. Rates of loss were calculated from the residue analyses of pesticides in treated wheat at monthly intervals during the storage period. Calculated half-lives and pseudo ®rst-order rate constants of these pesticides are discussed with reference to temperature and moisture.72000 Published by Elsevier Science Ltd.
Keywords:Stability; Pesticide residues; Half-lives; Chlorpyriphos-methyl; Pirimiphos-methyl; Permethrin
1. Introduction
Many research workers have studied the relationship between the moisture content of dierent cereal grains and their equilibrium relative humidity but the eects of temperature and moisture content on pesticide breakdown have not generally been described quantitatively (Kane and Green, 1968; Champ et al., 1969; Desmarchelier, 1977; LaHue and Kadoum, 1979; Quinlan et al., 1980; Davies and Desmarchelier, 1981; Bengston et al., 1983). According to Desmarchelier (1978), loss of fenitrothion from post-harvest application to wheat, oats, rice and sorghum followed a second-order rate process with rate of loss being proportional at a
Journal of Stored Products Research 37 (2001) 199±204
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®xed temperature to the amount of fenitrothion and the activity of water. Zongmao and Haibin (1988) presented growth dilution factors for pesticide persistence in the growing tea plant in their studies on the residual behaviour of 17 pesticides and their half-lives. In another study, Leahey and Curl (1982) studied degradation of pirimiphos-methyl on cereal grains under controlled moisture contents. LaHue (1974) concluded that the recovery of malathion residue was greater from grain treated with a dust formulation than from grain treated with an emulsion at 12.5% moisture content of the grain. Typical half-lives of two insecticides were studied by Rowlands and Wilkin (1975) at 208C and two moisture contents, 12 and 18%. Noble et al. (1982) applied four pyrethroids to wheat stored for a ®xed period at two dierent temperatures and moisture contents and concluded that loss of these protectants is slow especially under milder conditions.
Studies presented herein deal with loss of organophosphate and pyrethroid pesticides on wheat grains during 52 weeks storage at four dierent temperatures and two moisture contents. The half-lives of the pesticides were also evaluated.
2. Material and methods
Fresh wheat was cleaned and fumigated with phosphine for 1 week to remove pests. The initial moisture content was 10.6% as determined by the standard ISO air oven method. Grain moisture was adjusted to 13% m.c. according to Winks (1983). A sun drying process was used to adjust wheat grains to 10% m.c. Three kilogram lots of wheat at each moisture content were treated at recommended dosages with chlorpyriphos-methyl (10 mg kgÿ1), pirimiphos-methyl (4 mg kgÿ1) and permethrin (2 mg kgÿ1). Three replicates of each pesticide treatment at each moisture content were prepared. The pesticides were diluted with 4 ml distilled water and applied as a ®ne spray using a Quick®t R-7/9 spray atomizer to 1 cm thick layers of wheat in a galvanized iron tray. The treated wheat was transferred to glass jars and tumbled vigorously for approximately 20 min and then stored in dried sealed glass jars at 25, 30, 35 and 408C. Sub-samples of 50 g each were drawn after storage periods of 13, 26, 39 and 52 weeks. The control and treated sub-samples were analysed in triplicate for pesticide residues. In order to economize cost and time, Becker's (1985) modi®ed procedure for organophosphorus pesticide residues in grains was further modi®ed in this laboratory (Masud and Parveen, 1991) to make it suitable for permethrin.
The method involved extraction of the pesticide from the grain with water+acetone, 1:8, and cleanup on a silica gel column. Each cleaned sample extract was analysed by gas chromatography (Varian AG, GC-3600) ®tted with thermionic speci®c (TSD) and63Ni electron capture (ECD) detectors against relevant pesticide standards. Three injections of each extract were made to check reproducibility of results. The eciency of analytical procedures was evaluated in model experiments with control samples of wheat. This established that the minimum detection limit of each of the three pesticides was 0.01 mg kgÿ1 and the recoveries at this level were in the range of 80.5±96.8%.
kt ln a
where a-x is the measured pesticide residue level (mg kgÿ1); a is the concentration of applied pesticide; k is a degradative constant (dayÿ1) and t is the time after treatment (days). Average half-lives and their 95% con®dence limits were calculated from the k values of the compounds when held under simulated conditions for 52 weeks. Half-lives (t1/2 ) were calculated using Eq. (2).
GC analyses of treated wheat samples from 0 to 52 weeks are presented in Table 1. In addition to the thermal eect, water activity also has a signi®cant in¯uence on the degradation rates (Table 1). From the results, it is evident that the residues of chlorpyriphos-methyl and pirimiphos-methyl were rapidly degraded at a m.c. of 13% and a temperature of 408C. The apparent rates of degradation in the initial storage period were higher than in the later stages and this could re¯ect a degree of loss of insecticide during treatment application. Both these OP compounds appear promising and are safe as their residues fell below the limit of detection after storage in admixture for 52 weeks even at 358C and m.c.'s of 10 and 13%. By contrast, residues of permethrin were highly persistent on stored wheat. During the ®rst 13 weeks storage period its residues persisted at 68.4 and 73.6% at both moisture contents and at 258C. At the higher temperature of 408C and at both moisture contents, however, residues fell to 3.03 and 1.36% respectively. Table 1 indicates that chlorpyriphos-methyl was lost most rapidly followed by pirimiphos-methyl, and permethrin was most stable.
Calculated pseudo ®rst-order rate constants and half-lives of the three pesticides at the four temperatures and two moisture contents are presented in Table 2. The data in Table 2 show that the loss of these pesticides is slow especially under milder conditions of temperature and moisture. The rate constant and half-lives show an order for the rate of loss on stored wheat to be chlorpyriphos-methyl > pirimiphos-methyl > permethrin. The estimated half-lives did not exceed the period (52 weeks or 365 days) over which analyses were conducted. This is re¯ected in the con®dence limits for the half-lives of chlorpyriphos-methyl and permethrin for storage at 258C and 10% m.c. In practice 408C and 13% m.c. is an extreme condition. The data also indicate that at the application rate of 2 ppm rather than 4 or 10 ppm, permethrin may provide a long useful period of protection from insects that attack under the aforesaid bulk
Table 1
Persistence of pesticide residues (% of dose remaining) on wheat held at dierent temperature and moisture content
Moisture content
10% 13%
Storage interval Storage temperature (8C)
Chlorpyriphos-methyl Pirimiphos-methyl Permethrin Chlorpyriphos-methyl Pirimiphos-methyl Permethrin
13 weeks 25 11.8 59.3 68.4 11.1 55.5 73.6
30 8.6 42.6 60.6 7.9 39.3 60.9
35 4.0 30.1 52.4 3.7 22.7 50.9
40 3.6 5.4 40.7 2.5 4.7 40.0
26 weeks 25 10.4 39.5 61.5 10.1 36.8 64.1
30 6.0 14.7 56.7 5.6 13.1 57.3
35 3.1 13.0 45.4 2.9 9.6 46.4
40 1.9 3.2 32.0 1.1 2.3 32.7
39 weeks 25 7.0 16.9 52.4 6.9 15.2 51.8
30 4.4 6.6 44.2 1.8 5.6 45.0
35 1.4 4.2 28.1 1.0 3.5 26.8
40 0.4 1.0 17.7 0.3 0.5 13.6
52 weeks 25 1.8 7.8 44.2 1.9 6.6 41.4
30 0.8 1.2 20.3 0.2 0.7 21.8
35 Nil Nil 10.4 Nil Nil 10.9
40 Nil Nil 3.0 Nil Nil 1.4
I.A.K.
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storage conditions. Overall the results show that the loss of protectants in bulk grain depends on grain moisture, grain temperature and time of storage.
References
Becker, C., 1985. Modi®ed procedure for the determination of organophosphorus pesticide residues in grain. Analyst 110, 765±768.
Bengston, M., Cooper, L.M., Davies, R.A.H., Desmarchelier, J.M., Hart, R.H., Phillips, M.P., 1983. Grain protectants for the control of malathion Ð resistant insects in stored sorghum. Pesticide Science 14, 385±398. Champ, B.R., Steele, R.W., Genn, B.G., Elms, K.D., 1969. A comparison of malathion, diazinon, fenitrothion and
dichlorvos for control of Sitophilus oryzae (L.) and Rhyzopertha dominica (F.) in wheat. Journal of Stored Products Research 5, 21±48.
Davies, R.A.H., Desmarchelier, J.M., 1981. Combinations of pirimiphos-methyl and carbaryl for stored grain protection. Pesticide Science 12, 669±677.
Table 2
Rate of breakdown of residues on wheat
Pesticide Rate of dosage
Permethrin 2 10 25 0.014 346 303±404
30 0.023 211 180±243
Desmarchelier, J.M., 1977. Selective treatments, including combinations of pyrethroid and organophosphorus insecticides for control of stored products Coleoptera at two temperatures. Journal of Stored Products Research 13, 129±137.
Desmarchelier, J.M., 1978. Loss of fenitrothion on grains in storage. Pesticide Science 9, 33±38.
Kane, J., Green, A.A., 1968. The protection of bagged grain for insect infestation using fenitrothion. Journal of Stored Products Research 4, 59±68.
LaHue, D.W., 1974. Pesticide residues of potential protectants of grain. In: Proceedings of Ist International Working Conference on Stored Products Entomology, Savannah, Georgia, 7±11, October 1974, pp. 635±638. LaHue, D.W., Kadoum, A., 1979. Residual eectiveness of emulsion and encapsulated formulations of malathion
and fenitrothion against stored grain beetles. Journal of Economic Entomology 72, 234±237.
Leahey, J.P., Curl, E.A., 1982. The degradation of pirimiphos-methyl on stored grains. Pesticide Science 13, 467± 474.
Masud, S.Z., Parveen, Z., 1991. Persistence of permethrin, pirimiphos-methyl and chlorpyriphos-methyl insecticides in wheat stored under simulated environmental conditions. Pakistan Journal of Science and Industrial Research 34, 240±246.
Noble, R.W., Hamilton, D.J., Osborne, W.J., 1982. Stability of pyrethroids on wheat storage. Pesticide Science 13, 246±252.
Quinlan, J.K., Wilson, J.L., Davidson, L.I., 1980. Pirimiphos-methyl as a protectant for high moisture stored wheat. Journal of Kansas Entomological Society 53, 825±832.
Rowlands, D.G., Wilkin, D.R., 1975. Metabolism of pesticides in stored cereals. Report of Director, Pest Infestation Control Laboratory, Slough UK, 1971±1973.
Winks, R.G., 1983. Laboratory culturing of some storage pests. In: Champ, B.R., Highley, E. (Eds.), Proceedings of Australian Development Assistance Course on preservation of stored cereals. Commonwealth Scienti®c and Industrial Organization, Canberra, pp. 220±221.
