State of California 

M e m o r a n d u m

To : Kean S. Goh, Program Supervisor             Date: 16 August 1995 
     EM & PM Branch 
                                                 Place: Sacramento 

                                                 Phone: 324-3904 

From : Department of Pesticide Regulation - J. Leyva, APRS 
       Registration Branch 


Malathion (O,O-dimethyl phosphorodithioate of diethylmercaptosuccinate), a broad-spectrum insecticide, is used in California to eradicate infestations of the Mediterranean fruitfly (Ceratitis capitata). In the 1989/90 Medfly Eradication Program in California, malathion was combined in a 4:1 ratio with Nu-Lure0 bait (v/v), a protein-based bait used to attract insects(Segawa 1991). This bait is a dark brown, slightly viscous liquid derived from plant sources (Miller Chemical and FertilizerCorp.). 

The extent to which pesticide residues may be removed from treated produce by washing is influenced by a variety of factors such as the chemical properties of the pesticide, the nature ofthe commodity, the length of time that the residue has been on the commodity's surface, and the formulation in which the pesticide has been applied. Smith et al. (1955) reported that washing lettuce with tap water for 30 seconds, 1 hour after malathion treatment removed 88% of the residues from 16.3 ug/g to 1.8 ug/g. Wallis et al. (1957) reported that washing parsleysamples for one minute with tap water one hour after malathiontreatment removed 92% of the residues from 36.2 ug/g to 3.0 ug/g. For some pesticide products, washing of produce with detergents increased the proportion of pesticide residue removed. Detergent wash significantly improved removal of DDT residues from spinach(73%) compared with water rinse (48%) (Farrow et al. 1972). 

The dislodgeable residue analytical method for vegetation currently used by the California Department of Food and Agriculture (CDFA), Chemistry Laboratory, is intended to provide data for assessment of human dermal exposure after pesticide applications on crops. This data is then used to determine reentry intervals for farmworkers (Gunther et al. 1973; Gunther etal. 1974). Dislodgeable residues are removed with deionized water with 2% Sur-Ten (1 g/50 mL of Aerosols OT 75% aqueous).  This rinse method, however, may not be appropriate for assessing exposure by ingestion because the use of Sur-Ten may result in increased quantities of residues being removed compared to a water wash used by the consumer. This study was conducted to compare the concentrations of malathion and malaoxon, a malathion metabolite, in lettuce between two rinse procedures: 1) CDFA,Chemistry Laboratory Services Sur-Ten method to remove vegetation dislodgeable residues and 2) deionized water-rinse method, which typifies the procedure that consumers may follow to remove material from the surface of produce before consumption. 


On July 1993, baited malathion was applied in the evening in Sacramento, CA to simulate the usual conditions under which malathion is aerially applied during an eradication program.  Malathion (Clean Crop Malathion ULV, 94.9% purity, Platte Chemical Co.) was combined in a 4:1 ratio with Nu-Lure bait mixture and applied to 40 heads of green-leafy lettuce as discrete droplets using an electronic pipetter (EP-100, Rainin Instruments Co., Woburn, MA). In order to simulate the malathion deposition from an aerial application (2,212 ug ai/929 cm2), it was assumed that one lettuce head covered an area of 929 cm2.Hence, every lettuce head would receive 9 uL of malathion bait deposition (2,092 ug) in 1-uL discrete droplets. Prior toapplication, each lettuce head was weighed and placed vertically in a 10.2 cm x 25.4 cm 3-L glass jar. The 40 lettuce heads ranged in weight from 139.5 g to 231.8 g with an average value of 184.52 g. The baited malathion mixture was kept on a stir plate and was continuously agitated throughout the application. Then, the 3-L jars containing the lettuce heads were placed outdoors on blue ice in open ice chests. The mouths of the jars were covered with cheese cloth to protect the lettuce from animals. 

Twelve hours after application, ten lettuce heads were collected for rinsing dislodgeable residues with the deionized water-rinse method and ten heads collected for rinsing with the Sur-Ten method. The 20 remaining 3-L jars each with one lettuce head were stored in a refrigerator at 10oC, collected at 36 hr after application, and rinsed using the two procedures (10 for each rinse method). Where as the 12-hr sample collection simulated residues deposited on lettuce the morning after an aerial application and the 36-hr sample collection simulated the lapse time required to transport samples from the field site in Southern California, in the event of an aerial application, to the Sacramento laboratory. 

Rinsing solutions were 1.93-L deionized water (water-rinse method) or 1 ml of 2% Sur-Ten in 1.93 L deionized water (Stir-Ten method). Rinsing solution was added to the 3-L container with the treated lettuce and the jar was capped and gently inverted 12 times for a 15-second period. Immediately following this procedure, the rinsate was transferred to an empty 1.93-L glass jar. The rinsate was extracted and analyzed for dislodgeable malathion and malaoxon residues. The whole washed lettuce head was blended with dry ice, then extracted, and analyzed for penetrated malathion and malaoxon residues. All heads were analyzed for % moisture prior to determination of penetrated residues. 

Chemical analyses for malathion and malaoxon residues were conducted by CDFA, Chemistry Laboratory. Rinsate was extracted with methylene chloride and lettuce slurry was extracted with ethyl acetate. Both extracts were analyzed for malathion and malaoxon residues (Lee 1993). Malathion and malaoxon were determined by gas chromatography, Hewlett Packard Model 5890 Gas Chromatograph with a HP-1 10m x 0.53mm x 2.0 um column and a helium flow rate of 10 mL/min. Results were reported in ug/g(wet weight basis). The method detection limit was 1 ug/sample for both malathion and malaoxon. 

The data were analyzed using SASS General Linear Model procedures to compare log-transformed mean malathion concentrations at each level (SAS 1987). Significant effects were tested on malathion concentrations with two way analyses of variance (ANOVA). 


There was no significant difference in malathion concentrations of rinsate between the Sur-Ten and water-rinse procedures at 12 or 36 hours post-treatment. Twelve hours after application, mean dislodgeable malathion concentration in rinsate was 3.50 ug/g for the Sur-Ten procedure and 3.49 ug/g for the water-rinse method.  Thirty-six hours after application, mean dislodgeable malathion concentrations were 3.33 ug/g for the Sur-Ten method and 3.35 ug/g for the water-rinse Method (Table 1). The percentage of dislodgeable malathion removed was calculated by dividing ug/g dislodgeable by ug/g applied. Consequently, twelve hours after application, 30.8% of applied malathion was removed with either rinsing method. Thirty-six hours after application, 29.5% of applied malathion was removed with either procedure. Malaoxon dislodgeable and penetrated residues were not detected in any of the samples. 

Twelve hours after application, mean penetrated malathion residues in washed lettuce were 4.22 ug/g for the Sur-Ten method and 4.15 ug/g for the water-rinse method. After 36 hours, mean penetrated malathion concentration was 5.97 ug/g for the Sur-Ten method and 5.82 ug/g for the water-rinse method (Table 1).  Values for the average penetrated malathion levels increased significantly for both rinse methods at 12 to 36 hours after application (p = 0.0003). This effect was not due to wilting(water loss) because before the chemical analysis the % moisture for both 12 and 36-hour samples was approximately the same, 95%. 


Since there was no difference between the use of surfactant and water for rinsing malathion-treated lettuce heads, deionized water should be used to remove dislodgeable malathion residue.  In addition, the water-rinse procedure is less complicated than the Sur-Ten method. 

Further studies are needed to investigate the 20% increase of malathion residues in/on lettuce from 12 to 36 hours after application.


cc: B. Rollins 
    T. Leffingwell 
        R. Sava 


Farrow RP, Elkins RE, Rose WW, Lamb FC, Ralls JW, Mercer WA(1972) Canning operations that reduce insecticide levels inprepared foods and in solid food wastes. National Canners Association pp. 73 104. 

Gunther FA, Westlake WE, Barkley JH (1973) Establishingdislodgeable pesticide residues on leaf surfaces. Bull EnvContam Toxicol 9:243 249. 

Gunther FA, Barkley JH, Taylor WE (1974) Worker EnvironmentResearch. II. Sampling and processing techniques fordetermining dislodgeable pesticide residues on leaf surfaces.Bull Env Contam Toxicol 12:641 644. 

Lee P (1993) Malathion residues in lettuce and in water.(Unpublished method). California Department of Food and Agriculture, Chemistry Laboratory Services. Sacramento, CA. 

SAS Institute, Inc. (1987) SAS/STAT_ guide for personalcomputers, Version 6 edition, Cary, NC, SAS Institute, Inc. 

Segawa R.T, Sitts JA, White JH, Marade SJ, Powell SJ (1991)Environmental monitoring of malathion aerial applications usedto eradicate mediterranean fruit flies in Southern California,1990. California Department of Food and Agriculture (NowCalifornia Environmental Protection Agency, Department ofPesticide Regulation). EH 91-03. 

Smith FF, Giang P. Taylor EA (1955) Reduction of malathionresidues by washing. J Econ Entomol 48:209 210. 

Wallis RL, Smith FF, Wheeler HG, Taylor EA (1957) Malathionresidues on vegetable, berry, and tobacco crops. J EconEntomol 50:362 363. 

Table 1. Plant weight and concentration of dislodgeable and penetrated malathion residuesin lettuce-head samples 12 and 36 hours after application. Residues are compared betweenheads rinsed with surfactant or only water.

                                       Dislodgeable Residue            Penetrated Residue
                                      Plant         Malathion               Plant             Malathion
                                      Weight       Concentration        Weight         Concentration

                                        (g)             (ug/g)                    (g)                 (ug/g) 

12 Hours After Application 

Water rinse                     195 + 20     3.49 + 0.12 a         195 + 20         4.15 + 0.13 a 

Surfactant rinse               175 + 15     3.50 + 0.17 a         175 + 15         4.22 + 0.17 a 

36 Hours After Application 

Water rinse                     170 + 25      3.35 + 0.23 a         170 + 25         5.82 + 1.07 b 

Surfactant rinse                 199 + 26     3.33 + 0.32 a        199 + 26         5.97 + 1.61 b 

Same letters within a column are not significantly different at p < 0.05.