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
From : Department of Pesticide Regulation - J. Leyva, APRS
Subject: REMOVAL OF MALATHION RESIDUES FROM LETTUCE FOR MEDFLY
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
MATERIALS AND METHODS
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).
RESULTS AND DISCUSSION
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
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.
12 Hours After Application
195 + 20 3.49 +
0.12 a 195 +
20 4.15 +
175 + 15 3.50 +
0.17 a 175 +
15 4.22 +
36 Hours After Application
170 + 25 3.35 +
0.23 a 170 +
25 5.82 +
199 + 26 3.33 +
0.32 a 199 +
26 5.97 +
Same letters within a column are not significantly different at p <