417. Pirimiphos-methyl (Pesticide residues in food: 1977 evaluations)

PIRIMIPHOS-METHYL JMPR 1977

Explanation

Pirimiphos-methyl was evaluated in 1974 and 1976 (FAO/WHO 1975 ,
1977), an acceptable daily intake of 0.01 mg/kg for humans was
established and maximum residue limits in a range of commodities were
recommended. In 1976 a request was made for results from commercial
trials on other commodities. The 9th session of the Codex Committee an
Pesticide Residues considered these proposals and requested
clarification of the question of intake by humans and the distinction
between commodities destined for animal feed and for human consumption
(ALINORM 78/24 Para. 154).

Additional information concerning the level and fate of
pirimiphos-methyl residues on sorghum and in milled products from
wheat has come available and is summarized the following monograph
addendum.

No new toxicological information has become available since the 1976
Meeting.

RESIDUES IN FOOD AND THEIR EVALUATION

USE PATTERN

In 1974 the usefulness of pirimiphos-methyl as a grain protectant for
barley, oats, maize, rice, rye and wheat was evaluated and maximum
residue limits in these grains were recommended. Since then there has
been extensive experience from bulk trials and commercial use of
pirimiphos-methyl on these raw cereal varieties and also on sorghum.

The rate of application depends upon a number of considerations of
which the following are the most important: -

a) anticipated period of storage

b) temperature of grain

c) moisture content of grain (as an indicator of relative
humidity in the inter-grain space)

d) type of storage structure

e) severity of insect pest problems

f) intended use of the grain

There are many situations where cool (temperature below 15°C), dry
(moisture content less than 10%) grain is to be stored for short
periods in sound storage structures where insect pest numbers are
relatively low. Under such conditions the application of
pirimiphosmethyl at rates of the order of 4 mg/kg or less would

provide adequate protection against most, if not all, of the
stored-product pests likely to be encountered. This applies equally to
all varieties of grains though it is known that some varieties are
preferred by certain species of insects.

However, in many countries, particularly in the tropics and
semi-tropics, grain with a high temperature (above 30°C and ranging
above 40°C) and high relative humidity must be stored for long periods
(one year or more) under conditions that lead to high incidence and
high levels of insect infestation. Under such conditions many
countries sustain losses of over 20% of their harvested crop.

It is therefore apparent that some lots of barley, maize and oats may
need to be treated with pirimiphos-methyl at rates that could produce
residues higher than the maximum residue limit of 7 mg/kg recommended
in 1974.

RESIDUES RESULTING FROM SUPERVISED TRIALS

Because of the development of resistant strains and the failure of
malathion and dichlor vos to effectively protect sorghum from insect
attack, pirimiphos-methyl has been evaluated as a grain protectant for
sorghum. Bengston at al. (1977c) evaluated pirimiphosmethyl along with
a number of other organophosphorus insecticides in combination with
bio-resmethrin, d-phenothrin and pyrethrins. Pirimiphos-methyl was
applied at the nominal rate of 4 mg/kg at two separate sites. The
sorghum contained 11.8 -12.6% moisture and had a temperature of 25 -
27°C. The rate of decline of pirimiphos-methyl residues is given in
Table 1.

TABLE 1. Pirimiphos-methyl residues on sorghum held in storage

Site/Weeks 1 4 8 12 18 24

A 3.9 3.6 3.5 3.1 3.1 3.0
B 3.9 4.9* 3.5 3.4 2.4 4.9*

* samples contained appreciable quantities of grain dust

Bioassay tests indicated that at the end of 12 weeks the treated
sorghum would still withstand infestation by a variety of
stored-product pests selected to be resistant to malathion. The work
is continuing.

TABLE 2. Pirimiphos-methyl residues in stored wheat, 1976-77 Pilot studies (Australia)
nominal application rate 4 mg/kg

Residue, mg/kg
Bin
Site No. Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept.

A 8 1.8 2.4 1.8 2.1 1.6 1.5
B 1 1.6 1.1
C 8 2.4
D 8 2.4 2.4 2.2 1.6 1.8
E 4.5 6.2 5.5 5.6
F 1-3 3.1 3.3 4.3 3.8 2.5
" 2-3 3.0 2.9 4.2 3.6 2.5
" 3-3 2.9 2.4 3.6 3.3 3.3
G 93 2.2 3.4 2.4 2.6 2.1
" 98 0.8 1.2 1.0 1.2 1.0
" 99 1.0 0.9 1.1 1.2 1.0
H 2.3 1.1 1.8
I 2.6 3.2 1.9 3.4
J 3.1 2.6 2.2 2.5 3.0
K 0.9 1.2 4.2
L 1.8 1.2 1.8 1.6
M 1.8 2.1 2.1 2.9
N 3.6

Bengston at al. (1975) have reported on the performance and fate of
pirimiphos-methyl as a grain protectant against malathion-resistant
insects in stored wheat in laboratory studies and Bengston at al.
(1977a,b) have reported the results of extensive bulk trials and
commercial scale use of pirimiphos-methyl on wheat in Australia. Prom
these studies it was calculated that the half-life of
pirimiphos-methyl in grain at 25°C is of the order of 80 weeks. Table
2 gives a section of the analytical data from 18 separate storages
over a period of 9 months. In these studies the application rate was
deliberately low in order that the likelihood of failure to control
resistant species could be determined without waiting for
exceptionally long times.

Cerná and Bénes (1977) have provided results of a study carried out in
Czechoslovakia wherein 400 tons of wheat containing 12.6 -14.2%
moisture at a temperature of 8-10°C was treated with pirimiphos-methyl
at a rate equivalent to 4 mg/kg. The wheat was analysed at the time of
treatment and at intervals thereafter until discharge for milling at
the end of 286 days. The results of these analyses are given in Table
3.

TABLE 3. Pirimiphos-methyl residues after treatment of wheat
in the silo at 4 mg/kg

Days after treatment Sampling Residues, mg/kg

0 during treatment 3.70
51 top 3.11
}3.55
bottom 3.35

105 top 4.51
}3.50
bottom 2.50
160 bottom 2.27
286 (average before grinding) 1.62

In a trial in the Philippines pirimiphos-methyl was applied either as
a dust or emulsifiable solution to maize (shelled) at rates calculated
to leave 10, 15 and 30 mg/kg on the grain. Samples were analysed after
3, 6 and 9 months. The results obtained are given in Table 4
(Kagallona, 1977).

TABLE 4. Pirimiphos-methyl residues on maize (Philippines)

Formulation Rate of application Residue found after
mg/kg 3 6 9 months

E.C. 10 1.3 N.D.
15 1.5 1.5 N.D.
30 2.2 2.5 N.D.

Dust 10 0.6 0.4 N.D.
15 1.5 1.2 N.D.
30 2.8 2.5 N.D.

FATE OF RESIDUES

In stored products

Residues of pirimiphos-methyl on grains are degraded and detoxified by
hydrolysis of the phosphorus-ester side chain, the rate of hydrolysis
increasing with increases in moisture and temperature (FAO/WHO 1975,
Page 502).

Desmarchelier (1977) has shown by an extensive series of experiments
with different grains held at a series of controlled temperatures at
different relative humidities that the loss of insecticide from
post-harvest application to various grains is predictable. These
studies have shown that the loss is a second-order process with the
rate of loss being proportional, at a fixed temperature, to the amount
of insecticide and the equilibrium partial pressure of watervapor, or
in the inter-grain space.

The predictions of Desmarchelier (1977) have been plotted against
residues of pirimiphos-methyl expressed as a percentage of the
application rate using a log scale. Fig. 1 shows the rate of
degradation predicted for "typical Australian conditions" and based on
80% recovery of the calculated application against the mean of
analytical results from many separate samples from 20 separate sites.
Temperature and humidity conditions were neither uniform nor
consistent for all sites.

From this work it is clear that the rate of degradation doubles for
each 5°C increase in temperature over the range 15 -35°C. In other
words the residual life of deposits is twice as long at 25°C as at

30°C. This information will enable grain storage authorities to treat
their grain with protection whilst possibly avoiding excessive
residues. However, it is often impossible to determine in advance how
long the grain will remain in store.

In processing

Results of experiments in Czechoslovakia (Cerná and Bénes 1977)
indicate that the residues in grain which has been in store for 9
months are substantially removed by the milling process. Table 5 shows
both the residue level and total weight of pirimiphos-methyl in the
various mill fractions. The bulk of the residue was removed in the
bran, there being no substantial difference in the concentration in
the different bran fractions. When the white flour was made into white
bread there was a further loss of approximately 50% so that the
concentration of the residue in the bread was only 10-15% of its
concentration in the raw grain. Only 10-18% of the total weight of
residue on the raw grain found its way into the white flour. Some
residue was lost (destroyed?) during milling.

In bran

The Codex Committee on Pesticide Residues at its 9th Session asked for
reconsideration of the recommended maximum residue limit in bran, in
view of the use of bran in human diets (ALINORM 78/24 para. 154).

With the exception of the study by Cerná and Bénes (1977) there are no
data on the level and fate of residues in diet bran additional to that
reported in the 1974 monograph. Most workers, including those whose
work was reviewed in 1974 and 1976, have concentrated on studying the
transfer of residues from grain to bread, biscuit, pasta and similar
cereal foods or wholemeal products. The small dietetic use of bran has
not apparently been studied separately.

In the 1974 monograph it was reported that although bran represents
only about 25% of the mass of the original grain the concentration of
residues in the bran was of the order of 2.5 times the concentration
in the whole wheat. In the studies reported by Cerná and Bénes the
concentration in bran was 2.8 times that in the whole wheat from which
it was milled. Since the rate of application of pirimiphos-methyl
ranges from 4 to 6 mg/kg, with higher rates only where high
temperature and moisture are likely to contribute to rapid
degradation, it can be assumed that in commercial practice with
typical storage periods, crude bran of the type used for dietetic
purposes will contain about 5 mg/kg or less of pirimiphos-methyl.
Wheat from several sources is usually blended in order to obtain the
qualities desired in the flour and baked goods. Such blending will
usually lead to a dilution of the residues, especially those
segregated in bran.

TABLE 5. Pirimiphos-methyl residues in wheat, milling fractions and white bread

A B
pirimiphos-methyl pirimiphos-methyl

Weight of mg/kg Total weight Weight of mg/kg Total weight
Fraction fraction, in fraction, fraction, in fraction,
g mg g mg

wheat before grinding 1000.0 1.62 1.62 1000.0 1.94 1.94
crude bran 172.4 4.64 0.80 207.6 4.84 1.00
fine bran 61.2 4.90 0.30 77.4 3.01 0.23
pollard flour 215.7 0.32 0.07 215.0 0.45 0.10
white flour 550.7 0.525 0.29 500.0 0.371 0.19
white bread 0.27 0.19
total after grinding 1000 1.46 1000 1.52
decrease by grinding 0.16 0.42
(9.9%) (21.7%)

A: wheat treated in the silo (dose 4 mg/kg) 286 days earlier
B: wheat treated in the laboratory (dose 4 mg/kg)

METHODS OF RESIDUE ANALYSIS

Desmarchelier at al. (1977) as the result of a collaborative study of
the analysis of residues of a number of grain protectant insecticides,
have pointed to the importance of the extraction solvent and procedure
in obtaining consistently high recoveries of residues from grain and
cereal products. They found a simple shaking of unmilled grain with
methanol at room temperature to be the most convenient and consistent
procedure. The extract can be used for direct injection into the gas
chromatograph without further clean-up.

NATIONAL MAXIMUM RESIDUE LIMITS REPORTED TO THE MEETING

The following national maximum residue limits have been reported to
the Meeting.

Country Commodity MRL mg/kg

Australia Bran 20
Wheat, rye, rice
(in husk) 10
Barley, maize, oats 7
Wholemeal flour 5
Rice (dehusked),
Wheat flour (white) 2
Bread (wholemeal),
rice (polished) 1
Bread (white) 0.5
Meat, milk and eggs 0.05*

* at or about limit of determination

APPRAISAL

In 1976 a request was made for results from commercial trials with
pirimiphos-methyl in further commodities and the Codex Committee on
Pesticide Residues at its 9th (1977) Session sought clarification of
the intake by humans, the high level of residues in bran and the
distinction between commodities destined for animal feed and human
food.

Additional information concerning the level and fate of
pirimiphos-methyl residues on Sorghum, maize and in milled products
from wheat has become available.

The rate of application of pirimiphos-methyl post-harvest for grain
protection depends upon the degree of protection desired, the length
of storage, the temperature and humidity of the grain and the
anticipated period of storage. At 25°C and low relative humidity the
half-life of pirimiphos-methyl on grain is of the order of 80 weeks.
Under hot humid conditions, the half-life is comparatively short and

therefore higher initial deposits are required to achieve acceptable
protection against insect damage.

A model has been developed whereby it is possible, given the
temperature, relative humidity and proposed application rate, to
predict the storage life of the deposit and to estimate the residue
level at any desired date. Residue data from extensive pilot and
commercial trails have been compared with the model and there has been
remarkably good agreement.

The fate if residues of pirimiphos-methyl on grain subjected to
milling has been revealed in several studies. These show that the bulk
of the residue is removed in the bran and that only about 10% of the
amount present on the grain finds its way into bread. It is recognized
that small amounts of bran are used for dietetic purposes but it is
considered that the amount contributed to the intake from this source
is acceptable because only occasional batches of grain will contain
residues approaching the recommended maximum residue limit.

Attention is drawn to the importance of the extracting solvent and
extraction technique on the recovery of residues from raw grains,
milling products and cooked cereals.

RECOMMENDATIONS

The existing maximum residue limits for barley, maize, oats, rice (in
husk), rye and wheat are replaced by a limit for raw cereals.

Commodity Limit, mg/kg
Raw cereals (except rice, hulled or polished) 10

FURTHER WORK OR INFORMATION

DESIRABLE

1. Results of studies now in progress on the residues in peanuts and
peanut products.
2. Results trim commercial trials on other commodities.
3. Further information on the level and fate of residues in food at
the point of consumption following the use of pirimiphos-methyl
for the control of various stored product pests.

REFERENCES

Bengston, Cooper, L.M., and Grant-Taylor, F.J. (1975) A comparison of
bioresmethrin, chlorpyrifos-methyl and pirimiphos-methyl as grain
protectants against malathion resistant insects. Queensland J. agric.
anim. Sci. 32, 51-78.

Bengston, Connell, M., Crook, I.D., Desmarchelier, J.M., Hart, R.J.,
Phillips, M., Snelson, J.T. and Sticka, R. (1977a) Field trials to
compare chlorpyrifos-methyl, fenithrothion, pirimiphos-methyl

malathion and methacrifos for the control of malathion-resistant
insects infesting wheat in Australia. J. Stored Prod. Res. (In press).

Bengston, M., Connell, M., Davies, R., Desmarchelier, J., Elder, B.T.,
Hart, R., Phillips, M., Ridley, E., Ripp, E., Snelson, J. and Sticka,
R. (1977b) Chlorpyrifos-methyl plus bioresmethrin, methacrifos,
pirimiphos-methyl plus bioresmethrin and synergised bioresmethrin as
grain protectants for wheat (In press).

Bengston, M., Cooper, L.M., Davies, R.A.H., Desmarchelier, J.M., Hart,
R/J. and Phillips, M. (1977c) Grain protectants for the control of
malathion resistant insects in sorghum. J. Stored Prod. Res. (in
press).

Cerná, V. and Bénes, V. (1977) Residues of pirimiphos-methyl in wheat
mill products and white bread. Report by Czechoslovakian Institute of
Hygiene and Epidemiology. Prague - June 30, 1977.

Desmarchelier, J.M. (1977) (CSIRO, Division of Entomology, Canberra,
Australia.) Loss of insecticide deposits on grains in storage. (In
preparation)

Desmarchelier, J., Bengston, M., Connell, M., Minett, W., Moore, B.,
Phillips, M., Snelson, J., Stiaka, R., and Tucker, K. (1977a) A
collaborative study of residue of CGA 20168, chlorpyrifos-methyl,
fenitrothion, malathion and pirimiphos-methyl.
(1977b) II. Rates of decay - Pestic. Sec. (in press).

Magallona, E. (1977) Fate of pirimiphos-methyl applied to stored corn.
Report to FAO from Pesticide Residue Laboratory University of the
Philippines.

FAO/WHO (1975) 1974 evaluations of some pesticide residues in food.
AGP:1974/14/11; WHO Pesticide Residues Series, No. 4.

FAO/WHO (1977) 1976 evaluations of some pesticide residues in food.
AGP:1976/M/14.

    See Also:
       Toxicological Abbreviations
       Pirimiphos-methyl (WHO Pesticide Residues Series 4)
       Pirimiphos-methyl (Pesticide residues in food: 1976 evaluations)
       Pirimiphos-methyl (Pesticide residues in food: 1979 evaluations)
       Pirimiphos-methyl (Pesticide residues in food: 1983 evaluations)
       Pirimiphos-methyl (Pesticide residues in food: 1992 evaluations Part II Toxicology)