Revista Brasileira de
Engenharia Agrícola e Ambiental
v.13, n.6, p.729–733, 2009
Campina Grande, PB, UAEA/UFCG – http://www.agriambi.com.br
Protocolo 001.07 – 18/01/2007 • Aprovado em 08/04/2009
Biological activity and persistence of pirimiphos-methyl
applied to maize grain at different temperatures
Rodrigo D. Silveira1, Lêda R. A. Faroni2, Raul N. C. Guedes3, Maria E. L. R. Queiroz4 & Marco A. G. Pimentel3
ABSTRACT
The expansion of dryeration may impose a further problem for insect control with protectants – the high grain tempera-
tures during insecticide spraying. To assess the impact of this procedure on insecticide activity, maize grains at different
temperatures (25, 30, 35, 40 and 45 °C) were sprayed with pirimiphos-methyl. Residue analyses were carried out every
30 days and insecticide biological activity towards Sitophilus zeamais and Tribolium castaneum was assessed every 15
days throughout the experimental period of 90 days. Insect mortality was evaluated after 48 h. Pirimiphos-methyl resi-
due decreased with increased storage time and grain temperature during spraying. Similar trends were also observed for
mortality of S. zeamais and T. castaneum, which dropped from around 100% for lower grain temperatures, shortly after
spraying, to mortality values around 0% for higher temperatures and after 90 days of storage. These results indicate the
drastic effect of grain temperatures during spraying, which compromises the efficiency of grain protectants for insect
pest control on stored grains.
Key words: Sitophilus zeamais, Tribolium castaneum, organophosphate, residue, insect control
Eficácia biológica e persistência do pirimifós-metil aplicados
sobre grãos de milho em diferentes temperaturas
RESUMO
A técnica de seca-aeração pode causar problema no controle de insetos devido à alta temperatura do grão durante o
processo de pulverização do inseticida. Com este trabalho objetivou-se avaliar o efeito imediato e latente da temperatura
do grão durante o processo de pulverização sobre a persistência e eficácia biológica do inseticida pirimifós-metil no
controle de Sitophilus zeamais e Tribolium castaneum. Para tal, pulverizou-se o inseticida pirimifós-metil sobre os grãos
de milho quando se apresentavam nas temperaturas de 25, 30, 35, 40 e 45 °C. A análise de resíduo foi realizada após
a pulverização e a cada 30 dias, até 90 dias. Para avaliação da eficácia biológica 20 insetos adultos de S. zeamais e de
T. castaneum foram colocados em uma placa de Petri contendo grãos tratados e, após 48 h de exposição dos insetos aos
grãos, logo depois da pulverização e a cada 15 dias, até 90 dias, foram realizadas as avaliações. Observou-se que a
eficácia biológica do pirimifós-metil reduziu durante o período de armazenamento e que o aumento da temperatura do
grão no momento da pulverização também contribui para a redução da eficácia deste inseticida. Observou-se, também,
que S. zeamais apresentou menor sensibilidade ao pirimifós-metil que T. castaneum.
Palavras-chave: Sitophilus zeamais, Tribolium castaneum, organofosforado, resíduo, controle de inseto
1 UFVJM, CEP 39100-000, Diamantina, MG. Fone: (38) 3532-1226. E-mail: rodrigo-ufvjm@hotmail.com
2 DEA/UFV, CEP 36571-000, Viçosa, MG. Fone: (31) 3899-1874. E-mail: lfaroni@ufv.br
3 DBA/UFV, Fone: (31) 3899-4008. E-mail: guedes@ufv.br, marcoagp@gmail.com
4 DEQ/UFV, Fone: (31) 3899-1430. E-mail: meliana@ufv.br
730 Rodrigo D. Silveira et al.
R. Bras. Eng. Agríc. Ambiental, v.13, n.6, p.729–733, 2009.
INTRODUCTION
Several factors may contribute to the degradation of in-
secticides used for controlling insect pests in stored grains.
Among them, air and grain temperature and moisture con-
tent are better studied and of major importance (Arthur et
al., 1992; Wintersteen & Foster, 1992; White & Leesch,
1996; Fleurat-Lessard et al., 1998; Hamacher et al., 2002;
Pimentel et al., 2004). High grain temperatures, for instance,
cause fast breakdown of many insecticides, mainly by stim-
ulating grain hydrolytic enzymes (Rowlands, 1975; Orth &
Minett, 1975).
Dryeration techniques are increasingly used in Brazil and
may impose a further problem for insect control with pro-
tectants in the tropics – the high grain temperatures during
insecticide spraying. These techniques aim at energy savings,
high drying capacity and lower thermal damage by long grain
exposure to high temperatures (Silveira, 2002). If using dry-
eration, the warm grain is removed from the drier with up
to 2.5% higher moisture content than recommended for stor-
age. The grain is then transferred to a bin where it remains
from 4 to 10 h to allow moisture distribution within the grain
by residual heat. This process facilitates the removal of ex-
cess moisture during about 12 h of aeration, which will take
place afterwards until the grain reaches the desired moisture
content. However, the grain usually remains in the same bin
after the resting period for the aeration, and the insecticide
spraying takes place while conveying the grain from the drier
to the bin, when grain temperature reaches over 45 °C
(Mourier & Poulsen, 2000).
The organophosphate pirimiphos-methyl, one of the main
grain protectants used against stored product insects in Bra-
zil, is frequently used in these operations (Andrei, 1999) and
the procedure described above (spraying at high grain tem-
peratures) is a potential problem for stored product protec-
tion because organophosphate degradation is usually faster
at high temperatures (Arthur et al., 1992; White & Leesch,
1996; Daglish, 1998; Fleurat-Lessard et al., 1998; Faroni et
al., 2002; Hamacher et al., 2002; Pimentel et al., 2004) com-
promising even further the already difficult control of stored
product insects in tropical areas. Therefore, the objective of
the present study was to assess the impact of high grain tem-
peratures during spraying on pirimiphos-methyl persistence
and activity towards two important pest species of stored
maize in the tropics – the maize weevil and the red flour
beetle.
MATERIAL AND METHODS
The investigation was carried out in the Viçosa County,
State of Minas Gerais, from December 2000 to January 2002,
where maize grains of the variety AG-1051 harvested with
20% m.c. were dried to 15% m.c. in batches of 20 kg using
an experimental drier with 1 m s-1 of drying air speed. Dried
grains (15% m.c.) at different temperatures (25, 30, 35, 40
and 45 °C) were placed in a conveyor belt 1.0 m long and
0.2 m wide (Faroni et al., 2002; Hamacher et al., 2002). The
belt capacity was 8.25 t ha-1 activated by an induction tripha-
sic 0.5 hp motor coupled with a 0.5 hp motoreductor (with
24 x reduction). A container with flow regulation was cou-
pled to the conveyor belt to guarantee uniform grain flow in
a thin layer throughout its length. The sprayer used for in-
secticide spraying at the conveyor belt was equipped with a
single flat fan nozzle (Teejet TP650067) at 0.15 m high and
regulated to deliver an application rate of 11.5 mL min-1 at
2 bar pressure.
The organophosphate insecticide pirimiphos-methyl (Ac-
tellic® 500EC) was applied at the recommended concentra-
tion of 4.0 mg a.i. mL-1 and 1.5 mL of insecticide solution
was used per kg of maize (= 1.5 L per tonne of maize). The
grain batches were left to rest for 6 h after spraying and then
they were aerated for 12 h until reaching 13% m.c. Four
batches of maize grains were sprayed at each temperature
and 2.0 kg samples were taken from each batch and stored
at 27 ± 1 °C and 56 ± 5% r.h. until the residue analysis and
bioassay tests. Grain samples sprayed with water only were
also used for the residue analysis and bioassay tests. The tests
were carried out at the same environmental conditions of
storage.
Insecticide residues on grain samples sprayed at the dif-
ferent temperatures were analyzed 0, 30, 60 and 90 days
after spraying using three replicates. Non-sprayed grain
samples were also analyzed for pirimiphos-methyl residues
at the same storage intervals. Only analytical standards of
the solvents were used for the residue analysis. Technical
grade pirimiphos-methyl (91.5%) was obtained from Zen-
eca Brasil (Holambra, SP, Brazil). The extraction method-
ology was adapted from Hamacher et al. (2002) and Luke
et al. (1975). The efficiency of this extraction technique was
assessed in three maize samples fortified with 1.0 mL stan-
dard solution of pirimiphos-methyl at 100.0 mg mL-1, al-
lowing 86.1% recovery of the active ingredient. Ten grains
of maize from each sample where mixed with acetone
(25 mL), hexane (10 mL) and dichlorometane (15 mL) in
an Erhlenmeyer flask and shaken for 30 min. The extract
was filtered through filter paper with 20 g of sodium sul-
fate and 15 mL dichlorometane was used to wash the pa-
per at the end of the filtration. The extract was concentrat-
ed in a rotary evaporator at 40 °C and recovered to 4.0 mL
with hexane. Sample extracts were analyzed in a gas chro-
matograph (Shymadzu CG-17A, Kyoto, Japan) equipped
with a BP-5 column (poly – 5% diphenyl/95% dimethylsi-
loxane, 30 m X 0.25 mm i.d. and film thickness of 1 mm)
and a flame ionization detector (FID) (Faroni et al., 2002;
Hamacher et al., 2002). Temperature of the column was
220 °C (7 min; isothermic), while the injector was main-
tained at 250 °C and the FID detector was maintained at
300 °C. The flow of the carrier gas (N) was 1.2 mL min-1,
the split ratio was 1:5 and the injected volume was 1 mL.
Under these conditions, the pirimiphos-methyl retention
time was about 4 min. The setup values for the hydrogen,
air and nitrogen make-up gas for the FID were 60 kPa (ap-
prox. 50 mL min-1), 50 kPa (approx. 500 mL min-1) and
75 kPa (approx. 30 mL min-1), respectively. The residues
were quantified directly from the calibration curve
731
established within the detector linearity range, by compar-
ing the sample peak areas with the peak areas of the exter-
nal standards. The detector showed reasonable linearity
within the work range from 0.2 to 10.0 mg mL-1, with
R2 = 0.994, detection threshold of 0.2 mg mL-1 and quan-
tification threshold of 0.6 mg mL-1.
The biological activity of pirimiphos-methyl sprayed at the
five different temperatures was independently evaluated us-
ing non-sexed adults of maize weevil, Sitophilus zeamais
Motschulsky (Coleoptera: Curculionidae), and of red flour
beetle, Tribolium castaneum (Herst) (Coleoptera: Tenebrion-
idae). These insects were from laboratory colonies maintained
at 27 °C and 55% r.h. on non-treated maize. Four 50 g sam-
ples of treated maize grains were taken from each treatment
every 15 days up to 90 days after the insecticide spraying.
These grain samples were placed in Petri dishes and infest-
ed with 20 insects of either species. They were maintained
at 27 °C and 55% r.h. for 48 h, after which the insect mor-
tality was assessed. Mortality was corrected for the natural
mortality on samples sprayed with water only.
Multiple regression analysis (SAS Institute, 1997) were
used to determine whether the different grain temperatures
at spraying and storage periods affected residue levels of
pirimiphos-methyl on maize grains and the biological ac-
tivity of this insecticide towards the maize weevil and the
red flour beetle. Non-linear regression analyses were used
to establish the relationship between residue levels and bi-
ological activity of pirimiphos-methyl. These analyses were
carried out using the curve fitting procedure of SigmaPlot
(SPSS INC, 2000).
RESULTS AND DISCUSSION
The residue level of pirimiphos-methyl on the grains
shortly after spraying (2 h) were significantly smaller when
this operation was carried out at higher grain temperatures
with a nearly 80% difference between the extreme tempera-
tures (2.5 ± 0.0 ppm at 45 °C and 4.3 ± 0.4 ppm at 25 °C)
(Figure 1). The differences remained, with time getting even
greater at the end of the storage period, 90 days after spray-
ing (0.2 ± 0.0 ppm at 45 °C and 0.5 ± 0.1 ppm at 25 °C). The
decrease in residue level was inversely related to grain tem-
peratures and storage time. The effect of the air temperature
favoring degradation of organophosphate insecticide residues
on stored grains is widely recognized (Arthur et al., 1992;
White & Leesch, 1996; Fleurat-Lessard et al., 1998; Afridi
et al., 2001).
High air temperatures, even if only during spraying, also
showed a drastic effect on reducing pirimiphos-methyl res-
idues on the grain surface and compromising insect con-
trol as demonstrated by Faroni et al. (2002) and Pimentel
et al. (2004). In contrast, the effect of grain temperature in
insecticide degradation was seldom explored. Wintersteen
& Foster (1992) did not observe any significant effect of
grain dried under different systems in malathion degrada-
tion, unlike what was reported for pirimiphos-methyl. Such
differences may be due to structural differences between
both compounds, but the high storage temperature used in
the present study (27 °C) was a likely contributor to max-
imize insecticide degradation. The effect of the grain tem-
perature during insecticide spraying on the degradation and
activity of pirimiphos-methyl, demonstrated in our study,
was expected based on previous studies assessing the ef-
fect of temperature speeding up insecticide degradation
(Arthur et al., 1992; Daglish, 1998; Fleurat-Lessard et al.,
1998; Afridi et al., 2001; Hamacher et al., 2002; Pimentel
et al., 2004). However, residue levels of 50 to 80% higher
at 25 than at 45 oC immediately after spraying, with 2- to
4-fold drop during the 90-day storage period, were a sur-
prise. The high grain temperatures at spraying may have
led to insecticide losses through its suspension in the warm
air surrounding the warm grain. This may have prevented
the insecticide from reaching the grain surface and proba-
bly favored degradation on the grain surface as well. Evap-
oration of a volatile carrier (water in our case) can result
in a decrease in droplet size to the point at which small
droplets solidify, remaining suspended in the air and fail-
ing to adhere to the grain surface (Johnstone, 1985).
The decrease in the residue levels of pirimiphos-methyl
in stored maize was also inversely related to mortality of the
maize weevil and the red flour beetle (Figure 2). Insect mor-
tality dropped quickly below 2 ppm, going from nearly 100%
mortality to 0% for both insect species. Higher residue lev-
els had negligible impact on insect mortality, which was al-
ready close to 100%. High temperatures generally result in
fast insecticide degradation, which seems particularly true
in the case of organophosphates (Arthur et al., 1992; White
& Leesch, 1996; Daglish, 1998; Fleurat-Lessard et al., 1998).
The lower persistence of pirimiphos-methyl residues on maize
grains was translated into lower efficiency in controlling in-
festations of the maize weevil and red flour beetle. These re-
lationships were non-linear, showing a great decrease in
Figure 1. Effect of grain temperature, during spraying, and storage period
(days) on the res idue level of pirimiphos-methyl on maize grains.
(y = 7.81 – 0.08x – 0.13z + 0.001xz; where y = residue level (ppm),
x = storage period (days), and z = grain temperature during spraying
(°C); R2 = 0.67; F = 24.26; p < 0.0001; dferror = 31)
R. Bras. Eng. Agríc. Ambiental, v.13, n.6, p.729–733, 2009.
Biological activity and persistence of pirimiphos-methyl appliod to maize grain at different temperatures
732
mortality with a decrease in residue levels from 2 to 1 ppm.
Arthur et al. (1992) and Hamacher et al. (2002) also observed
an increase in insect survival with a decrease in organophos-
phate residue levels on stored grains. The rate of reduction
of residue levels indicates the length of protection provided
by the insecticide. In all the instances investigated, with in-
secticide applications taking place with warm grains (from
25 to 45 °C) in a tropical area, the grain protection period
will be greatly compromised.
Insect mortality, as expected, decreased with an increase
in grain temperature during spraying and period of storage
(Figures 3A and 3B). Mortality was highest right after in-
secticide application for the highest temperature under in-
vestigation (i.e., 45 °C). The decrease in mortality was uni-
form with storage time and reached the lowest mortality
levels faster when grains were sprayed at the highest tem-
peratures for both insect species. There was no significant
interaction between grain temperature at spraying and stor-
age time for either insect species. A fast decrease was ob-
served in residue levels and biological activity of pirimiphos-
methyl during a 90-day storage period that was accentuated
by the high grain temperatures during insecticide spraying.
This finding is of importance because organophosphate
spraying in tropical areas is frequently carried out on warm
grain subjected to dryeration, which favors organophosphate
degradation. A solution to minimize this problem is to avoid
using the same bin for resting (allowing moisture distribu-
tion in the grain) and aeration (completing the grain dry-
ing). The use of two bins, one for resting the grain and an-
other for aerating it, will allow spraying of the grain after
its cooling at the resting bin while transporting it by the
conveyor belt to the aerating bin. Such procedure will favor
the residual effect of the insecticide providing suitable pro-
tection against insect-pests for longer storage periods.
CONCLUSION
These results clearly indicate the drastic effect of grain
temperatures during spraying compromising the efficiency
of grain protectants for insect pest control on stored grains.
ACKNOWLEDGEMENTS
To Dr. M.T. Martins (Department of Agricultural Engi-
neering) from the Federal University of Viçosa, for his tech-
nical assistance; to Dr. D. Pedroni (Zeneca Brasil, Holam-
bra, SP, Brazil) for providing the technical grade insecticide;
Figure 2. Relationship between residue levels of pirimiphos-methyl on
stored maize and mortality of Sitophilus zeamais (open triangule; dotted
line) and Tribolium castaneum (open circle; solid line). (S. zeamais:
y = 105.54/(1 + (x/1.71) -6 .95, R2 = 0.80, F = 32.35, p < 0.0001,
dferror = 14; T. castaneum: y = 103.68/(1 + (x/1.25)
-4 .16, R2 = 0.72,
F = 21.58, p < 0.0001, dferror = 14)
A.
B.
Figure 3. Effect of grain temperature during spraying with pirimiphos-methyl,
and storage period (days) on the mortaltity of Sitophilus zeamais  (A)
(y = 141.65 - 1.15x - 0.98z, R2 = 0.87, F = 116.96, p < 0.0001,
dferror = 32) and Tribolium castaneum (B) (y = 188.32 - 1.29x - 3.95z +
0.05z2, R2 = 0.84, F = 62.91, p < 0.0001, dferror = 31)
R. Bras. Eng. Agríc. Ambiental, v.13, n.6, p.729–733, 2009.
Rodrigo D. Silveira et al.
733
and to CAPES, FAPEMIG and CNPq for providing us with
financial support for the present investigation.
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