Laboratory evaluation of chlorpyrifos, bifenthrin, imidacloprid ...

Pak. Entomol. Vol. 28, No.2, 2006
Insecticides
Following five insecticides were used in the
bioassay of termites,
1. Chlorpyrifos 40EC
2. Bifenthrin 10EC
3. imidacloprid 50SL
4. Thiamthoxam 70WSC
5. Flufenoxuron 10DC
1000 ppm, 500 ppm, 250 ppm, 125 ppm, 62.5 ppm,
31.25 ppm, 15.62 ppm and 7.81 ppm solutions of
these insecticides were prepared in order to evaluate
their effects against Microtermes obesi.
Soil sampling
Soil samples from 3 Districts (Faisalabad, Layyah
and Chakwal) were collected. The soil was sifted
through a 30-mesh screen and moisture was
determined with the help of Oven Wet and dry
method. More amount of water was added up to 50%
of water holding capacity of soil to avoid mortality
of termites due to dehydration.
Two types of bioassay were carried out.
a. Mixing insecticide concentrations in soil
Toxicity tests were done in Petri dishes of 8.75 cm
dia containing 10g sifted soil, strips of sugarcane
having dimension 1.5 x 6 cm. were placed in each
Petri plate to keep the termites alive. Experimental
units were oven dried for 24 h for solvent
evaporation. Control units were treated with acetone
only. 40 workers of termites were released in the
Petri dishes having treated and untreated soils. These
dishes were placed under controlled conditions
28 + 2 0 C and 80 + 5% r.h. in a growth chamber.
Termites were examined after every 2 hours up to 12
hours, later, and then after each 24 hours until 100%
mortality occurred.
b. Direct exposure by Micro applicator
0.1 µl (microlitre) drop of different concentrations
was applied to the thorax of the termites by using
Burkard Micro Applicator. The termites after
treatment were put into the Petri dishes containing
filter paper moistened with distilled water. Mortality
was noted after 24 hours of treatment.
Statistical Analysis
Probit analysis (Polo-Pc) was used to make
toxicological inferences.
RESULTS
Table 1 shows LT 50 values of different
concentrations of chlorpyrifos, bifenthrin,
imidacloprid and thiamethoxam on Microtermes
obesi in soils from Faisalabad, Chakwal and Layyah.
LT 50 values of different concentrations of
chlorpyrifos and bifenthrin were in minutes while
imidacloprid and thiamethoxam showed LT 50 values
in hours. These values increased with decline in
concentrations. However, small difference was
found in LT 50 values of four insecticides in three
soils. For example, 198, 222 and 227 min were
values at 7.81 ppm of chlorpyrifos in soils from
Faisalabad, Chakwal and Layyah. Bifenthrin
differed from chlorpyrifos as 222, 136 and 123 were
values in these soils. There was no difference in
LT 50 values of imidacloprid and thiamethoxam in
three soil types.
Topical application of different concentrations of
chlorpyrifos, bifenthrin, imidacloprid and
thiamethoxam did not show difference in LT 50
values recorded in case of when these were applied
in the soils. To some extent these values were less in
control treatment (Table 2).
The lethal time of flufenoxuron to kill 50%
population of Microtermes obesi was high (3.62 to
11) as Chakwal than at two other places (2.5 to 7).
However direct exposure to flufenoxuron showed
similar pattern as was observed in case of other four
insecticides (Table 3).
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