and Integrated Pest Management - part - usaid

1'ESTICIDE RISISTANCE PROBLEMS IN TIIAILAND 283
monogenic (or monofactorial resistance) or polygenic (or polyfactorial
resistance).
RESEARCI NEEDED
Detection of the Resistant Strain
Failures in field application of pesticides may often be due to reasons other
than the development of resistance. And conversely, although one strain of a
pest may have developed resistance, that does not necessarily mean that field
failures will occur. Ingeneral, the only way to confirm resistance is to compare
the sensitivity of a sample to a control. The most common method for detecting
resistant strains is to use increasing doses of pesticides to determine the
relationship between (lose of toxicant and mortality. Regression analysis will
then give the LD 50 value from the derived equation. The level of resistance may
be quantified as a Resistance Index, calculated as
L.D5 0 of resistant strain
LD 50 of susceptible strain
In some cases resistance may be dletermined using bioassays or biochemical
methods, assuming the mechanism of resistance is well accepted. The enzymes
involved may be hydrolases, carboxylesterases or Gusathione-S-transferase
(Miyata ct al. 1985, Oppenoorth & Welling 1976). An electrophoretic technique
is often used in the rapid detection of resistance in the brown planthopper in
Japan.
Identification of the Level of Resistance
After the level of resistance has been identified as discussed above, then it
would be feasible to proceed with mapping the level of resistance among the
colonies collected from different locations. This is the most important area for
which research is urgently needed.
Unfortunately, there are few Thai toxicologists or entomologists who are
working on this aspect of the pesticide resistance phenomenon. There is a lot to
challenge Thai researchers, in addition to the research needed on the mechanisms
of resistance and the development of cross resistance in the key pests in the
country.
CONCLUDING REMARKS
For serious pests of crucifcrous crop and cotton in Thailand such as the
DBM lleliothisand Spodoptera exigua, research to strengthen our knowledge of
pesticide resistance must be emphasized. It is impractical to consider a regular
alternation with new insecticides as the solution to the present situation. As a
matter of fact, it is probably difficult to introduce alternative insecticides with no
known cross resistance when insects already show resistance to certain
insecticides. Miyata et al. (1985) discussed the use of synergists to delay the rate
of resistance development but, so far, an effective synergist to overcome