Charle Patrick Garcia and K.L. Heong
International Rice Research Institute, Los Baños, Philippines
As new insecticides are introduced to control pests, the pest populations keep adapting and develop resistance to these new insecticides. In intensive rice production systems, where insecticide use is excessive, resistance has been an important factor causing the decline of a pest management strategy. Reports of multiple fold developments in resistance and parallel resistance are found in China and Vietnam (Read: Matsumura and Sanada-Murimura (2010).
Resistance is a genetic change in response to selection by insecticides. Over 500 pest insect species have evolved resistance to at least one insecticide in the last 40 years, and the increase in numbers of resistant species is growing exponentially. Resistance management is thus an essential part of IPM. As a new management tactic is deployed, such as a new chemical or a new rice variety, it should be utilized in a manner that is designed to prevent or slow the development of resistance. The goals of resistance management are to avoid resistance, slow the rate of resistance development, and cause resistant populations to revert to more susceptible populations.
We collected BPH from Bicol, evaluated their LD50s after 3 generations, reared them insecticide free on TN 1 rice plants in the insectary for another 10 generations and measured the LD50s again. The toxicological tests were conducted by standardized methods described in Heong et al (2010). There were significant reductions in LD50s in the four insecticides tested (Table 1). Among the four insecticides, the highest reversion was found in fenobucarb, 6.10 folds (4.26-8.72), followed by fipronil, 3.77 (2.86-4.97), isoprocarb, 2.89 (2.04-4.04) and imidacloprid, 1.77 (1.35-2.32).
Table 1: LD50s of 4 insecticides on BPH from Bicol, Philippines reared on TN (1) at generations 3 and 13 after field collection.
The high and rapid reversion after just 10 generations in an insecticide-free environment indicates that when the insecticide is used sparingly or withdrawn, resistance can be reverted. In China where the use of fenobucarb had been reduced in the last few years, LD50s had remained high of around 45 mug/g or 6 to 7 times that in the Philippines. It is thus likely that if the use of an insecticide is not well managed insecticide resistance when established cannot be reverted.