Patrick Garcia and K.L. Heong, International Rice Research Institute, Los Baños, Philippines
Fenobucarb or BPMC has been used in rice fields in the Philippines for several decades for brown planthopper (BPH) management. In some areas the BPH had acquired resistance of more than 5 folds that of the most susceptible area. Insect populations acquire insecticide resistance when the insecticide pressure is high. For instance in China where insecticide use if extremely high, BPH from Bicol and reared in insecticide free on TN 1 rice plants in the insectary for 10 generations were found to have significant reductions in LD50s or reversion of about 6.10 folds (4.26-8.72). To determine the stability of resistance, we measured the LD50s periodically of insects reared for 30 generations. Stability is an estimate of how well resistance persists in a pest population once the pesticide is no longer used. The rate of reversion to a susceptible state varies enormously. But when the use of insecticides with similar modes of action cease, selective pressure for resistance is removed, and over time resistance will often be reduced. BPMC, like other carbamates and organophosphates act by inactivating the enzyme acetylcholinesterase in the nerve synapse and give rise to immediate hyperactivity and paralysis and eventually death.
We collected BPH from 4 locations in the Philippines and reared them through 30 generations. LD50s were obtained from the population at 2-5, 14, 25 and 30 generations. The probit lines of 3 locations, IRRI, Bicol and Davao were parallel and thus could be compared. Data from Isabela were highly variable. We found that LD50s of the Davao population from declined significantly from 28.54 microgram/gram to 6.41 microgram/gram from the 2-5th generation to 14th generation (about 4.45 folds) and remained stable at between 6.41 to 5.90 microgram/gram. Similarly the Bicol population dropped from 7.63 to 2.61 (about 3 folds) and remained stable at between 2.62 – 2.12 microgram/gram. For the IRRI populations LD50 values were fluctuating but had remained between 5.00 and 6.90 microgram/gram.
Resistance stability of BPMC in the Philippines for BPH probably lies between 2.12 and 6.90 microgram/gram. It is much lower than resistance levels in China (44.79 microgram/gram) and Vietnam (30.44 microgram/gram). BPMC has also been used in China and Vietnam for several decades and had lately been replaced by other compounds. The rapid rate of reversion probably indicates that resistance in the Philippines was due to first stage biochemical factors likely esterases rather than stage 2, target site mutation.