International Rice Research Institute, Los Baños, Laguna, Philippines
The brown planthopper (BPH) is a monophagous pest, dependent on rice cultures. Earlier studies showed that the toxicities of 5 insecticides to be variable in different parts of the Philippines. In general, populations from Bicol and Davao were more tolerant. However the LD50s were obtained from insects collected over a 12 month period and since significant reversion can occur in just 10 generations, such comparisons may not reflect current field situations. For us to have better comparisons, we collected insects from various locations in Luzon Island within a period of a week, reared them in the insectary for 1 generation and used them for LD50 evaluation. At each site at least 200 females were collected, placed in mylar cages, labeled with the name of the town and the geographic position (GPS) and brought back to the laboratory. For the LD50 determinations 7-8 concentrations of the insecticide active ingredient were topically applied to 3 replications, each with 20 one-day old females. Details of the procedure are available in Heong et al 2011. Three insecticides representing 3 different modes of action for BPH control were evaluated.
The probit lines for 12 sites were parallel and thus permit direct comparisons. The LD50 values varied from 0.014 mcg/g in Camarines Sur to 0.092 mcg/g in Sorsogon or 6.6 folds in the same region of southern Luzon. Low toxicities were distributed throughout the island with high LD50s only in a few sites. When compared to previous measures done in 2009, toxicities seem to be considerably lower as it was about 0.245 mcg/g then. It is not clear if the lower LD50s in this study is due to active reversion because the insecticide is less used. BPH populations in the Philippines appear to be remain largely susceptible to imidacloprid when compared to those in China (6.8 mcg/g) and Vietnam (3.1 mcg/g).
The probit lines from all sites except for Banaue were parallel. LD50s varied from 0.019 mcg/g in Laguna to 0.353 mcg/g in Albay or 18.6 folds. The most susceptible BPH was from Laguna with LD50 much lower than BPH populations in Pila collected in 2010. Since BPH populations can undergo rapid reversions, this may suggest that fipronil use might have declined in the area. In Albay the high resistance might be due to increase in use of fipronil.
In the analysis of toxicities to fenobucarb, the two populations from Sorsogon were not parallel with the others, although the most resistant population (LD50 = 5.14 mcg/g) was obtained. In the remaining sites, the LD50s ranged from 0.31 to 3.471 mcg/g representing a 11 fold difference. The most susceptible population was in Cagayan while the most resistant population was in Cavite. Since reversion in resistance to this insecticide is high, the variations in susceptibility to fenobucarb probably represent variations in application frequencies of the compound. When compared to previous LD50s obtained for nearby Pila in 2010 (1.7 mcg/g), there seems to be slight increase in Cavite. When compared with BPH populations in China, Philippines BPH remain at least 10 times more susceptible.
The test insects were collected within a week and the tests were done in the 1st generation. This might have eliminated the effects of populations collected over a period of months and the variation in LD50s might represent the intensity of use of the insecticide in the area. The lower LD50s probably indicate low use, at least in the last year or so. The higher LD50s were from Sorsogon and Albay area, which probably imply high use of the 3 insecticides there.
Heong, K.L., Tan, K.H., Garcia, C.P.F, Fabellar, L.T. and Lu, Z. 2011. Research methods in Toxicology and Insecticide Resistance Monitoring of Rice Planthoppers. International Rice Research Institute, Los Baños, Philippines.