by
Wantana Sriratanasak and Sukanya Tepandung, Rice Department, Thailand
Patrick Garcia and Lilia Fabellar, International Rice Research Institute, Los Baños, Philippines
Brown planthoppers. Photo credit: Jirapong Jairin, Rice Department
BPMC or fenobucarb, a carbamate insecticide, has been used for rice planthopper control in Asia for the last 30 years. It is still in use nowadays so its toxicity to BPH was evaluated. The brown planthopper (BPH) populations were collected from the provinces of Chainat, Ang Thong and Nakhon Ratchasima and reared in the laboratory. After 3 generations, the toxicity of BPMC to BPH was evaluated using the standardized procedure and compared with BPH populations in the Philippines and China (Table 1). The data were analyzed using PoloPlus program and the relative potency ratios computed.
Table1. LD50 values in nanograms (ng )* and related statistics
| Locations | LD50(ng/g) | Fiducial limit(95%) | Slope (+se) | Heteroge-neity | Relative Potency Ratios |
| Chai Nat | 634.11 | 369.97 – 935.84 | 1.653 (0.161) | 1.62 | 1.0 |
| Ang Thong | 1418.51 | 1099.24 – 1744.20 | 2.061 (0.230) | 0.55 | 2.2 |
| Nakhon Ratchasima | 1904.74 | 1545.34 – 2271.65 | 2.488 (0.291) | 0.15 | 3.0 |
| San Pablo | 847.16 | 627.46 – 1058.06 | 2.244 (0.295) | 0.76 | |
| Jinhua | 44791.92 | 24248.54 -61856.09 | 2.460 (0.338) | 2.13 |
* 1 nanogram = 1.0 x 10 -4 microgram = 1.0×10 -6 milligrams
The analysis showed that the LD50s of the three BPH populations were significantly different (chi-square 79; p<0.05). The Nakhon Ratchasima and Ang Thong populations had similar slopes (chi- square 3.06 p<0.05) while the slope for Chai Nat population was significantly lower. The LD50s for Ang Thong and Nakhon Ratchasima were 2.2 and 3.0 times higher, respectively, than that of Chai Nat indicating that the populations in Ang Thong and Nakhon Ratchasima were more tolerant or resistant to BPMC, however, the populations with different slopes may not be comparable. The Ang Thong and Nakhon Ratchasima were more comparable with the latter population more tolerant or resistant by 1.35 times.
Topical application technique used to determine LD50
The LD50s obtained were also compared with BPH populations from San Pablo, Philippines and that from Jin Hua, China with those from Nakhon Ratchasima and Ang Thong in the same batch analysis. The LD50s were significantly different but the slopes were not and thus the populations might be comparable. We used the lowest LD50 of San Pablo as the base and found that the Jin Hua population was 52.87 times more resistant and the populations in Ang Thong and Nakhon Ratchasima were 1.67 and 2.25 more resistant, respectively (Figure 1).
Figure1. Probit lines for BPH populations in Thailand, Philippines and China
The result indicated that in 2009 BPH populations in Thailand were about 2 times more tolerant or resistant to BPMC than those in the Philippines. The BPH population in China was the most resistant, about 53 times and 25 times higher than population in the Philippines and Thailand, respectively. The level of insecticide resistance clearly reflects the intensity of insecticide use, with China using BPMC far more intensively than Thailand and Philippines and Thailand more intensively than in the Philippines. Thai farmers use abamectin and cypermethrin frequently, research is needed to determine if they can stimulate resistance development in carbamates or other insecticides.
