Insecticide resistance reversion of three compounds against BPH in Central Thailand

by moni on October 2, 2012

by
Wantana Sriratanasak & Sukanya Arunmit, Rice Department, Bangkhen, Thailand and
Patrick Garcia, International Rice Research Institute, Los Baños, Laguna, Philippines
 

Topical application method used for monitoring insecticide resistance levels of BPH. A: Female short winged form (photo credit S. Villareal)

When insect population is exposed to high selection pressure of insecticides, invariably insecticide resistance is selected or developed.  Often the developed resistance may decline or reverse when the insecticide resistant insects are reared through a few generations free from any selection pressure.

We collected BPH from the field to explore the possibility of resistance reversion against 3 commonly used insecticides.  The BPH were reared through 14 generations; and the LD50s were determined for populations at 2nd to 5th and 14th generations.

Standardized methods as described by Heong et al (2011) (link to  h ) were used. The data obtained were analyzed using the POLO plus program. A summary of the data analyses is shown in Table 1.

Table 1:  LD50s (in microgram/gram insect weight) of 3 common insecticides on BPH population at the 2nd – 5th and 14th generations (G) after field collections in Central Thailand. 2012.

 Insecticide

LD50

Fiducial limits 95%

Slope (+se)

Heterogeneity

Sig

Rel. Potency

Fenobucarb 2-5 G

1.905

1.545 to 2.272

2.49 (0.29)

0.15

ns

1.20

Fenobucarb 14 G

1.586

1.101 to 2.109

1.84 (0.31)

0.64

Imidacloprid 2-5 G

3.437

2.428 to 4.435

2.15 (0.33)

0.72

*

1.80

Imidacloprid 14 G

1.908

1.327 to 2.548

1.70 (0.25)

0.06

Fipronil 2-5 G

0.159

0.121 to 0.203

1.64 (0.19)

0.67

*

1.83

Fipronil 14 G

0.087

0.062 to 0.112

2.21 (0.34)

0.69

*Significant at 5%.

The data we obtained were homogenous and all paired probit lines were parallel. Thus, the LD50s of each insecticide tested against the two generations may be compared and the relative potencies determined. Toxicity of fenobucarb was relatively low at LD50 =1.905 microgram/gram and it declined to 1.586. Although the difference was not significantly different, the LD50 declined about 20% after 10 generations. In the Philippines, the resistance stability of fenobucarb seems to lie between 2.12 and 6.90 microgram/gram. Stability is an estimate of how persistent resistance in a pest population remains when the insecticide pressure is removed. In Thailand stability is probably at less than 2.0 and is slightly lower than that in the Philippines and  much lower than resistance levels in China (44.79 microgram/gram) and Vietnam (30.44 microgram/gram).

For imidacloprid and fipronil, differences in LD50 values between the early generations and the 14th generation were both significantly different. The LD50 values were 80 and 83% lower when insecticide pressures were removed. Resistance stability seems to be around 1.908 and 0.087 microgram/gram for imidacloprid and fipronil, respectively.

In the Philippines LD50s for imidacloprid probably lie between 0.014 and 0.092 microgram/gram which are about 20 to 136 folds lower. However, in China and Vietnam the LD50s were 6.8 and 3.1 microgram/gram, respectively. These values are much higher than that for BPH populations in Thailand and Philippines.

In China, the LD50 for fipronil although banned recently, remains high at 2.5 to 5.6 microgram/gram or 29 to 64 folds that of BPH populations in Thailand.

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