Toxicity of 5 Insecticides to BPH in the Philippines

by
L. Fabellar and P. Garcia
Crop and Environmental Sciences Division
International Rice Research Institute, Los Baños, Philippines
 

Map of the Philippines showing the 6 sites where BPH were collected in 2010

Insecticide use in the Philippines has remained relatively low as compared to other Southeast Asian rice growing countries like Vietnam and Thailand. The brown planthopper (BPH) problem has been minimal in recent years and this might be reflected in the BPH’s responses to insecticides. We quantified the toxicity of 5 insecticides to BPH collected in 2010 from 6 sites from as far south as Davao to as far north as Isabela (see map above).

Treating standardized BPH adults with a micro applicator

BPH adults were collected from rice fields in the respective sites, brought back to IRRI research station and reared under standard conditions in seedling boxes in the insectary.  After 5 generations, 3 day old macropterous BPH females were randomly selected as test insects.  Insecticide active ingredients diluted with technical grade acetone were used for the tests.  Insects were anaesthetized with CO² and doses of the insecticides were applied onto the insects using a micro applicator.  For details of standard procedure click here.

BPH populations from the south, Bicol and Davao, were significantly most tolerant to fipronil than the other 4 sites. (Table 1 below). In the case of imidacloprid, the Davao population was significantly more susceptible than populations from the other 5 sites. For BPMC the Davao population was significantly more tolerant than the rest. For chlorpyrifos and isoprocarb the 5 populations we tested appeared to have similar responses. The order of toxicity based on the LD50s of populations from the different sites to the 5 chemicals is shown below:

Table 1. The LD50 values of the 5 chemicals to BPH populations collected in 2010.

Fipronil

Imidacloprid

BPMC

Chlorpyrifos

Isoprocarb

BPH populations from the south, Bicol and Davao, were significantly most tolerant to fipronil than the other 4 sites (Table 1 above). In the case of imidacloprid, the Davao population was significantly more susceptible than populations from the other 5 sites. For BPMC the Davao population was significantly more tolerant than the rest. For chlorpyrifos and isoprocarb the 5 populations we tested appeared to have similar responses. The order of toxicity based on the LD₅₀s of populations from the different sites to the 5 chemicals is shown below:

Fipronil                        Isabela > Nueva Ecija > Pila > IRRI >>> Davao > Bicol
Imidacloprid               Davao >>> Nueva Ecija > Bicol > Isabela > Pila > IRRI
BPMC                          Pila > Isabela > Nueva Ecija > IRRI > Bicol >>> Davao
Chlorpyrifos                Pila > Bicol > Isabela > IRRI > Davao
Isoprocarb                   IRRI >Davao > Pila > Isabella > Bicol

Note: > indicate greater but not significantly different and >>> indicate significantly
greater.

Since the comparison between two dose-mortality lines is only valid when they are parallel (Busvine 1971) we only used those lines where the slopes are not significantly different for comparison and discussion. For fipronil populations from Davao was most resistant and was about 13 times more tolerant than populations from Isabela. When compared with the recent report on populations from Tien Giang, Vietnam (Read: Post by Tuong et al), the slopes were similar, the BPH from Tien Giang were 14 and 34 times more resistant than those from Davao and Isabela, respectively.

In the case of imidacloprid, the Davao and IRRI populations had similar slopes and the IRRI population was 21 times more tolerant. Tien Giang population also had a similar slope and was 12 times more tolerant than that of IRRI and Tien Giang BPH was 251 times more resistant than that found in Davao.

For BPMC the slopes of IRRI and Davao populations were similar and Davao populations were 5.7 times more tolerant than that from IRRI. Davao populations were rather similar in response to that from Tien Giang, Vietnam, which was 6 times more tolerant than the IRRI population.

Responses of BPH populations to the 5 insecticides in the Philippines were mixed.  For chlorpyrifos and isoprocarb, there was no significant difference between populations in the north and south. Since data from previous studies was not available we could not infer if BPH populations had acquired tolerance over the years. Fipronil and imidacloprid are relatively new compounds and multiple folds of tolerance between locations had been observed. This probably reflected the intensity of use in the different sites although we could not obtain any report on use patterns. BPMC had been used widely in rice fields for several decades. The level of resistance between sites seemed to have remained rather low (less than 10).

Based on the toxicities of the 6 compounds, the insecticide resistance problem in the Philippines is low. This is probably because rice farmers’ insecticide usage has remained relatively low in the 1990s and 2000s as compared with that in Vietnam, Thailand and China. There are however indications of high tolerance to the new chemicals, fipronil and imidacloprid, in some areas, perhaps due to higher usage.  Since resistance development to these two insecticides had been very rapid (Read: Matsumura et al 2009 paper), there is need for Philippines to be cautious in their introductions. Incorporating these insecticides as seed dressing extends the duration of exposure and thus intensifies resistance selection and has been cited as a factor to the rapid resistance development. Perhaps for the Philippines these two compounds should not be sold for seed dressing. Clearly the 2010 data shows the beginnings of resistance development in the Philippines of up to 14 and 21 times in fipronil and imidacloprid, respectively.

References

Busvine, J.R. 1971. A critical review of the techniques for testing insecticides. Commonwealth Agricultural Bureaux, London.

Matsumura,  M., Takeuchi, H., Satoh, M., Sanada-Morimura, S., Otuka, A., Watanabe, T. and D. V. Thanh 2009. Current status of insecticide resistance in rice planthoppers in  Asia. Pp 233-244. In Heong KL, Hardy B, editors. 2009. Planthoppers: new threats to the sustainability of intensive rice production systems in Asia. Los Baños (Philippines): International Rice Research Institute.

Tagged as: toxicity of insecticides