by
P. Garcia, L. Fabellar and K.L. Heong
International Rice Research Institute, Los Baños, Philippines
BPH – Long winged form female and male
In the field, insecticides are often sold in packages pre mixed with two or more active ingredients, like chlorpyrifos + fenobucarb (or BPMC). Many farmers mix two or more insecticides together (“cocktails”) into their spray tanks before they apply to control brown planthoppers. Presumably the purpose is to simultaneously expose the target pests to two biocides to increase the spray potency or its kill spectrum or both. We can use PoloMix available from LeOra Software (http://www.LeOraSoftware.com/ ) to evaluate if the toxicity of each is affected by the other and if the mixture has increased or decreased potency on BPH. The mixture we used was 66% chlorpyrifos + 34% fenobucarb and results are presented in Table 1 and Figure 2 shows the 3 probit lines for the Pila population.
Table 1: Toxicities of chlorpyrifos, fenobucarb and mixture of the two to BPH populations from Pila and IRRI.
| IRRI population | Chlorpyrifos | BPMC | Mixture |
| LD50 (95% CL) | 7.52 (5.78-8.94) | 4.98 (3.59-6.70) | 4.98 (3.05-7.20) |
| Slope (SE) | 3.58 (0.72) | 1.65 (0.26) | 2.04 (0.21) |
| Heterogeneity | 0.781 | 0.561 | 1.77 |
| Chi square joint action | 23.66 | ||
| Degree of freedom | 7 | ||
| Probability | 0.0013 | ||
| Pila population | |||
| LD50 (95% CL) | 5.68 (4.89-6.62) | 2.15 (1.68-2.61) | 2.89 (1.90-3.90) |
| Slope (SE) | 2.74 (0.26) | 2.63 (0.32) | 2.32 (0.28) |
| Heterogeneity | 0.38 | 0.39 | 1.02 |
| Chi square joint action | 23.18 | ||
| Degree of freedom | 7 | ||
| Probability | 0.0015 |
PoloMix calculates a chi square and when the calculated is greater than the tabulated (at p=0.05) at the specified degrees of freedom, the null hypothesis on independent joint action should be rejected. The toxicity of each component was affected by the other. Individually the BPH from the IRRI population was 1.5 times more susceptible to fenobucarb than chlorpyrifos and the mixture had the same toxicity as fenobucarb thus implying that the chlorpyrifos had little effects on BPH mortality in the mixture.
Probit lines for fenobucarb, chlorpyrifos and a mixture of the two.
For the Pila population, the BPH was 2.6 times more susceptible to fenobucarb than to chlorpyrifos individually. Toxicity of the mixture seemed to be 74% lower than fenobucarb alone indicating that might be some antagonistic effects. The mode of action of both insecticides is through the inhibition of acetylcholine esterase resulting in the accumulation of the neuro transmitter, acetylcholine in the insects’ nerve endings.
It appears that mixing chlorpyrifos and fenobucarb had no added value in potency to BPH and in some populations it might be antagonistic and potency could be reduced by 26%. Further research of other mixtures, particularly those where insecticides with different modes of action are mixed will provide more details about mixing insecticides in applications.
Chlorpyrifos is a chemical that can cause BPH resurgence (Heong and Schoenly 1998) and it is 3 times more toxic to the egg predator, Cyrtorhinus than to BPH (Suvaparp 1993). Thus this mixture not only has reduced potency to BPH, it is also more toxic to natural enemies and can cause BPH resurgence. Farmers who use such mixtures or “cocktails” are unknowingly making their rice crops more vulnerable to BPH outbreaks.
References
Suvaparp, R. 1993. Effects of sublethal doses of insecticides on the mirdi preadator, Cyrtorhinus lividipennis (Reuter) (Heteroptera: Miridae). MSc thesis submitted to Graduate School, Kasetsart University, Thailand.
Heong, K.L. and Schoenly, K.G. 1998. Impact of insecticides on herbivore-natural enemy communities in tropical rice ecosystems. Pp 381-403 ( P. T. Haskell and P. McEwen Eds.) Ecotoxicology: Pesticides and Beneficial Organisms. Chapman and Hall, London.
