Xu Hongxing, Yang Yajun, and Lu Zhongxian, Zhejiang Academy of Agricultural Sciences (ZAAS), Hangzhou, P.R. China
In China insecticides are applied in prophylactic sprays, often in “cocktails” with several active ingredients. One of the most popular mixture than is marketed for use in rice is buprofezin, a chitin synthase inhibitor, and chlorpyrifos, an organo phosphate that acts as an acethylcholinesterase (AChE) inhibitor. These two modes of actions are known to affect brown planthoppers. However resistance to a similar AChE insecticide such as fenobucarb has been found to be very high while resistance to buprofezin is still low. Presumably this mixture should have high efficacy and more effective in managing rice planthoppers. We evaluated the two active ingredients separately and as mixtures in various ratios as those sold in the market under various trade names. The software PoloMix available from LeOra Software was used to evaluate if the toxicity of each is affected by the other and if the mixture has increased or decreased potency on BPH.
The toxicities were determined using the stem dipping method with solutions at various concentrations. Details of the stem dipping method is available in https://ricehopper.files.wordpress.com/2011/09/buprofezin-methodology.pdf. BPH mortality was observed after 5 days and the LC50s were computed using POLO plus.
The table below shows the toxicities of the buprofezin, chlorpyrifos and 5 different mixtures.
Table 1: Toxicities (in LC50) with related statistics of buprofezin, chlorpyrifos and 5 different mixtures on the brown planthopper. Ant = Antagonistic. Equal = No significant antagonistic or synergistic effects.
[table id=20 /]
The hypothesis of parallelism (equal slopes) was not rejected (Chi Sq = 8.43, p = 0.21) indicating that all the 7 probit lines are parallel and thus the LC50s are comparable. Chlorpyrifos has virtually no effect on BPH. When it is mixed with buprofezin, the mixtures have either equal or reduced efficacies or antagonistic effects. BPH efficacy reduction was as much as 74% in the mixture I, while in mixture III the efficacy was about equal or slightly higher by about 4%. Efficacy was lowest in mixtures with 1 % buprofezin. Mixtures with buprofezin at 5% and 7% had efficacies about equal to that of burpofezin alone.
Similar to the evaluation of the fenobucarb+chlorpyrifos, the mixtures of buprofezin with chlorpyrifos had antagonistic effects on their BPH efficacies. Chlorpyrifos is known to have resurgence effects on BPH (Heong and Schoenly 1998) and is 3 times more toxic to the egg predator, Cyrtorhinus than to BPH (Suvaparp 1993). Thus such mixtures not only have reduced efficacies to BPH, they are also more toxic to natural enemies, like the egg predators and parasitoids and can cause BPH resurgence. The mixtures we evaluated had proportionately much higher concentrations of chlorpyirifos. Farmers who use such mixtures or “cocktails” are unknowingly making their rice crops more vulnerable to BPH outbreaks.
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.
Suvaparp, R. 1993. Effects of sublethal doses of insecticides on the mired predator, Cyrtorhinus lividipennis (Reuter) (Heteroptera: Miridae). MSc thesis submitted to Graduate School, Kasetsart University, Thailand.