Toxicity studies of insecticides on lucerne aphids and their predators

The insecticides pirimicarb, fenvalerate, chlorpyrifos and demeton-S-methyl were tested against lucerne aphids Acyrthosiphon kondoiShinji and A. pisum (Harris) and against the natural enemies Austromicromus tasmaniae (Walker) and Coccinella undecimpunctata L. by topical application. LD₅₀ values were determined. Pirimicarb was 1000 – 10000 times more toxic to the aphids than to the predators while the equivalent factors for fenvalerate and demeton-S-methyl were 5 – 1000 and 4 – 100 respectively. Chlorpyrifos showed the same order of toxicity to both predators and aphids. The effect of post-treatment temperatures of 10 and 25°c on the toxicity of pirimicarb, fenvalerate and chlorpyrifos to the adult lacewing A. tasmaniae was investigated. A marked increase in toxicity of fenvalerate was observed at 10°c. A brief investigation of possible sublethal effects of fenvalerate on lacewing fecundity, and of pirimicarb and fenvalerate on the length of the larval developmental stage, revealed no significant effects