Studies of insecticide toxicity to lucerne aphids and their predators

Abstract

The insecticides pirimicarb, chlorpyrifos, fenvalerate, and demeton-S-methyl were topically applied in acetone to the lucerne-infesting aphids Acyrthosiphon kondoi and A. pisum and to two of their natural enemies, the lacewing Austromicromus tasmaniae and the beetle Coccinella undecimpunctata. LD₅₀ values were determined. Pirimicarb was 1000–10 000 times more toxic to aphids than to predators; the equivalent factors for fenvalerate and demeton-S-methyl were 5–1000 and 16–400 respectively. Chlorpyrifos was equally toxic to aphids and predators. Application of aqueous solutions of pirimicarb, chlorpyrifos, and fenvalerate to the lacewing confirmed the order of toxicity of these three chemicals by topical application. Pirimicarb, fenvalerate, and demeton-S-methyl were topically applied to adult lacewings at the previously determined LD₅₀ rate, and groups of insects were subjected to post-treatment temperatures of 10°c or 25°c. With fenvalerate there was a marked increase in toxicity (approximately 3 ×) at the lower temperature.

Keywords:

• lucerne
• aphids
• lacewings
• coccinellids
• organophosphorus insecticides
• synthetic pyrethroid insecticides
• mortality
• temperature