Susceptibility of the Cowpea Aphid (Hemiptera: Aphididae) to Widely Used Insecticides in Australia

Globally, 27 aphid species have evolved resistance to almost 100 insecticide active ingredients. A proactive approach to resistance management in pest aphids is needed; this should include risk analysis, followed by regular baseline susceptibility assays for species deemed at high risk of evolving resistance. The cowpea aphid (Aphis craccivora Koch) has evolved insecticide resistance to multiple insecticides outside Australia and was recently identified as a high-risk species in Australia. In this study, we generated toxicity data against four insecticides (representing four unique chemical Mode of Action groups) for populations of A. craccivora collected across Australia. Alpha-cypermethrin was the most toxic chemical to A. craccivora in leaf-dip laboratory bioassays with an average LC50 value across nine populations of 0.008 mg a.i./L, which was significantly lower than dimethoate (1.17 mg a.i./L) and pirimicarb (0.89 mg a.i./L). Small, but significant, differences in sensitivity were detected in some populations against pirimicarb and dimethoate, whereas responses to alpha-cypermethrin and imidacloprid were not significantly different across all aphid populations examined in this study. For all insecticides, the field rate controlled 100% of individuals tested. The data generated will be important for future monitoring of insecticide responses of A. craccivora. Proactive management, including increased reliance on non-chemical pest management approaches and routine insecticide baseline sensitivity studies, is recommended for A. craccivora.