Whole-plant systemic bioassay method for assessing susceptibility to neonicotinoid insecticides in Aphis glycines

The soybean aphid is a limiting pest of soybeans in North America. Yield losses are often associated with the decreased photosynthetic rate, reduced number of pods, and virus transmission. Seed treatments and host plant resistance are the most common strategies to manage soybean aphids. The large-scale adoption of both management practices suggests the need to understand the susceptibility of soybean aphid to thiamethoxam when used in combination with a resistant variety. Resistant traits of soybean germoplasms are reduced when tested aphids are evaluated on excised leaves in comparison to whole-plants. Therefore, the objective of this study is to develop a whole-plant systemic bioassay that can measure the effect of neonicotinoid insecticides and resistant soybean accessions on A. glycines populations. This bioassay will allow researchers to determine the potential for cross-tolerance between both management practices. The soybean aphid biotype 1 and V4 soybean seedlings were used to perform the bioassays. Cone-tainers were placed inside plastic cups with different thiamethoxam concentrations. Plants were allowed to take up the insecticide solutions for 48 hours followed by the transfer of 10 wingless aphids to the V4 trifoliate leaf. Mortality was recorded every day during 7 days. A LC₅₀ of 6.22 ng/ml was generated at the last day of the experiment. A new range of concentrations will be developed in order shorten the period of bioassay, thereby reducing the effects of other factors on soybean aphid mortality. The whole-plant bioassay provides a potential alternative to measure cross-tolerance between neonicotinoid insecticides and soybean resistant varieties on A. glycines survival.