Are Japanese beetles (Popillia japonica) attracted to neonicotinoid seed treated soybeans?

During 2012 adult Japanese beetles (JB), Popillia japonica Newman (Coleoptera: Scarabaeidae) emerged approximately 3 weeks earlier than usual in central Iowa and unusually large populations of JB were observed in soybean plants in early vegetative stages. During this period, JB adults appeared to preferentially congregate in higher numbers in soybean plots planted with the neonicotinoid seed treatment thiamethoxam (Cruiser 5FS) applied to the seed versus untreated plots. To confirm this observation, we compared the abundance of JB adults collected in replicated plots of soybeans either grown with or without seed treatments of thiamethoxam. Plots were sampled with a sweep-net at 42, 49, and 57 days after planting.  We observed significantly more JB in plots grown with the thiamethoxam than without (400-500% more) at the 42 day sampling date. A possible explanation for this observation is that the seed treatments improved the vigor and overall health of the plants early in the season, thereby making them more attractive to JB. In 2013, we tested this hypothesis with multiple lines of both treated and untreated soybean in a choice test. Pairs of treated and untreated plants were grown in cages and artificially infested with JB adults at 35 days after planting. Both the number of beetles per plant and defoliation were measured.  Although JB are not generally considered an early season pest of soybeans, our results may suggest that the impact of this polyphagous pest may be altered with the use of neonicotinoid seed treatments.