Insect Pest and Beneficial Species Response to the Timing and Severity of Dicamba Injury in Soybean

Off-target movement of dicamba (3,6-dichloro-2-methoxybenzonic acid) has been one of the most significant and controversial issues to affect the production of non-dicamba-resistant (DR) soybean ( Glycine max (L.) Merr.) in recent years. Although the yield response of non-DR soybean to off-target movement of dicamba has been extensively studied, few studies have examined the effects of dicamba injury on insect population dynamics in non-dicamba-resistant soybean. In a series of field experiments conducted during 2018 and 2019, we assessed the effects of dicamba injury to non-DR soybean on insect abundance. Dicamba was applied to non-DR soybean at rates that correspond to 1/10 th , 1/100 th , 1/1,000 th , and 1/10,000 th fractions of the labeled use rate (560 g ae ha -1 ) at either the third trifoliate (V3) or beginning bloom (R1) stage of growth. Dicamba at 1/1,000 th and 1/10,000 th the labeled rate increased bean leaf beetle ( Cerotoma trifurcata ) abundance following beginning bloom applications. However, the overall effect was reduced insect abundance after third trifoliate or beginning bloom applications of dicamba at 1/10 th or 1/100 th the labeled rate compared to the non-treated control. This response is likely related to the reduction in soybean growth and biomass observed following treatment with the higher rates of dicamba, which in turn influenced the habitat preferences exhibited by the species observed in this study. As the use of dicamba increases in U.S. soybean production, it is important to understand the effects that the off-target movement of dicamba has on insect species in agricultural production environments