Weed resistance to glyphosate and development of new GM crops tolerant to 2,4-dichlorophenoxyacetic acid (2,4-D) and dicamba is expected to lead to increased use of these herbicides in cropland. The lady beetle, Coleomegilla maculata is an important beneficial insect in cropland that is commonly used as an indicator species in safety evaluations of pesticides. Here, we examined the lethal and non-lethal effects of 2,4-D and dicamba active ingredients and commercial formulations to this lady beetle species, and tested for synergistic effects of the herbicides. Second instars of lady beetles were exposed to an experimental treatment, and their mortality, development, weight, sex ratio, fecundity, and mobility was evaluated. Using similar methods, a dose-response study was conducted on 2,4-D with and without dicamba. The commercial formulation of 2,4-D was highly lethal to lady beetle larvae; the LC90 of this herbicide was 13 % of the label rate. In this case, the "inactive" ingredients were a key driver of the toxicity. Dicamba active ingredient significantly increased lady beetle mortality and reduced their body weight. The commercial formulations of both herbicides reduced the proportion of males in the lady beetle population. The herbicides when used together did not act synergistically in their toxicity toward lady beetles versus when the chemistries were used independently. Our work shows that herbicide formulations can cause both lethal and sublethal effects on non-target, beneficial insects, and these effects are sometimes driven by the "inactive" ingredients. The field-level implications of shifts in weed management practices on insect management programs should receive further attention.
Keywords: Coleomegilla maculata; GM crop; Herbicide tolerant crop; Pesticide; Risk assessment; Sublethal effects.