Unintended effects of the herbicides 2,4-D and dicamba on lady beetles

Authors: FREYDIER, L - Agrocampus Ouest; LUNDGREN, J - Former ARS Employee

Submitted to: Ecotoxicology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2016
Publication Date: 6/9/2016
Citation: Freydier, L., Lundgren, J.G. 2016. Unintended effects of the herbicides 2,4-D and dicamba on lady beetles. Ecotoxicology. 25:1270-1277. doi:10.1007/s10646-016-1680-4.

Interpretive Summary: Many weeds are now resistant to glyphosate. The development of new genetically modified (GM) crops tolerant to the herbicides 2,4-D and dicamba is expected to facilitate increased use of these herbicides in cropland. The lady beetle known as Coleomegilla maculata is an important beneficial insect in cropland and 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, and tested for synergistic effects of the herbicides. Second stage immatures of the lady beetle were exposed to the herbicides, 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 dosage of this herbicide that killed 90 percent of larvae on average 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 effects of the herbicides when used together was no greater than the sum of their individual toxicities toward lady beetles. Our work shows that herbicide formulations can cause both lethal and sublethal effects on a non-target, beneficial insect, 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.

Technical Abstract: 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.