Unintended Effects of the Herbicides 2, 4-D and Dicamba on Lady Beetles
New genetically modified crops that are resistant to 2,4-D and dicamba are expected to lead to increased use of these herbicides. This study examined lethal and non-lethal effects of 2,4-D and dicamba active ingredients (a.i.) and commercial formulations the lady beetle (Coleomegilla maculata), an important beneficial insect species. The effects of the herbicides on development were evaluated. A dose-response study was also conducted on 2,4-D with and without dicamba.
C. maculata second instars were assigned to one of eight treatments: the insecticide esfenvalerate (Asana XL); 2,4-D commercial formulation (2,4-D LV4); 2,4-D a.i.; dicamba commercial formulation (Dicamba DMA); dicamba a.i.; 2,4-D and dicamba commercial formulations; water control; or acetone control. Commercial products were diluted in water per label instructions, and the a.i.s were diluted in acetone at rates equivalent to the amount of a.i. in the commercial products.
Larvae treated with the commercial 2,4-D had a significantly shorter larval durations than all other treatments. Larvae treated with dicamba a.i. alone or the combination of both commercial herbicides were significantly smaller than their respective untreated controls. Commercial formulations of both herbicides reduced the proportion of males in the population. Treatments also affected larval longevity, with those treated with dicamba a.i., both commercial herbicides, 2,4-D commercial, or the insecticide dying significantly sooner than the controls. Larvae administered the combined herbicides produced significantly smaller pupae than the water-treated control. No synergism between the two herbicides was shown, but the mixture treatment had an intermediate survival rate. The authors noted that the intermediate mortality seen is likely because tank mixing reduces the recommended amount of herbicide applied – not only improving the resilience of weed management programs, but also possibly reducing effects on non-target species.
Dicamba a.i. significantly reduced longevity and survival of larvae relative to the acetone control but the commercial formulation did not. The commercial formulation of 2,4-D was highly lethal to the lady beetle larvae, with the LC90 at 13% of the label rate. In this case, the inactive ingredients were a key driver of toxicity, either because they were more toxic than the a.i. or because they increased its toxicity. The results show that effects of herbicides can be significant and sometimes driven by inactive ingredients in the formulations. Both active ingredients and the commercial herbicides had sublethal effects on the lady beetles that survived the initial doses (most lethality occurred within a few days of herbicide exposure). While size reductions were seen in some treatment groups, the authors note that it is unclear if they were biologically meaningful. The skewed sex ratios could have been the result of sex-specific effects of the herbicides or that the larger females were better able to tolerate the herbicides.
