Effects of a glyphosate-based herbicide and temperature on foraging of a common arthropod predator

Background/Question/Methods

When anthropogenic chemicals affect the behavior of non-target organisms there can be unexpected consequences in the food web. For example, herbicides are applied to alter primary production, and it is generally assumed that this is the only avenue by which they impact animals. However, if herbicides have independent and direct effects on activities of herbivores or carnivores, they can have unexpected impacts on trophic interactions. We tested the hypothesis that a glyphosate-based herbicide affects foraging success of the wolf spider, Pardosa milvina, the dominant predator in agroecosystems in our area. We also examined if these effects differ with temperature, as this herbicide is applied at multiple times during a growing season. In the laboratory, individual spiders were allowed to forage on crickets for 24 hours in an arena with the substrate treated uniformly with herbicide or water, at temperatures of 15, 25, and 35°C. In the field, five spiders were enclosed with prey in mesocosms with three patches treated with herbicide or water, experiencing natural daily temperature variation. The numbers of crickets killed and left alive were counted, and body measurements of each spider were taken before and after trials as a proxy for consumption.   

Results/Conclusions

In the laboratory, there was no effect of herbicide on the number of crickets killed by Pardosa or on our measure of consumption. However in the field, Pardosa maintained with herbicide killed fewer crickets but consumed more as evidenced by increases in abdomen size. In the laboratory, it was possible to quantify the number of prey that spiders killed and left behind and there were more of these superfluous kills with herbicide present. Temperature effects were similar in the laboratory and field, and no interactive effects were found between temperature and herbicide. As temperature increased, Pardosa captured more crickets, but decreased consumption. These results suggest that following herbicide application Pardosa’s impact on the food web may be dampened, leaving more contaminated prey alive due to increased consumption of those that are killed. The role of Pardosa in the food web could be further affected by changing temperature, due to increased capture but decreased consumption of prey killed in warmer conditions. These results support the hypothesis that herbicide affects foraging success of Pardosa, and demonstrate the importance of evaluating its effects in both the laboratory and field, as predators may respond to anthropogenic chemicals differently under more realistic conditions.