Predator cues and an herbicide impact activity, emigration, and survival in a wolf spider

Background/Question/Methods

Animals living in highly disturbed systems must cope with multiple environmental stressors. When individual stressors are combined their impacts can be amplified or mitigated, making it difficult to predict their effects in nature. Animals in agroecosystems cope with a range of anthropogenic stressors that might affect how they interact with competitors or predators. Our goal was to explore how a common herbicide similar to Round-up (active ingredient glyphosate), affected the interactions between intraguild predators. The wolf spider Pardosa milvina is numerically dominant in agricultural systems across the eastern United States, and often falls prey to or competes with the larger wolf spider, Hogna helluo and/or the carabid beetle, Scarites quadriceps.  We tested the effects of these intraguild predators and exposure to herbicide on the activity, emigration, and survival of Pardosa using full-factorial laboratory and mesocosm field experiments. In the laboratory, we measured distance traveled and time to emigration for Pardosa on substrates exposed to predator cues (Hogna or Scarites) and/or herbicide. In a similar field mesocosm experiment, we evaluated emigration in response to Scarites and/or herbicide, and maintained Pardosa in the laboratory for 60 days to determine the impacts of treatment on survival.

Results/Conclusions

In the laboratory, both predator cues and herbicide led to a decrease in movement by Pardosa. Specifically, Hogna cues caused a decrease in distance traveled, regardless of herbicide. However, Scarites cues only led to a decrease in distance traveled when herbicide was also present. Presence of glyphosate and predator cues each decreased time Pardosa spent active, with the impact of Hogna cues being larger than Scarites. Hogna cues also lengthened time to emigration, but neither Scarites cues nor herbicide (alone or in combination) had any impact. In the mesocosm experiment Pardosa did not change emigration rate in response to presence of Scarites and/or herbicide. Surprisingly, Pardosa exposed to Scarites cues alone exhibited increased survival while those exposed to herbicide alone had a marginally significant decrease in survival. Together, these results indicate that predation risk and herbicide application likely interact in complex ways to affect the movement and survival of a major arthropod predator in agricultural systems, and thus may have complex effects on the food web.