Exotic grass invasion reduces survival of Amblyomma americanum and Dermacentor variabilis ticks

Background/Question/Methods

Exotic plants often invade areas of high human activity, such as along trails, roads, and forest edges, and in disturbed riparian areas. These same habitat types are also favored by ticks. This convergence suggests that habitat modifications caused by exotic plant invasions may mediate disease vector habitat quality, indirectly affecting human disease risk at the local spatial scale. We tested the hypothesis that experimental invasions of Microstegium vimineum (Japanese stiltgrass ) alter soil surface microclimate conditions, thereby reducing habitat quality for ticks. Microstegium is an exotic annual grass that is highly invasive throughout the eastern United States, where the disease vectoring ticks Amblyomma americanum and Dermancentor variabilis occur. Ticks (n = 100 per species) were housed individually in mesh bags and introduced into experimental invasion plots and native vegetation control plots (n = 5 per treatment) established two years prior to this experiment.  Tick survival was  assessed  11 times over 40 days and soil surface microclimate conditions were measured twice.

Results/Conclusions

In invaded plots D. variabilis mortality rate increased 178% and A. americanum mortality increased 72% relative to control. These survival differences corresponded with changes in abiotic conditions known to increase tick mortality.  Microstegium invasion resulted in a 13.8% increase in temperature and an 18.8% reduction in humidity compared to controls. We predict that areas naturally invaded by Microstegium will have lower densities of host-seeking ticks, and are testing this hypothesis with ongoing field sampling. Our results emphasize the potentially complex consequences of biological invasions, the role of invasive species in mediating local human disease risk, and the need for integrative management strategies to simultaneously address exotic plant invasions and vector-borne disease.