Global change factors such as climate change, invasive species, and land use change may alter vector-borne disease risk but few studies have experimentally quantified how these drivers might affect disease vector survival. In particular, plant invasions may alter habitat conditions in ways that enhance the survival, and thus, abundance of ticks, which are important human disease vectors. We tested the hypothesis that cogongrass (Imperata cylindrica) invasions increase the survival time of off-host lone star ticks (Amblyomma americanum) by altering microclimate conditions. The lone star tick is widely distributed in the Southeast US and transmits zoonotic diseases that affect human health, including Rocky Mountain spotted fever, ehrlichiosis, and tularemia. We established the tick survival experiment in a longleaf pine forest where cogongrass was adjacent to native plant communities. In each of 12 plots in invaded and native plant communities, we placed a mesh bag with 10 adult- and 10 nymph-stage ticks. The bags were situated on the ground, protected by a wire cage, and oriented to allow ticks to move vertically. We evaluated tick survival every two weeks and logged temperature and relative humidity at each plot to assess differences in microclimate conditions between the plant communities.
Results/Conclusions
After 200 days, 82% of adult ticks were alive in the cogongrass invasion but only 8% survived in native plant communities. The nymph life stage suffered greater mortality over this time period, but the effect of the invasion was similar, with 8% of nymphal ticks surviving in the invasion and approximately 1% surviving in native plant communities. Relative humidity was consistently higher in the invaded plant communities compared to adjacent native plant communities. The primary cause of tick mortality is through desiccation, so the higher relative humidity in invaded areas likely is the critical mechanism by which the invasion extended tick survival time. The greater tick survival in cogongrass invasions suggests that this invasive species may increase human risk of exposure to vector-borne disease in these areas. Invasive species have been shown to reduce biodiversity, alter ecosystem services, and shift disturbance regimes, and now these results demonstrate that invaders may pose an additional threat to public health by extending the lifespan of disease vectors.