Predator effects on host-parasite interactions

Background/Question/Methods

By selectively culling infected hosts, some predators can reduce the spread of disease within the host population. However, not all predators select prey in the same way, hence predator effects on host-parasite interactions may vary with the species of predator. Since 2002, we have been following epidemics of the parasitic fungus (Metschnikowia bicuspidata) in 19 lake populations of Daphnia dentifera. Epidemics only occur in nine populations, and only in the late summer and early fall. Predators likely play a role in the timing and distribution of this disease. Visually-orienting fish predators are highly selective on infected individuals and hence may inhibit epidemics in some lakes by selectively removing infected hosts. Invertebrates such as the midge Chaoborus are also effective predators in lakes, but their potential role in disease transmission is unknown. We hypothesized that Chaoborus may facilitate disease transmission by liberating spores from infected hosts. However, this potential effect depends on the degree of preference with which Chaoborus consumes infected hosts and the rate at which spores are liberated from dead Daphnia. Results/Conclusions

We found that areal abundances of Chaoborus were higher in lakes that experienced Metschnikowia epidemics. In the laboratory, we are completing experiments that test the role of Chaoborus feeding in disease transmission and the selectivity of Chaoborus on infected prety. Although vertebrate predators can reduce disease in Daphnia by "keeping the heards healthy" our preliminary results suggest that this many not be the case for predatory invertebrates.