Avoidance of parasitic fungi and nematode threats by Tribolium castaneum

Just as for predators, many animals have developed a suite of defenses to counter the ubiquitous threat posed by parasites. Behavioral avoidance is a key strategy to reduce the likelihood of encountering situations with a relatively high degree of risk. However, because such avoidance likely incurs costs (i.e., non-consumptive effects), individuals should engage in risk assessment to gauge their threat of parasitism based on relevant cues. Combinations of cues may signal particularly risky situations, especially as animals often face concurrent natural enemies. In the first study to consider host choice in the presence of different parasites, we investigated behavioral avoidance in red flour beetles (Tribolium castaneum) given various foraging options involving two entomopathogens, the fungus Beauveria bassiana and the nematode Steinernema carpocapsae. Combinations included single and dual parasite threats, as well as whether parasites were represented by the presence of infectious stages and/or conspecific cadavers that were euthanized or killed by infection. Beetles generally did not exhibit behavioral avoidance of either parasite or the parasite-killed conspecifics, but rather, tended to be attracted to the latter if nematode-infected. They also did not avoid conditions with mixed cues related to the threat of infection, suggesting that they do not gauge or respond to parasitism risk in this way. Our findings have implications for understanding avoidance as an anti-parasite behavior across different host taxa, but also for the use of biocontrols such as fungi and nematodes for pest insects.