We Are Dying to Eat You: Cannibalism and Disease Transmission Under Global Climate Change

ABSTRACT In terrestrial ecosystems, climate change may alter host‐pathogen dynamics along with interactions between hosts due to rising global temperatures and carbon dioxide levels. As carbon dioxide levels increase, plants may decrease their nitrogen‐carbon ratio, which may decrease plant quality for herbivores and alter species interactions. Here, we examined disease transmission, a host‐pathogen interaction, and cannibalism, an interaction between hosts, using the fall armyworm ( Spodoptera frugiperda ), a cannibalistic agricultural pest, and its species‐specific lethal baculovirus, Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV), under various potential challenges of climate change. We used three temperature treatments (cooler, optimal, warmer) based on the fall armyworm's thermal performance curve to simulate a changing climate and four artificial diets with various protein‐to‐carbohydrate ratios (low, equal, high, standard) to manipulate resource quality due to changes in nitrogen‐carbon ratio from increasing carbon dioxide levels. After molting to the fourth instar, experimental larvae were presented with a virus‐infected or uninfected conspecific. Cannibalism incidence and final infection status of the experimental larvae were recorded. For the experimental larvae that survived and did not become infected, pupal mass was measured as a fitness proxy. We found that higher temperatures increased cannibalism in larvae on all resource types. Larvae cannibalized more when provided with a lower protein diet. Warmer temperatures led to slightly higher disease transmission, though diet type did not influence infection results in cannibals. Changes in global temperatures and carbon dioxide levels could cause more frequent cannibalism in the fall armyworm and similar species, though disease transmission from cannibalism alone may not increase substantially. We also found significant differences in fitness estimates between each temperature and diet treatment; individuals fed more carbohydrates (lower protein) and kept at cooler temperatures had larger pupal masses, indicating potentially higher fitness, while individuals fed more protein and kept at warmer temperatures had lower fitness potential. Under climate change, increases in temperature and carbon dioxide levels affect both host‐pathogen dynamics and intraspecific interactions between hosts with varied effects on host fitness. By examining the multiple potential effects of climate change instead of examining each factor in isolation, a clearer picture of how climate change will affect ecological systems can be more thoroughly developed.