Plant-chewers turned flesh-eaters: Altered macronutrient ratios promote cannibalism in a generalist herbivore pest (Trichoplusia ni)

Insect pests pose a significant challenge for crop management strategies in agricultural systems. One way of reducing insect damage is promoting predation, either between species or within a species. Studies have shown that plant chemical defenses can reduce plant damage by promoting cannibalistic behavior in herbivores, leading to a reduced amount of total plant tissue consumed and fewer pests surviving to adulthood. The aim of this study was to test the hypothesis that differences in macronutrient ratios promote cannibalistic behavior independently of other facets of plant defense. We performed an experiment where we reared the generalist herbivore pest, Trichoplusia ni, on artificial diet to examine the effects of altering macronutrient ratios on cannibalism. We varied the ratio of protein and carbohydrates, essential macronutrients for metabolic processes and growth. When macronutrient ratios differ from optimum levels, frequency of cannibalism increased regardless of which nutrient was present in greater quantities. Caterpillars had higher levels of cannibalism in surplus protein and surplus carbohydrate treatments relative to the optimum ratio control treatment. Caterpillars given a surplus ratio of carbohydrates: protein cannibalized more rapidly than caterpillars in the surplus protein: carbohydrate treatment. Survival of cannibals in surplus nutrient diet treatments was much lower compared with the optimal balanced diet treatment. Our results demonstrate that imbalances in the ratio of macronutrients drive changes from herbivory to cannibalism. We propose that understanding how macronutrients influence cannibalism offers new insight into the dietary mechanisms that facilitate shifts in feeding behavior and host plant choice in plant-chewing herbivores.