LAUSR

Cannibalism is relatively common in herbaceous insects, but studies looking at life-history traits on conspecific versus herbaceous diets reveal a mix of outcomes. In some taxa, fitness is highest on a conspecific diet, but in others, fitness is highest on an herbaceous diet. Identifying factors that maintain the cannibalistic feeding strategy has been complicated by the interaction between the direct effect of consuming conspecifics and the indirect effect of releasing future competition. Here, we isolate and study the direct fitness consequences of consuming conspecifics in the tea tortrix (Adoxophyes honmai) by comparing a flexible choice diet against three fixed-composition diets (0, 50, and 100% conspecific). While overall rates of cannibalism were low in the flexible choice treatment (17%), these cannibalistic individuals ingested a greater amount of resources than any other treatment. Increased ingestion is strongly correlated with greater pupae mass suggesting that these trait changes are a relatively straightforward consequence of resource acquisition. In contrast, development time was only weakly correlated with ingestion suggesting a more complex relationship with resource quantity and quality. Interestingly, individuals feeding on either pure conspecific or pure herbaceous diet revealed a trade-off between pupae mass and development rate, whereas the trade-off disappeared for individuals consuming mixed diets. Combining all life-history traits, fitness was maximal on the flexible choice diet compared to all three fixed-composition diets. Our manipulations reveal that only low levels of cannibalism confer a direct fitness advantage. The direct fitness benefit likely emerges from the balance of ingestion rate and nutritional value for each food type.Significance statementCannibalism is common in herbaceous insects, but there is contradictory evidence as to whether the feeding strategy has a higher fitness than pure herbivory. Identifying the direct effects of cannibalism on fitness is challenging as most common experimental approaches do not disentangle these effects from the indirect effect of releasing future competition. Here, we study the direct effect of cannibalism on fitness in a tea tortrix moth and show that low levels of cannibalism have the highest fitness compared to either pure herbivory or pure cannibalism. By measuring ingestion rates, we show that benefits to life-history traits come about from a combination of increased ingestion under low levels of cannibalism and relaxation of the trade-off between development time and pupae mass. As a result, our study investigates the proximate mechanisms that explain the maintenance of low levels of cannibalism in an herbaceous insect.