Influence of victims' diet on the life-history traits in cannibalistic Menochilus sexmaculatus

Multitrophic interactions are common in nature and play a major role in modulating life-history traits in predatory insects. The sequestration of chemicals and the transfer of nutrients from one trophic level to a higher trophic level have been reported previously. However, the effect of cannibalism, which is prevalent in the food chain, has not been explored yet. In this study, we hypothesised that the varying nutritional condition of the victim would significantly influence the life-history traits of adult Menochilus sexmaculatus Fabricius (Coleoptera: Coccinellidae). To test this hypothesis, we provided fourth-instar individuals with first-instar victims of varying nutritional quality for cannibalism. To create victims of varying nutritional quality, first-instar victims were fed on high-quality aphids [Aphis craccivora Koch (Hemiptera: Aphididae)], low-quality aphids (Aphis nerii Boyer de Fonscolombe) and conspecific eggs. The cannibalistic groups were tested against the control group, in which the fourth instars were provided with their natural prey, A. craccivora. Our results revealed a significant difference in the consumption of prey by the fourth instars between the diet groups, with the control group consuming the most prey. The developmental duration of fourth instars was not significantly influenced by the victims' diet or the amount of victims cannibalised. Furthermore, neither the victims' diet nor the amount of victims cannibalised affected the mating (time to commence mating and copulation duration) and reproductive parameters (egg viability and fecundity) in M. sexmaculatus. Cannibalism here might have compensated for the low-quality victims, resulting in insignificant effects on the life-history traits of M. sexmaculatus. Otherwise, given the short duration of the first instars and their potentially limited ability to sequester significant amounts of plant allelochemicals, it is possible that their diet did not affect the development, mating and reproductive attributes of the cannibals. Thus, the use of higher instars as victims may yield substantially different outcomes.