Costs and Benefits of Wax Production in the Larvae of the Ladybeetle Scymnus nubilus

SIMPLE SUMMARY: Scymnus nubilus Mulsant (Coleoptera: Coccinellidae) is a tiny ladybird reaching a maximum body weight of about 1.5 mg. Despite its small body size, the individuals of this species are able to thrive in aphidophagous guilds with other predator species with stronger competitive abilities and potential to exert intraguild predation (IGP). In this study, we explore to what extent that the wax layer of S. nubilus larvae produced by dorsal epidermal cells is an effective defensive mechanism. We predict that wax production by larvae is a trait selected by adaptive evolution where some benefits (eventually protection against intraguild predation) were traded with some costs. In manipulative experiments, we found that waxless S. nubilus larvae (waxes removed artificially) were more susceptible to IGP by lacewing larvae of Chrysoperla agilis (Neuroptera: Chrysopidae). We also found that adults originating from waxless larvae were lighter than the ones originating from wax larvae, demonstrating a metabolic cost resulting from a constant need of wax production. The results indicate the potential existence of a trade-off between growth and protection associated with wax production in beetles. ABSTRACT: BACKGROUND: Larvae of the minute aphidophagous Scymnus nubilus Mulsant (Coleoptera: Coccinellidae) are common predators in apple orchards, covered by a wax layer that might act as a defense mechanism against natural enemies. However, the costs and benefits of protection conferred by wax remain to be assessed. We tested the following hypothesis: there is a trade-off in wax producing ladybeetles between the protection conferred by wax cover and the physiological or behavioral costs associated with its production. We predict that: (1) wax production is an efficient defensive mechanism (against intraguild predation), (2) wax production is associated with detrimental physiological (growth, reproduction) or behavioral effects (behavioral compensation: increased biomass consumption). RESULTS: Tests were carried out in the laboratory with wax and waxless larvae of S. nubilus, with and without lacewing larvae of Chrysoperla agilis (Neuroptera: Chrysopidae) being used as a potential intraguild predator of the coccinellid. Waxless individuals were more susceptible to intraguild predation by lacewing larvae. Adults originating from waxless larvae were lighter than the ones originating from wax larvae, suggesting a metabolic cost resulting from a constant need of wax production. Body-weight gain and conversion efficiency were lower in waxless larvae. Biomass consumption was similar, showing that waxless larvae did not compensate for the physiological cost by eating more aphid biomass. CONCLUSION: The results indicate the potential existence of a trade-off between growth and protection associated with wax production.