Factors influencing egg parasitism in sub-social insects: insights from the treehopperAlchisme grossa(Hemiptera, Auchenorrhyncha, Membracidae)

Abstract

Subsocial insects rely on maternal care in order to enhance offspring survival. Subsocial Hoplophorionini treehoppers defend their egg masses from predatory and parasitoid attacks. Egg parasitic wasps are important enemies of the Membracidae. We examined parasitism patterns and female defence mechanisms in the Hoplophorionini treehopper Alchisme grossa (Fairmaire). We examined the effect of aggregations of egg-guarding females, spatial aggregations of host plants and female A. grossa, and host plant identity on the incidence of egg mass parasitism. Additionally, we measured the effect of female maternal care strategies on reducing the amount of parasitism on their egg masses. Aggregation of egg-guarding females on a single host plant was the most important factor affecting egg mass parasitism. Increased numbers of aggregated females lowered both parasitism risk and parasitism levels on their egg masses. When spatial clumping of host plants and egg-guarding females occurred simultaneously, parasitism risk was also reduced. Maternal care strategies such as coverage of the egg mass by the females' pronota and intensity of female defensive behaviour also seemed to lower parasitism on egg masses. A reduction of parasitism by female aggregation on host plants can be explained by a selfish herd dilution effect. This dilution effect was potentiated by a possible cooperation between aggregated egg-guarding females on the host plant. The present study provides a new dimension to sub-sociality in Hoplophorionini treehoppers.