An investigation of demographic and component Allee effects in the Madagascar hissing cockroach

Abstract

Population density affects the behaviour and population dynamics of insects, but the nature of that effect may depend on the degree of sociality. We experimentally manipulated initial population density of a subsocial insect, the Madagascar hissing cockroach, Gromphadorhina portentosa Schaum, 1853 (Blattodea: Blaberidae), to determine its effect on vital rates and population dynamics. In accordance with the predicted Allee effect, we hypothesised that intermediate cockroach population density will increase per capita birth rates and cause a higher population growth rate relative to smaller and larger population densities. After tracking 12 experimental populations (three replicates of four initial population densities) for 15 months, we found evidence of Allee effects in this insect. Per capita birth rates showed a quadratic response, with rates highest at intermediate population densities, suggesting a component Allee effect. Contrary to expectations for a demographic Allee effect, population growth rates showed negative density dependence. The proportion of adult females in the population increased with population density, but the mechanism for this increase is not known. Our findings provide evidence for at least one form of Allee effect in G. portentosa and shows a possible connection between subsocial behaviour and population-level responses.