Density and disease resistance in group-living insects: Do eusocial species exhibit density-dependent prophylaxis?

Density-dependent prophylaxis (DDP) is a mechanism of disease resistance found in some insects that adjust their immune defense according to population density. The demonstration of DDP in several insect taxa suggests that it might be a general mechanism of defense against infection in insects that experience variable but frequently high densities. We tested for DDP in a eusocial species, the dampwood termite Zootermopsis angusticollis, by contrasting the survivorship of individuals nesting at relatively high and low densities prior to a challenge with the entomopathogenic fungus Metarhizium anisopliae. Our results show that living in a high-density group did not significantly affect termite survivorship following challenge exposures to either 5.3 × 10⁴ or 5.3 × 10⁵ conidia/ml. The apparent lack of DDP in Z. angusticollis may be due to the multiple individual and prophylactic social adaptations such as allogrooming and other hygienic behaviors, antimicrobial glandular secretions and communication systems that are inherent in this termite and may occur in social insects in general. The innate immunological responses that are typically associated with the DDP of gregarious insects may be induced independent of nestmate density in eusocial species.