Social immunity and housekeeping behaviors in social aphids

Group-living is inherently vulnerable to the increased risk of pathogens and disease transmission. To combat this, eusocial organisms have evolved cooperative immune responses known as social immunity.  Everything from the design of a hive or ant mound to the ejection of an infected individual is considered an aspect of social immunity. Identifying these cooperative immune responses is a keystone in understanding how groups reduce the risks of living together and transmitting pathogens. Fortress defenders, such as social aphids and thrips, live in galls, tumor-like growths of plant material.  In these galls, aphid soldiers will fiercely defend the nest from invaders. While these soldiers act as altruistic defenders of the gall, they also exhibit a “housekeeping” behavior. This housekeeping behavior includes actively removing honeydew, waste and exuviae from the gall. This behavior is common among eusocial organisms, but has only been described a few times in social aphids.  To explore this behavior further, we quantified housekeeping activity near the galls’ entrances and measured the rate of honeydew and exuvia ejection of three closely related aphid species, which vary in the degree of sociality. By comparing species that vary in their degree of sociality, we can identify if this behavior is universal among gall-living aphids or if this behavior is unique to those species that possess soldiers.