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Eusocial insect societies possess complex multilevel disease defences, including individual level protection conferred by physical (e.g. cuticle) and immunological obstacles and colony level protection mediated by collective behaviours (social immunity). It remains unclear whether and how these two levels of disease protection are related to one another in jointly driving colonies’ susceptibility to disease. Here, we examine whether a relationship exists between individual worker survival after exposure to a fungal pathogen (a proxy for immunity) and corpse removal (a colony level social immunity metric) in the acorn ant Temnothorax curvispinosus. Since behavioural avoidance is the first line of defence against infection, we also tested whether individual ants exhibited parasite avoidance behaviour during exploration and whether colonies exhibit avoidance behaviour during foraging. We found that individual level and colony level immunity were negatively correlated: colonies that removed corpses more rapidly contained workers with weaker individual defences. We did not detect parasite avoidance behaviour by individual workers or whole colonies, nor were these two factors related. These data suggest that individual immunity and social immunity may trade off, regulating overall parasite protection. Alternatively, optimized social immunity at the colony level may compensate for disease vulnerability to infection at the individual level, and thus provide a protective benefit in overall colony defence in the absence of pathogen avoidance.

The invasive plant, garlic mustard (Alliaria petiolata), has the potential to affect soil microbial communities and ecosystem processes in temperate hardwood forests primarily through the release of allelopathic chemicals into the soil. These forest soils are also often affected (directly and indirectly) by the high abundance of white-tailed deer (Odocoileus virginianus), which can alter plant community composition and productivity. We examined the joint effects of deer and garlic mustard on soil microbial communities, soil nutrients and a native plant species’ vital rates in a temperate forest 8 years after initiation of a paired plot deer exclusion/access study where garlic mustard was either removed from half of each plot or remained at ambient level in the other plot half. We examined soil microbial communities using DNA-based techniques and quantified nutrient availability and physicochemical properties. Deer exclusion affected the community structure of AM fungi, particularly when garlic mustard was present, but had no effect on soil chemistry. Garlic mustard removal plots showed no changes for soil fungi, but displayed higher soil carbon content. Interestingly, we found significant changes to native plant vital rates that mirrored soil responses; the presence of garlic mustard led to higher mortality of large, mature plants and reduced native plant cover and biomass. Our data suggest herbivore-plant-soil feedbacks and synergies can interact to negatively affect the soil ecology of forests. Management activities that reduce deer or invasive plant abundance may positively affect soil microbial communities and chemistry in temperate forests.