Samantha Straus

A defining feature of parasitism is the harm parasites cause to their host via a reduction in lifetime reproductive success. Harm, also referred to as “virulence,” may involve host mortality or sublethal effects, such as a decreased body condition or protracted development of immature individuals. We considered a system where colonies of the social spider Anelosimus eximius serve as hosts to confamilial kleptoparasitic spiders that steal food resources. In a laboratory experiment with parasitized and non‐parasitized colonies, we tested whether this host–parasite interaction meets the criterium of harm to host individuals and colonies. We assessed survival and measured scaled mass indices (SMI) before and after the experimental period. Linear mixed effects models demonstrated that colonies exposed to kleptoparasites had lower SMI values at the end of the experiment compared to controls, but found no effect of kleptoparasitism on mortality in treatment and control groups. We conclude that kleptoparasites meet the criterium of harm to their host to be considered parasitic and provide the first empirical measure of virulence for this study system.