Grooming and avoidance of contaminated areas are among the behavioural defences employed by
animals against parasites. Antiparasite defence behaviour is costly in terms of time, energy and/or food
foregone and therefore animals are expected to modulate their defences depending on the risk of attack
and/or the severity of the symptoms caused.We tested the hypothesis that an insect host invests more in
defence against more virulent (more likely to cause death) than less virulent parasites. We tested
avoidance and grooming of adult pine weevils, Hylobius abietis, in response to infective juveniles of two
species of entomopathogenic nematodes, the more virulent Steinernema carpocapsae and less virulent
Heterorhabditis downesi. Weevils avoided feeding on a substrate contaminated with S. carpocapsae but
not H. downesi. Weevils also groomed more when their bodies were contaminated with S. carpocapsae
than either H. downesi or water. We also made direct observations of nematodes on weevils. When equal
numbers of nematodes were applied to weevils more S. carpocapsae than H. downesi moved actively on
the weevil’s cuticle. Thus, the differential response of weevils to the two nematode species can be
explained by the weevils detecting the more aggressive behaviour of S. carpocapsae than H. downesi,
which corresponds to a higher probability of death
