Host‐age discrimination during host location by Cotesia glomerata, a larval parasitoid of Pieris brassicae

Abstract

AbstractSome parasitoids are restricted with respect to the host stage that they attack and even to a certain age within a stage. In this paper we investigate whether the parasitoid Cotesia glomerata can discriminate between old and young caterpillar instars of its host, Pieris brassicae, before contacting these hosts, since contacts with older instars are very risky with a chance of being killed, due to the aggressive defensive behaviour of the caterpillars.Flight chamber dual choice tests showed that volatile chemicals emitted by Brussels sprouts plants (Brassica oleracea var. gemmifera) after feeding damage by 1st and 5th larval instars are equally attractive to the wasps. Simulated herbivore damage by 2nd and 5th larval instars, obtained by treating mechanically damaged leaves with carterpillar regurgitant, was also equally attractive, even when the wasps were exposed to repeated experience on different larval instars to increase their discriminatory ability.In contrast, single choice contact bioassays showed that the time spent searching on a leaf with feeding damage of 1st instar larvae was significantly longer than the time spent on 5th instar feeding damage or on mechanically damaged leaves. Both flight and contact bioassays did not show any effect of egg‐related infochemicals. The results demonstrate that C. glomerata can discriminate between young and old larval instars of P. brassicae, without contacting the caterpillars. This is not done through volatile herbivore‐induced synomones but through cues that are contacted after arrival at a caterpillar‐infested leaf.