The parasitoid Cotesia glomerata responds differently to plant volatile emission induced by parasitized caterpillars

To locate herbivorous hosts, parasitoids use herbivore-induced plant volatiles (HIPVs) that are emitted by plants after herbivore attack. HIPVs may even provide information about host quality, such as whether hosts are infested with competing parasitoid larvae, allowing parasitoids to discriminate between parasitized and unparasitized hosts. However, although discrimination behaviour has been explored in controlled laboratory settings, its occurrence under field conditions remains uncertain. We conducted experiments under both controlled laboratory conditions and natural field conditions to investigate the preference of female Cotesia glomerata parasitoids for plants emitting HIPVs as defence induced by unparasitized cabbage white butterfly, Pieris brassicae , caterpillars or by P. brassicae caterpillars parasitized by conspecific or heterospecific parasitoids. In wind tunnel choice tests, C. glomerata preferred plants induced by caterpillars parasitized by the solitary parasitoid Hyposoter ebeninus over plants induced by unparasitized caterpillars. In contrast, plants induced by unparasitized caterpillars were preferred over those induced by caterpillars parasitized by conspecifics. Parasitoids did not discriminate between HIPVs when plants were damaged by Cotesia rubecula -parasitized caterpillars. In the field, caterpillars found on plants induced by H. ebeninus -parasitized caterpillars were less often parasitized than those on plants damaged by unparasitized caterpillars or those parasitized by C. glomerata or C. rubecula . Plant damage did not explain these results, suggesting that qualitative rather than quantitative differences in HIPVs support C. glomerata decision making, but we cannot rule out other simultaneously interacting factors involved in host acceptance at close range. Our study suggests that C. glomerata shows limited discrimination between parasitized and unparasitized hosts. Herein, we discuss the potential evolutionary factors that may be responsible for this behaviour and the importance of confirming laboratory results with field studies. • Parasitoids may use plant volatiles to identify the presence of competitors. • Responses depend on the parasitoid species already inside the host caterpillar. • Preference tests in the laboratory do not fully predict outcomes in the field. • More studies on parasitoid host acceptance and patch behaviour are needed.