Caterpillar-induced plant volatiles remain a reliable signal for foraging wasps during dual attack with a plant pathogen or non-host insect herbivore

Plants respond to herbivory with the emission of plant volatiles, which can be used by the herbivores' natural enemies to locate their hosts or prey. In nature, plants are often simultaneously confronted with insect herbivores and phytopathogens, potentially interfering with the attraction of the herbivores' enemies as a result of modifications of the induced volatile blend. Here, we investigated parasitoid (Cotesia glomerata) attraction to volatiles of plants challenged by different attackers, either alone or in combination with Pieris brassicae caterpillars, hosts of C. glomerata. We used a natural system consisting of Brassica nigra plants, eggs and larvae of P. brassicae, Brevicoryne brassicae aphids and the bacterial phytopathogen Xanthomonas campestris pv. campestris. In all cases, parasitoids successfully located host-infested plants, and wasp foraging behaviour was unaffected by the simultaneous presence of a non-host attacker or host eggs. Analysis of the volatile emissions show that the volatile blends of caterpillar-infested treatments were different from those without caterpillars. Furthermore, dually attacked plants could not be separated from those with only caterpillars, regardless of non-host identity, supporting the behavioural data. Our results suggest that, in this system, indirect plant defences may be more resistant to interference than is generally assumed, with volatiles induced during dual attack remaining reliable indicators of host presence for parasitoids.