Effects of non‐host traces on behavioural responses of an egg parasitoid to substrate‐borne semiochemicals in a multi‐herbivore scenario

Insect parasitoids commonly rely on air-borne and substrate-borne semiochemicals during the host location process. In addition, parasitoids commonly face the challenge of exploiting semiochemicals in complex environments, where the presence of non-host resources interspersed with host resources may interfere with their foraging efficiency. While numerous investigations have shown that non-hosts may interfere with the ability of parasitoids to exploit air-borne semiochemicals, research on substrate-borne semiochemicals has lagged behind. In open arena behavioural assays, we studied the response of the egg parasitoid Trissolcus basalis to walking traces left on the substrate by the host Nezara viridula either alone or in combination with larval traces left by the non-host Spodoptera littoralis. The presence of non-host larval traces affected parasitoid responses to substrate-borne semiochemicals left by N. viridula hosts, but the interference depended on the hierarchical value of the host walking traces. In fact, non-host larval traces significantly reduced the residence time and increased the linear speed of T. basalis when exploring patches contaminated by gravid N. viridula females. However, the presence of non-host larval traces did not have any apparent effect on parasitoid responses to patches contaminated by N. viridula males. In a follow-up experiment restricted only to gravid N. viridula females, we found that the disruptive effects on egg parasitoid responses to substrate-borne semiochemicals were also influenced by non-host density and developmental stage, with the strongest interference occurring in the presence of traces left by fifth-instar S. littoralis larvae. Our results show that non-host herbivores can disrupt egg parasitoid exploitation of substrate-borne semiochemicals with potential consequences for foraging efficiency of biological control agents in multi-herbivore environments.