Can parasitized diamondback moth larvae avoid ant attacks?

Although koinobiont parasitoids exploit their hosts without killing them immediately, consuming the body tissues of hosts that avoid predation via high motility should increase the risk of the parasitoids being preyed upon along with their hosts. Cotesia vestalis (Haliday) (Hymenoptera: Braconidae) is a koinobiont parasitoid of the diamondback moth Plutella xylostella (L.) (DBM). Although DBM larvae exhibit predation avoidance behaviours based on their high locomotor ability, we hypothesized that DBM larvae parasitized by C. vestalis might not be able to maintain this ability and, hence, are exposed to higher predation pressure than unparasitized larvae. We compared locomotor and predation avoidance abilities of parasitized and unparasitized DBM larvae.

Compared to unparasitized DBM larvae, parasitized DBM larvae had lower wriggling ability to mechanical stimuli and were more likely to be preyed upon by the black wood ant Formica japonica (Motschoulsky) (Hymenoptera: Formicidae) in situations where they had to avoid predation by wriggling. In contrast, hanging ability did not differ between parasitized and unparasitized DBM larvae, nor in predation avoidance ability in situations where they had to avoid F. japonica predation by hanging.

The results demonstrate that parasitism by C. vestalis reduces the wriggling ability of DBM larvae, which consequently exposes the parasitoid to higher intraguild predation risk under some conditions. This finding also suggests that the conventional research methods used to reveal the mortality factors of DBM larvae may have underestimated parasitism because they did not take into account parasitized DBM larvae missing due to predation. Host species with vigorous defensive behaviours are likely to have such behaviours impaired when parasitized, while whether parasitized hosts are actually more vulnerable to predation depends on the specific defensive strategies used by the hosts.