Can parasitized diamondback moth larvae avoid ant attacks?

Although koinobiont parasitoids exploit their hosts without killing them, 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 behaviors 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. It seemed that host species with vigorous defensive behaviors are likely to have such behaviors impaired when parasitized, while whether parasitized hosts are actually more vulnerable to predation depends on the specific defensive strategies used by the hosts.

Methods
Wriggling ability

Wriggling ability in response to physical stimuli was compared between parasitized and unparasitized DBM larvae. It was practically impossible to confirm directly whether the living DBM larvae were parasitized by C. vestalis. Therefore, DBM larvae that had been exposed to C. vestalis were used as provisionally parasitized larvae and were reared individually after the experiment to confirm parasitism, i.e., the emergence of the parasitoid. The parasitoid emerged within 5 days, and C. vestalis larvae emerge 9 days after oviposition at 25°C (Yu et al. 2008), so the time elapsed after oviposition of the parasitoid must have been 4 ~ 9 days. DBM larvae from the study population that had never been exposed to C. vestalis were used as ‘unparasitized’ specimens.

We placed each parasitized or unparasitized fourth instar DBM larva on a piece of graph paper, and stimulated its head with a fine brush to make it wriggle backward. We measured the straight-line distance between the larval head position before and after the wriggling movement. Each larva was measured five times. The longest and average wriggling distances were compared between parasitized (n = 40) and unparasitized (n = 40) larvae. In addition, to compare the wriggling ability of parasitized DBM larvae with respect to parasitism progression by C. vestalis, we compared the longest and average wriggling distances between parasitized larvae from which the parasitoid emerged within 1 day (i.e., < 1 day before emergence; n = 21) and after that time (i.e., > 1 day before emergence; n = 19).

Hanging ability

We fixed a 1 cm × 1 cm cabbage leaf square horizontally in the air with wire and placed a parasitized or unparasitized fourth instar DBM larva on the upper leaf surface. The larval head was mechanically stimulated several times with a fine brush. The number of stimulus times it took for the larva to hang from the leaf, and also the length of the silk thread from the leaf to the larva, were recorded. We compared 1) the proportion of larvae that successfully hung down, 2) the number of stimulation events before hanging, and 3) the silk thread length between parasitized (n = 40) and unparasitized (n = 27) larvae. Larvae that failed to hang down (i.e., that fell off) were excluded from the analysis for parts 2 and 3.