Abstract
In the arms race between plants, herbivores, and their natural enemies, specialized herbivores may use plant defenses for their own benefit, and variation in plant traits may affect the benefits that herbivores derive from these defenses. Pieris brassicae is a specialist herbivore of plants containing glucosinolates, a specific class of defensive secondary metabolites. Caterpillars of P. brassicae are known to actively spit on attacking natural enemies, including their main parasitoid, the braconid wasp Cotesia glomerata. Here, we tested the hypothesis that variation in the secondary metabolites of host plants affects the efficacy of caterpillar regurgitant as an anti‐predator defense. Using a total of 10 host plants with different glucosinolate profiles, we first studied natural regurgitation events of caterpillars on parasitoids. We then studied manual applications of water or regurgitant on parasitoids during parasitization events. Results from natural regurgitation events revealed that parasitoids spent more time grooming after attack when foraging on radish and nasturtium than on Brassica spp., and when the regurgitant came in contact with the wings rather than any other body part. Results from manual applications of regurgitant showed that all parameters of parasitoid behavior (initial attack duration, attack interruption, grooming time, and likelihood of a second attack) were more affected when regurgitant was applied rather than water. The proportion of parasitoids re‐attacking a caterpillar within 15 min was the lowest when regurgitant originated from radish‐fed caterpillars. However, we found no correlation between glucosinolate content and regurgitant effects, and parasitoid behavior was equally affected when regurgitant originated from a glucosinolate‐deficient Arabidopsis thaliana mutant line. In conclusion, host plant affects to a certain extent the efficacy of spit from P. brassicae caterpillars as a defense against parasitoids, but this is not due to glucosinolate content. The nature of the defensive compounds present in the spit remains to be determined, and the ecological relevance of this anti‐predator defense needs to be further evaluated in the field.
Highlights
The idea that plants affect their consumers, and the interactions between the second and third trophic levels has received considerable attention since Price et al.’s seminal article in 1980
To test the hypothesis that host plant secondary chemistry affects the efficacy of regurgitant as an anti-predator defense in P. brassicae, and to investigate whether regurgitant efficacy is directly linked to glucosinolate concentration in plants, we conducted two experiments under laboratory conditions with young P. brassicae caterpillars and C. glomerata parasitoids
Our working hypothesis was that C. glomerata behavior should be more strongly impacted by regurgitant originating from plants with higher glucosinolate concentrations, and we tested this prediction at three different levels: across all host plants, within the three B. rapa populations, and between the two A. thaliana lines
Summary
The idea that plants affect their consumers, and the interactions between the second and third trophic levels has received considerable attention since Price et al.’s seminal article in 1980. Studies on the effects of plant secondary chemistry on anti- predator defenses have almost exclusively focused on sequestered toxins (Hartmann, 2004; Heckel, 2014), largely missing out on other strategies deployed by herbivores to fend off attackers (Karban & Agrawal, 2002) Most of these studies concern just a few host plants with a limited range of chemical profiles (Calcagno, Avila, Rudman, Otero, & Alonso-Amelot, 2004; Francis et al, 2001; Sword, 2001). To test the hypothesis that host plant secondary chemistry affects the efficacy of regurgitant as an anti-predator defense in P. brassicae, and to investigate whether regurgitant efficacy is directly linked to glucosinolate concentration in plants, we conducted two experiments under laboratory conditions with young P. brassicae caterpillars and C. glomerata parasitoids. Our working hypothesis was that C. glomerata behavior should be more strongly impacted by regurgitant originating from plants with higher glucosinolate concentrations, and we tested this prediction at three different levels: across all host plants, within the three B. rapa populations, and between the two A. thaliana lines
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
