Direct and indirect defenses are both key aspects of plant resistance to herbivores and both can be induced by damage. Direct defenses, such as secondary chemicals, are known to deter herbivores, while indirect defenses, such as plant volatiles, often serve to attract predators and parasites to the vicinity of the herbivore. The interactions between plants, herbivores and natural enemies are often disrupted when involving exotic plants. The enemy release hypothesis leads to the prediction that the success of exotic plants is in part a function of escape from coevolved herbivores and of high resistance to resident herbivores in the nonindigenous range. Indeed, many studies have shown that exotic plants are more resistant to resident herbivores. How the third trophic level responds to exotic plant species is less studied. Here we examined how constitutive and damage-induced systemic volatiles released from three willow species, one native (Salix cinerea) and two exotics (S. viminalis and S. dasyclados), affect the behavior of the native anthocorid predator Anthocoris nemorum. Although A. nemorum is an omnivorous predator that can sustain itself on plant tissue, it has much higher performance when feeding on animal prey. We performed a series of choice tests with the predator to determine their attraction to volatiles emitted by undamaged plants and to volatiles emitted systemically after local damage by the herbivorous beetle Phratora vulgatissima, a common prey of the anthocorid
Results/Conclusions
We found that A. nemorum was attracted to volatiles from undamaged plants in the following rank order: S. cinerea > S. dasyclados > S. viminalis = air. Within species choice tests revealed that damage differentially altered the response of the predator to the volatiles. Both constitutive and damaged-induced volatiles from S. cinerea were equally attractive to the predator. In contrast, beetle feeding increased the attractiveness of the two exotic willows. Choice tests specifically comparing the two exotic species shows that beetle feeding increases the attractiveness of S. dasyclados more so than S. viminalis. (We note that A. nemorum were not attracted to P. vulgatissima only volatiles.) Given that direct resistance to P. vulgatissima is also higher for S. dasyclados, our results show that exotic species with high direct resistance may also have high indirect resistance. Moreover, these results highlight the fact that ecological mismatches not only affect herbivore behavior but also natural enemy responses.
