Plant perception of herbivore odors: Exposure of host plants to the putative pheromone of a specialist herbivore primes indirect defenses

Olfactory cues play important roles in plant defense against herbivores. Plants produce characteristic blends of volatile compounds in response to herbivore damage, and previous work has shown that these herbivore-induced plant volatiles (HIPVs) serve as olfactory cues for insects as well as other plants. Plant volatiles help recruit insect predators and parasitoids that can kill feeding herbivores, and some plants can perceive the odors given off by their insect-damaged neighbors and respond by priming their own anti-herbivore defenses. We recently discovered that some plants also utilize olfactory cues given off directly by their insect herbivores. Our previous work has shown that tall goldenrod Solidago altissima plants exposed to the putative sex attractant of the specialist gall fly Eurosta solidaginis exhibit enhanced anti-herbivore defenses, including reduced palatability and stronger induction of the defense phytohormone jasmonic acid. Here, we demonstrate that exposure to the E. solidaginis emission also primes herbivore-induced volatile production in S. altissima. This phenomenon appears to occur between co-evolved plant and insect species, as we did not observe any evidence of priming in non-host maize plants (Zea mays) exposed to the E. solidaginis emission. Additionally, a possible alternative explanation for reduced damage to S. altissima plants exposed to E. solidaginis is that the emission is directly deterrent to feeding herbivores. However, we found no evidence that emission-exposed maize or squash (Cucurbita pepo) plants repelled herbivores or were less palatable than unexposed plants. These results provide further evidence that S. altissima plants respond physiologically to the emission of a co-evolved insect herbivore and that the emission triggers host-plant defenses, but does not appear to be directly deterrent to insects.