The Impact of Herbivore-Induced Plant Volatiles on Parasitoid Foraging Success: A General Deterministic Model

Abstract

Parasitoids respond to volatiles that plants produce when injured by herbivores. A considerable body of literature addresses the chemical pathways of herbivore-induced volatile production. However, there is almost no theory or data on how timing of volatile release in relationship to host availability for parasitization impacts the utility of these cues to parasitoids and on the extent that this volatile release timing might increase or decrease the percent of herbivores that become parasitized. This kind of information is critical in judging the benefits that might accrue from a breeding program aimed at enhancing herbivore-responsive volatile production. We developed a general model to begin examining this issue by using available parameters from two tritrophic systems. The model uses herbivore oviposition, development, and mortality rates, linked to a range of plant volatile induction and cessation periods for calculating the proportion of plants in a field that are (1) not producing volatiles but occupied by suitable herbivore hosts, (2) producing volatiles and occupied by suitable herbivore hosts, (3) producing volatiles but not occupied by suitable herbivore hosts, and (4) not producing volatiles and not occupied by suitable herbivore hosts. The impact of the plant volatiles on parasitoid foraging success is then determined by comparing the expected number of hosts parasitized when the parasitoid focuses solely on the volatile-producing plants to when it forages randomly among all plants. Under some conditions, parasitoids can attack three times more herbivores if they focus on volatile-producing plants. However, when we simulate plants that take several days to cease volatile production after pupation or death of the herbivore, parasitization rate does not increase when parasitoids use volatiles as cues. The utility of the volatile cues is consistently greater when a smaller proportion of plants is occupied by herbivores, indicating that their usefulness may be reduced to zero in fields saturated with volatiles.