Differential parasitism by a generalist parasitoid is mediated by volatile organic chemicals of the herbivore's host

A native parasitoid, Apanteles polychrosidis, shifted hosts to exploit the invasive leaf miner, Caloptilia fraxinella, on horticultural ash, Fraxinus spp. in Edmonton, AB, Canada. A. polychrosidis has the potential to control populations of the invasive leaf miner, and parasitism rates are studied on two host plants, black ash, F. nigra, and green ash, F. pennsylvanica. Parasitism by A. polychrosidis of C. fraxinella differs on the two ash species. Parasitism is independent of leaf miner density on black ash, but is negatively density dependent on green ash. On green but not black ash, the host plant appears to mediate the numerical response of the parasitoid. Parasitoids are less effective at high host densities on green ash which may be because foraging behavior is not enhanced by leaf miner activity on green ash. Thirteen volatile organic chemicals (VOCs) released by green ash are detected by the antennae of A. polychrosidis, and eleven are identified here. The potential for host location mediated by VOCs is examined with olfactometer studies. Parasitoid females are differentially attracted to volatile cues of each ash species. Undamaged and mechanically damaged green ash leaflets attract female parasitoids, but in black ash, only leaflets mined by host larvae are attractive to parasitoids in olfactometer tests. These results suggest that A. polychrosidis uses host location cues induced by feeding damage on black ash but not on green ash. This differential attraction to VOCs from each ash species may mediate the differential parasitism observed in field studies.