Olfactory Responses of Plutella xylostella Natural Enemies to Host Pheromone, Larval Frass, and Green Leaf Cabbage Volatiles

The parasitoids Trichogramma chilonis (Hymenoptera: Trichogrammatidae) and Cotesia plutellae (Hymenoptera: Braconidae), and the predator Chrysoperla carnea (Neuroptera: Chrysopidae), are potential biological control agents for the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae). We present studies on the interactions between these bioagents and various host-associated volatiles using a Y olfactometer. T. chilonis was attracted to a synthetic pheromone blend (Z11–16:Ald, Z11–16:Ac, and Z11–16:OH in a 1:1:0.01 ratio), to Z11–16:Ac alone, and to a 1:1 blend of Z11–16:Ac and Z11–16:Ald. C. plutellae responded to the blend and to Z11–16:Ac and Z11–16:Ald. Male and female C. carnea responded to the blend and to a 1:1 blend of the major components of the pheromone, although no response was elicited by single compounds. Among the four host larval frass volatiles tested (dipropyl disulfide, dimethyl disulfide, allyl isothiocyanate, and dimethyl trisulfide), only allyl isothiocyanate elicited significant responses in the parasitoids and predator, but C. plutellae and both sexes of C. carnea did respond to all four volatiles. Among the green leaf volatiles of cabbage (Brassica oleracea subsp. capitata), only Z3–6:Ac elicited significant responses from T. chilonis, C. plutellae, and C. carnea, but C. plutellae also responded to E2–6:Ald and Z3–6:OH. When these volatiles were blended with the pheromone, the responses were similar to those elicited by the pheromone alone, except for C. carnea males, which had an increased response. The effect of temperature on the response of the biological agents to a mixture of the pheromone blend and Z3–6:Ac was also studied. T. chilonis was attracted at temperatures of 25–35°C, while C. plutellae and C. carnea responded optimally at 30–35°C and 20–25°C, respectively. These results indicate that the sex pheromone and larval frass volatiles from the diamondback moth, as well as volatile compounds from cabbage, may be used by these natural enemies to locate their diamondback moth host.