Behavioral and sensory responses of endemic braconid parasitoids to changes in volatile emissions induced by wheat stem sawfly, Cephus cinctus herbivory

Abstract

Bracon cephi (Gahan) and Bracon lissogaster Muesebeck are considered the most effective biological control agents of the wheat stem sawfly, Cephus cinctus Norton, probably the most serious wheat insect pest in the northern Great Plains of North America. The great difficulty in controlling this pest suggests an urgent need to improve our understanding of the chemical ecology of these parasitoids to enhance effective control of this pest. Chemical ecology has been demonstrated to be an effective tool in the control of important pests. Plants produce volatile compounds as defenses against herbivory. These compounds play an important role in host selection by herbivores and their natural enemies. Studies revealed that wheat plants infested by C. cinctus produce different amounts of specific compounds compared to uninfested plants. Synthetic compounds matching those produced by sawfly infested wheat plants were tested in three different concentrations against males and females B. cephi and B. lissogaster parasitoids using an electroantennogram system. Six compounds generated positive electrophysiological responses from the parasitoid antennae. Behavioral bioassays using the previous responsive compounds determined that three compounds were attractive to parasitoids of both sexes. To better understand parasitoid host seeking, it was important to quantify and determine the spatial distribution of these behaviorally active compounds as they were emitted from wheat plants. Volatile compounds were collected from infested and uninfested wheat plants at three different heights and two distances in the greenhouse. Greater amounts were collected from the infested plants immediately adjacent to the base of the plants. Field studies were performed to compare concentrations of volatile compounds between areas of heavy and light sawfly infestation. Greater amounts of behaviorally active volatile compounds were collected from areas of heavier sawfly infestation. In conclusion, results show that chemical ecology is a valuable tool in better understanding host seeking in this tritrophic system. This new information on volatile production and dispersal by sawfly infested plants and host seeking behavior of these parasitoids establishes the basis for future exploration of optimal blends of behaviorally active compounds used by parasitoids in host seeking, which may allow for more effective control of the wheat stem sawfly.