Potential semiochemicals of wheat (Triticum aestivum L.) induced by oviposition and feeding of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae)

Abstract

Cephus cinctus Norton (Hymenoptera: Cephidae), the wheat stem sawfly, is currently the most devastating insect pest of wheat production in Montana. Currently, no effective controls are in place to check its damage and spread throughout wheat fields in the northern Great Plains. Natural biological control of sawflies occurs primarily in the form of larval parasitoids which attack the sawfly larva in the stem; however, these parasitoids are not reliably effective in controlling sawfly populations. Insect damage induces chemical changes in plants, and often these changes are part of a defensive response to the insect injury. Some of these chemical changes are apparent in the volatile chemicals produced by the plants and may include semiochemicals used by sawflies and parasitoids. Identifying the changes in volatile production could enhance the understanding of sawfly-wheat plant-parasitoid interactions and lead to more effective control measures for the wheat stem sawfly. I investigated the differences in the volatile chemicals produced by sawfly-infested and uninfested wheat plants and endeavored to determine if those differences were qualitative or quantitative. Additionally, I wanted to determine if changes in volatile production induced by the wheat stem sawfly could be mimicked by wounding coupled with the application of sawfly cuticular wax to wheat stems or by the injection of frass-treated water into the internodes of wheat stems. Volatiles of infested and uninfested wheat plants were collected and compared, with the results indicating that sawfly damage induces quantitative changes in some volatile chemicals produced by wheat. These results are discussed regarding their context within sawfly-wheat plant-parasitoid interactions and implications for better sawfly control. Volatiles from sawfly-infested, uninfested, frass-water-injected, and pin-pricked/waxtreated plants were also collected, and differences in 11 compounds selected from the results of the 1st experiment were compared. The results of this experiment found that pin-pricked/wax-treated plants came closer to mimicking the volatile production changes induced by sawfly infestation, but neither frass-water injection nor pin-pricked/cuticular wax application reliably induced the same changes in wheat volatiles that sawfly infestation did. There was, however, a definite response of the wheat to the application of the sawfly cuticular wax, and its significance is discussed.