Unexpectedly high levels of parasitism of wheat stem sawfly larvae in post-cutting diapause chambers

Authors: Rand, Tatyana; Waters, Debra; Shanower, Thomas

Submitted to: The Canadian Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2011
Publication Date: 9/1/2011
Citation: Rand, T.A., Waters, D.K., Shanower, T.G. 2011. Unexpectedly high levels of parasitism of wheat stem sawfly larvae in post-cutting diapause chambers. The Canadian Entomologist. 143(5): 455-459.

Interpretive Summary: Previous work suggests that native parasitoid wasps that attack the wheat stem sawfly, a major pest of wheat in the northern Great Plains of North America, rarely attack larvae after they move to the base of the stem, and cut it to form overwintering chambers. As a consequence, it has been assumed that a lack of late season hosts after wheat harvest, when the second generation of native parasitoids are generally active, is a major factor limiting their numbers and potential success as biological controls. Here we examined rates of Bracon spp. parasitism of C. cinctus larvae in overwintering chambers collected over 8 years in eastern Montana, USA. We found that rates of larval parasitism in these chambers can be substantial, reaching a max of 46%, contrasting dramatically with previous findings from Canada which suggested that parasitism never exceeded 2.5%. Parasitism rates exceeded the previous maximum in 75% of the site/years examined in our study. Our results conclusively demonstrate that sawfly larvae are suitable hosts for the native Bracon spp. parasitoids, even after stem cutting to form overwintering chambers. They further suggest that total parasitism rates may be underestimated by sampling stems prior to harvest. Further research along these lines will be important in understanding what limits native parsitoid numbers, and ultimately how to manage systems to augment these potentially important biological controls of the wheat stem sawfly.

Technical Abstract: Previous work suggests that native braconid parasitoids of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), rarely attack larvae after they move to the base of the stem, and cut it to form diapause chambers. As a consequence, it has been assumed that a lack of late season hosts after wheat harvest, when the second generation of native parasitoids are generally active, is a major factor limiting their populations and potential success as biological controls. Here we examined rates of Bracon spp. (Hymenoptera: Braconidae), parasitism of C. cinctus larvae in diapause chambers collected over 8 years in Montana, USA. We found that rates of larval parasitism in diapause chambers can be substantial, reaching a max of 46%, contrasting dramatically with previous findings that parasitism never exceeded 2.5%. Parasitism rates exceeded the previous maximum in 75% of the site/years examined. Our results conclusively demonstrate that sawfly larvae are suitable hosts for the native Bracon spp. parasitoids, even after the formation of diapause chambers. They further suggest that total parasitism rates may be underestimated by sampling stems prior to harvest. Further research along these lines will be important in understanding Bracon spp. population dynamics, and ultimately managing these potentially important biological controls of the wheat stem sawfly.