Advancing biological control of the wheat stem sawfly (Cephus cinctus) – new strategies in a 100 year struggle to manage a costly pest in the Northern Great Plains

Authors: Portman, Scott; Jaronski, Stefan; WEAVER, DAVID - Montana State University; REDDY, GADI - Montana State University

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2018
Publication Date: 3/14/2018
Citation: Portman, S.L., Jaronski, S., Weaver, D.K., Reddy, G.V. 2018. Advancing biological control of the wheat stem sawfly (Cephus cinctus) – new strategies in a 100 year struggle to manage a costly pest in the Northern Great Plains. Biological Control. 111(3):85-91. https://doi.org/10.1093/aesa/say002.
DOI: https://doi.org/10.1093/aesa/say002

Interpretive Summary: The wheat stem sawfly (WSS) is a destructive insect pest of wheat, barely, and rye in the Northern Great Plains. WSS infestation levels >70% have been reported and economic loss from crop damage caused by this insect has been estimated at $80 million (USD) per year in the state of Montana alone. Contact insecticides are not effective against WSS because the grubs and pupae reside inside the plant stem, and therefore, protected from exposure to these chemicals. For decades, University and USDA-ARS scientists have been working to control WSS, however, no sure-fire method has been developed. Historically, management of WSS has focused on developing resistant (solid-stem) varieties of wheat and/or tillage practices that attempt to destroy the grubs and pupae. Unfortunately, solid-stem cultivars have been unsuccessful at controlling WSS, and there is evidence that WSS can develop better in some solid-stem varieties. Biological control offers the best hope for sustainable management of WSS. This article synthesizes current WSS biological control research and discusses new ideas for improving and advancing the use of three biological control systems, parasitic wasps, predacious roundworms, and insect-killing fungi. In particular, this article discusses using inoculative releases of parasitic wasps and habitat management techniques to increase wasp abundance, the use of adjuvants and chemical protectants to boost survival and prey locating ability of predacious roundworms, and improving the plant colonizing capability of insect-killing fungi. It is unlikely that a single agent will produce the desired results (i.e. magic bullet), but increasing pressure from these three biological controls can work together to produce significant reductions in the WSS population - eventually resulting in population declines that fall below economic threshold. Reducing populations of WSS below economic damage levels will be of great benefit to grain growers of the Northern Great Plains and will lead to higher economic returns on grain production.

Technical Abstract: The wheat stem sawfly (Cephus cinctus, Norton), has become a destructive pest of cereal crops in the Northern Great Plains, including: Montana, North Dakota, South Dakota, Minnesota, Saskatchewan, Alberta, and Manitoba. Wheat stem sawflies (WSS) typically infest wheat (Triticum sp.), but they also damage barley (Hordeum vulgare L.) and rye (Secale cereal L.). WSS larvae are “stem borers” that feed on the parenchyma tissue of wheat stems. WSS infestation levels >70% have been reported and economic loss from crop damage caused by this insect has been estimated at $80 million (USD) per year in the state of Montana alone. Contact insecticides are ineffective against WSS because the larvae and pupae reside inside the plant stem, and therefore, protected from exposure to these chemicals. Historically, management of WSS has focused on developing resistant (solid-stem) varieties of wheat and/or tillage practices that attempt to destroy the diapausing larvae. However, the effectiveness of solid-stem cultivars at reducing WSS has been inconstant, and there is anecdotal evidence that suggests that some populations of WSS are developing more successfully in some solid-stem cultivars. To date, existing control strategies have been unsuccessful at reducing the numbers of this persistent pest below levels that do not produce significant economic losses to growers, thus, biological control might be the best option for achieving economical and sustainable management of WSS. This article synthesizes current WSS biological control research and discusses new ideas for improving and advancing the use of three biological control systems, parasitoids, entomopathogenic nematodes (EPNs), and entomopathogenic fungi. It is unlikely that a single agent will produce the desired results (i.e. magic bullet), but increased pressure from different biological controls, such as parasitoids, EPNs, and entomopathogenic fungi, can work together synergistically to produce significant reductions in WSS populations. Eradication of WSS is not a realistic goal, but it is probable that WSS populations will begin to decline with continued advancements to biological control methods, eventually resulting in population declines that fall below economic threshold.