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Title: Impact of nutrient availability on losses due to wheat stem sawfly injury in spring wheat

item WEAVER, DAVID - Montana State University
item LAMB, PEGGY - Montana State University
item MILLER, JOHN - Montana State University
item WALSH, OLGA - Montana State University
item Delaney, Kevin
item CLAIN, JONES - Montana State University

Submitted to: Proceedings Great Plains Soil Fertility Conference
Publication Type: Proceedings
Publication Acceptance Date: 1/19/2012
Publication Date: 3/7/2012
Citation: Weaver, D.K., Lamb, P.F., Miller, J.H., Walsh, O.M., Delaney, K.J., Clain, J. 2012. Impact of nutrient availability on losses due to wheat stem sawfly injury in spring wheat. In: Schlegel A. and H. D. Bond, editors. Proceedings of the Great Plains Soil Fertility Conference, March 6-7, 2012, Denver, Colorado. 14: 141-147.

Interpretive Summary: The wheat stem sawfly is a major problem of wheat in the Northern Great Plains. However, soil nutrient status (nitrogen and phosphorus levels) of host wheat might influence the negative impact of wheat stem sawfly. Also, the impact of this pest can be reduced by parasitoid wasps that kill larvae that may prevent some yield reduction caused the sawfly pest, and can vary great across regions and even across fields in Montana. Infestation of wheat stem sawfly in small plots increased in wheat provided broadcast nitrogen as urea (120 lbs./acre), but was generally unaffected by banded phosphorus (43 lbs./acre) at one site in Montana that had low levels of parasitoids killing sawfly larvae. There was no significant impact of nitrogen and phosphorus at a second Montana site where fairly high parasitoid attack caused high wheat stem sawfly larval mortality. Patterns with stem lodging due to cutting injury generally matched patterns of wheat stem sawfly infestation level. Wheat stem sawfly infestation levels varied from < 10% to > 90% from different samples within the field at each site. There was a 8-9 bushel yield increase (~ 33% greater) at both sites due to nitrogen addition, but no affect from phosphorus fertilizer. Oddly, there was no significant yield difference between a highly yielding but highly infested hollow stem variety (‘Reeder’), moderate to high yielding but moderate infestation solid stem varieties (‘Choteau’ and ‘Ernest’), or a lower yielding variety but also with very low infestation level relatively unattractive variety (‘Conan’). There was no yield benefit from nitrogen addition when wheat stem sawfly infestation >80% because of the high yield loss caused by injury from this pest, but nitrogen addition was generally beneficial for wheat yields in the presence of wheat stem sawfly. Thus, benefits of fertilizer addition can vary depending on the nutrient considered (nitrogen or phosphorus), degree of wheat stem sawfly infestation, and degree of parasitoid caused mortality on wheat stem sawfly larvae. This makes it challenging to make fertilizer recommendations across regions where wheat stem sawfly is a serious economic pest, though nitrogen fertilization often helped to maintain or increase wheat yields.

Technical Abstract: The wheat stem sawfly (WSS) is a major pest of cereals grown in the northern Great Plains (NGP). Soil nitrogen (N) and phosphorous (P) deficiencies and their effect on cereal growth may be particularly important in areas with heavy WSS infestations where a significant percentage of soils can test medium to low for these nutrients, especially where cereals are typically grown in monoculture. Field studies were conducted to evaluate the effects of supplemental N (120 lbs acre-1) and P (43 lbs acre-1) across four spring wheat varieties at two sites planted on cereal residue. The varieties were selected for either agronomic performance or resistance to the pest via non-preference or stem-solidness. Crop samples were taken at harvest to determine infestation and lodging due to stem cutting by WSS. Yield and WSS killed by endemic parasitoids were also determined. Overall infestation by WSS was high at both sites. At one site, WSS infestation within variety was distinctly increased by the addition of N but not P. At another site only a single, most attractive variety showed a similar response to N only, with no interaction between variety and infestation. Varietal effects on WSS infestation were stronger than those due to nutrients. Lodging due to WSS stem cutting followed the pattern for infestation at the site with the clear N response and there was no effect of either N or P at the other site due to a large amount of WSS parasitism by native braconid wasps. These natural enemies were abundant at this site only. Yield showed a clear response to N at both sites across all varieties. Despite the fact that infestation could vary as much as 8-fold within a site, there was no difference in yield across varieties. There were significantly higher overall yields (˜33%) at both sites in response to supplemented N. As infestation reached severe levels (> 80%) there was no increase in yield due to the addition of N. The results indicate the complexity of making fertilizer recommendations and varietal selections in anticipation of varying WSS population size.