Effects of Landscape Composition on Wheat Stem Sawfly (Hymenoptera: Cephidae) and Its Associated Braconid Parasitoids

Several agroecological and integrated pest management strategies focus on landscape management to increase complexity and foster biodiversity. However, landscape complexity does not always enhance biological control and in some cases may lead to increased pest populations. We examined the prevalence of two Bracon parasitoids, Bracon cephi Gahan and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae), and their host the wheat stem sawfly Cephus cinctus Norton, a major pest of wheat. We assessed the degree of noncrop and crop host plant use and responses to landscape composition. We found no instances of parasitism by either Bracon species in our three-year, statewide winter wheat survey but found small populations of Bracon in noncrop landscapes throughout eastern and western Colorado. We used model selection to examine how local (500 m scale) and landscape (5 km scale) cover of suitable noncrop and crop habitats potentially affects abundances of Bracon and wheat stem sawfly. Our best fit model for wheat stem sawfly suggests that a decrease in noncrop cover at the landscape scale leads to an increase in wheat stem sawfly infestation. Our best fit model for Bracon parasitism suggests an increase in wheat cover at the local level results in the greatest increase in the odds of parasitism by either species of Bracon. Herbaceous cover at local and landscape scales were also significant predictors of Bracon parasitism. The results of this study suggest that pest and natural enemies respond differently to landscape composition and these responses should be evaluated before management decisions are made.