Conservation Biological Control of Wheat Stem Sawfly: Applying Agroecology to Enhance Integrated Pest Management.

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            Wheat stem sawfly adapted from prairie grasses to infest wheat shortly after annual cropping began. This pest causes an average annual loss of $65 million in winter and spring wheat in Montana (data from 2008 – 2013). Bivoltine endemic parasitoid wasps, Bracon cephi and B. lissogaster, can effectively decrease infestations of this pest from severe to minimal by killing the damaging stem-mining larvae. Killing these larger larvae also prevents the direct loss of yield in infested stems. Tillage of soil and decreased harvest heights kill parasitoids and exacerbate the pest problem.  These practices are also environmentally undesirable, reducing available moisture and increasing risk of erosion.

            Exploiting multiple sources of host plant resistance by trap cropping can decrease yield loss due to this pest. Perimeter traps of attractive solid stem wheat cultivars reduce losses by killing neonate larvae in the trap and reducing infestation in the less attractive interior crop. Parasitoid foraging is concentrated on fewer surviving larvae, decreasing stem cutting and increasing standing stubble. Parasitoids are also conserved in uncultivated surrounding refuge. One example is commonly-occurring smooth brome, which is highly attractive to ovipositing females. Mortality of larvae increases as the grass matures, with late senescence allowing for surviving larvae to be utilized by second generation parasitoids.  This grass is a sink for the pest and a source of overwintered parasitoids. 

            Finally, the cropped area was originally shortgrass prairie with a diversity of grasses, forbs and shrubs suitable for pollinators.  Recent increased cultivation of historically fallow ground to pulse and oilseed crops has many agronomic benefits. Greater pollinator diversity and ecosystem services occur because these crops produce nectar during flowering. This diverse system has greater similarity to native habitat. The available nectars contain sugars that increase parasitoid longevity and may increase fecundity, which will further decrease wheat stem sawfly populations.