Effect of different wheat production systems on the presence of two parasitoids (Hymenoptera: Aphelinidae; Braconidae) of the Russian wheat aphid in the North American Great Plains

Diuraphis noxia (the Russian wheat aphid) severely damages winter wheat, which is typically grown in alternating 30-70 m wide strips of wheat and fallow ground (traditional rotation) in the western region of the North American Great Plains. Diversification of this production system occurs by adding a spring sown plant such as sunflower into the strip rotation (diversified rotation). Converting idle wheat land to grassland adds to plant diversity. Wheat production in the North American Great Plains is of an impressive scale (for each field, possibly 10 or more strips of several kilometers in length), but plant diversity occurs, particularly in the western region where diversified rotations and idle wheat land conversion to grassland are becoming more common. The 'Enemies Hypothesis' predicts a greater number of polyphagous natural enemies of insects in areas of high plant diversity. This hypothesis appeared to be valid for the 'specialist' aphid parasitoids Diaeretiella rapae and Aphelinus albipodus that were released to control D. noxia. A. albipodus was often more abundant along edges of wheat and sunflower strips of the diversified rotation than along edges of wheat of the traditional rotation. D. rapae was less abundant and principally found on sunflower and wheat of the diversified rotation. Parasitoids were often found in grassland adjacent to wheat at levels similar to those of the diversified rotation. The abundance of five aphid species on wheat was low in both rotations, and aphids on sunflower did not pose a threat to wheat health. (C) 2002 Elsevier Science B.V. All rights reserved.