Stem height and harvest management influence conservation biological control of wheat stem sawfly by endemic parasitoids

The wheat stem sawfly (WSS; Cephus cinctus Norton (Hymenoptera: Cephidae)) is a serious pest of wheat ( Triticum aestivum L.) in the Great Plains of North America. Two species of parasitoids, Bracon cephi Gahan and Bracon lissogaster Muesebeck, effectively attack C. cinctus; therefore, agronomic practices must strive to conserve these endemic parasitoids. Our objectives were (1) to establish distribution and within host infestation patterns of Bracon parasitoids, and (2) to study parasitoid responses to genetics, stubble height and straw management at harvest. Field surveys and harvest management studies were conducted in the southern Prairies of Canada and in Montana, USA. Harvest management consisted of three harvesting heights, (1) low, harvest near ground level to cut and thresh all material above the soil surface, (2) harvest stubble height = 15 cm, and (3) spikes only and two levels of straw management, (1) Chopped straw, or, (2) Not chopped. Most overwintering parasitoid cocoons (>80%) occurred in the bottom third (0–15 cm) of standing wheat stems, and the common harvest practice of chopping straw reduced adult parasitoid emergence. However, the incremental benefit of not chopping straw does not justify elimination of the practice as residue management is critically important in conservation farming. Compared to a low cutting height, increasing harvesting heights to 15 cm, or higher to remove only the grain spikes, increased total emergence of Bracon cephi by 40% and 60%, respectively. Thus, the optimal conservation of wheat stem sawfly parasitoids can be achieved with easily-adopted practices on-farm without compromising sustainable production principles.