Smooth brome is an attractive alternative host for the wheat stem sawfly

Background/Question/Methods

Insect pests cause over $500 billion of annual damage to agricultural crops. Synthetic insecticides are the conventional pest control strategy in large-scale agroecosystems, but growing resistance in insect populations highlights the need for a more sustainable strategy. Trap cropping is an ecologically-based pest management practice using the concept of alternative host plants. If an alternative host is more attractive than the crop, then insects are lured out of crops and concentrated in the trap crop, where they can be efficiently managed through a number of low impact strategies.

The wheat stem sawfly (Cephus cinctus) causes $300 million of annual damage in wheat fields throughout the Northern Great Plains of North America. Insecticides are not effective in this pest system, but trap cropping may be an alternative strategy. Smooth brome (Bromus inermis) may be a viable trap crop, but no empirical data have been presented to demonstrate its effectiveness. To address this issue, we used a combination of laboratory analysis and field sampling. In the lab, we identified and quantified attractive volatile compounds from wheat and smooth brome using GC-MS. In the field, we sampled smooth brome adjacent to wheat fields and monitored the number of eggs, larvae, and parasitoids in each species through the growing season.

Results/Conclusions

Smooth brome produces the same attractive green leaf volatile compounds as wheat, but at 4- to 8-fold higher concentrations. In the field, a greater proportion of smooth brome stems are infested with wheat stem sawfly eggs than wheat stems in adjacent fields. Among infested stems, smooth brome had more eggs per stem than wheat.

These results suggest that smooth brome could be an effective trap crop for wheat stem sawfly management. Under field conditions, smooth brome is an attractive alternative host and a preferred oviposition site for sawflies. The volatile profile data suggest this preference may be due to chemical attractants. Future work will include sawfly preference trials to determine the role of volatile chemicals in host plant selection and life table analysis to assay sawfly survival in smooth brome and wheat. The results of this study will inform sawfly management practices and may significantly reduce annual wheat loss to sawflies. The development of ecologically-based pest management strategies, such as trap cropping, in other systems will be critical to the productivity and sustainability of agricultural systems in the future.