Systemic insecticide efficacy on wheat stem sawfly (Cephus cintus)

Cephus cinctus (Hymenoptera: Cephidae), or wheat stem sawfly (WSS) is an economically destructive pest of the great plains areas of the Midwest, and is a major contributor to crop loss in Montana state. Female WSS use serrated ovipositors to insert single eggs into relatively newer stem areas just above the joint at the early jointing stage (approx Feekes 6). Larvae cause substantial crop damage by internally feeding, which girdles and weakens the stem. Damage leaves stems highly susceptible to lodging (falling over) and therefore makes it difficult for producers to harvest the crop. Larvae overwinter in the girdled stem stubble, emerging in spring for approximately 5-7 days to mate. Targeting this pest with insecticides is extremely difficult to accomplish due to the often unpredictable and short time adults are active. With the concealed nature of the larvae within wheat stems and lack of reliable thresholds for adult activity, there is little potential for the use of contact foliar applications. Soil incorporation of insecticides at planting has been ineffective due to the late season oviposition timing of WSS exceeding the chemical persistence period in soil. To date there are no recorded insecticides that show control on WSS except for some organophosphates, whose persistence and environmental toxicity make them unattractive choices for modern agriculture.

We tested six systemic insecticides not traditionally used in spring wheat sawfly management on two wheat varieties in a Randomized Complete Block design. Insecticides were applied by backpack sprayer with calibrated and controlled application speed and output of 40 GPA across all plots. Application was timed prior to stem jointing to have systemic persistence in the plant at the time of larval feeding. A visual % lodging rating by plot was conducted at plant maturity and harvest to determine pest impact on yield.

Results of this study were inconclusive due to an uncharacteristic drought that reduced pest presence and also reduced pest damage independently of the insecticides tested. Lodging percent ratings by treatment were statistically insignificant. Repeating this study under more typical environmental conditions could potentially yield more conclusive results, resulting in a better understanding of the efficacy of a less environmentally toxic, single application method for reducing crop loss.