By Jonathan Kleinjan
Many wheat producers in South Dakota have adopted more intensive management practices in the last few years, including an early season application of fungicide and, in some cases, insecticide.
While there are several insect species capable of impacting production in SD wheat fields, the most prevalent found early in the growing season have been bird cherry oat aphids, and English grain aphids. The aphids do not overwinter in the state and migrate in from the south and most early-season damage is caused by direct feeding on foliage. The early season threshold for chemical application is 20-30 bird cherry oats per stem and 30-50 English grain aphids per stem. However, in most seasons, aphid populations in the region are very low, slow to develop, or are simply controlled by natural events such as a late frost or beneficial insects.
Pyrethroid class insecticides are commonly used to treat these insects. One pyrethroid, cyfluthrin, was tested at experiment station trials conducted in Minot, North Dakota. The trials demonstrated a general reduction in aphid populations when plots were treated with early insecticide, but the reductions did not have a significant impact on grain yields. In another study, performed in eastern ND, a 1.5 bu/acre increase in spring wheat yields was observed when adding insecticide to an application of fungicide just prior to stem elongation, despite a lack of insect pressure. In some cases, synergistic effects between fungicide and insecticide products, specifically, pyrethroids, have been noticed to increase the toxicity of insecticides. However, yield increases in an insect-free environment is most likely the result of a beneficial prophylactic effect for insecticide on spring wheat, rather than a synergistic increase in fungicide efficacy.
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