Soybean aphid - I’m not kidding!
Editor’s note: This article is from the archives of the MSU Crop Advisory Team
Alerts. Check the label of any pesticide referenced to ensure your use is
You all know I love aphids, and that I can talk endlessly about them. But I really wasn’t kidding the last few weeks when I reminded you to watch fields for my favorite pest. Populations per plant in most of my field plots remained low throughout July, but the percent of infested plants was high, and has increased steadily - nearly 100 percent in some trials last week.
Within the last several weeks, aphid populations increased in southern Minnesota, producing winged migrants that have been caught in the Midwest aphid suction trap network. We definitely had an aphid flight into Michigan last week, perhaps from the infestation in Minnesota. On July 31, I found winged aphids with babies on plants on campus. By the following day, Friday, August 1, we were finding tiny aphid babies on most of the plants in an MSU efficacy trial. This means even if you didn’t have aphids, you probably have them now. Furthermore,Bruce MacKellar in southwest Michigan, reports a few fields over threshold in southwest Michigan, with a high enough population to produce winged migrants. These fields are in the infamous “K-deficient” belt that in the stretches across several counties. Potassium deficiency is a well-documented soybean aphid risk factor. Target such fields for scouting immediately to catch any infestations heading over threshold.
The soybean aphid threshold remains at 250 per plant, until plants are in the later R-stages (R5 and beyond).
Should you spray at a lower threshold because of high crop prices?
No. The aphid threshold was developed taking into consideration a range
of crop prices and input costs. Even given higher returns, lowering the
threshold doesn’t make good biological sense. We have never detected
yield differences in treated and untreated plots when fewer aphids are
present. And at lower aphid numbers, there still appears to be a battle
going on between the aphids and predators. Pulling the trigger early
wipes out the predators, so the aphids certainly win.
Should you spray at a lower threshold because of the additional impact of defoliators, such as bean leaf beetles and Japanese beetles?
No. The threshold was developed using data from plots in eight states over three years, including plots in Michigan. We did not keep bean leaf beetles or Japanese beetles out of these plots during the studies, they were there right along side the aphids. The yield from untreated plots in the study included the impact of both aphid sucking and beetle defoliation. The yields in treated plots included the impact of killing both aphids and beetles. Thus the soybean aphid threshold already includes the impact of defoliating pests, because we didn’t exclude them.
What about ignoring the threshold altogether and spraying an insurance application?
Bad idea. Killing beneficial insects can actually flare, or increase, an aphid population that was otherwise under control. Insurance applications may also flare spider mite (see accompanying article), which are making an appearance this week. Insurance applications also smoke honey bees, which have taken a beating in the last several years. See the August 4 edition of the Ohio State CORN newsletter for a cautionary article by my colleague Ron Hammond, “Spraying Insecticides on Soybeans and Honey Bees”.