Entomologists now confirm what Chritian Krupke suspected after he and an Illinois entomologist did an informal survey of sites full of buckthorn, host plant for soybean aphid over winter, last fall. Huge numbers of aphids reached the overwintering sites, but extremely low numbers survived.
"We expect soybean aphid number sot be low in Indiana this spring," say John Obermeyer, also a Purdue entomologist. What happened to soybean aphids last fall is rather unusual in the insect yield.
Due to very late-planted and slow maturing soybeans in southern Indiana, aphids hit southern Indiana hard very late in the season, such as in early to mid-September. Obermeyer says. Until then, insect activity for soybean aphids was extremely low. And when it picked up, it exploded in the southern half of the state, not the more traditional aphid counties in northern and northeast Indiana.
The flights were concentrated, making headlines since swarming insects actually bothered people on the golf course or taking a stroll on the sidewalk. As far as farmers go, most decided that the soybeans were too mature at that point to justify spraying the aphids.
Krupke expected to find high numbers of aphids in the overwintering sites later in the fall, he explains. But instead, they found large numbers of dead females on host plant leaves.
Apparently two things happened, Obermeyer believes. First, a parasitic disease that can kill aphid larvae ramped up at extremely high levels and attacked larvae on the plants. At the same time, there was a very limited number of male aphids, not nearly enough to mate with a large population of aphid females, that arrived at the overwintering sites. Entomologists don't have an explanation for the limited number of males that appeared there. Dying insects and the lack of males for breeding resulted in extremely low numbers of soybean aphids appearing to survive the winter on the buckthorn sites.
Obermeyer cautions that insects are unpredictable, and greatly influenced by weather conditions. What happens this spring in terms of weather patterns could impact soybean aphid numbers. Favorable weather conditions could lead to build-up of aphids during the summer.
It's also important to note that this situation isn't necessarily the same in more northern states, including Michigan and Minnesota. Those are the more typical areas where aphids live and overwinter.
"What we know now is that we don't believe soybean aphids will be around in very big numbers at the start of the season," Oberemeyer concludes.