The dust has settled a little from the shock wave that hit some Northern soybean fields last summer. That shock wave was soybean aphids.
Discovered in the U.S. for the first time in mid-July, their populations crashed near season's end. But by then they had been identified in 10 states: Wisconsin (where they were first discovered), Michigan, northern Illinois, Minnesota, Iowa, Indiana, Missouri, Ohio, West Virginia and Kentucky.
The explosive nature of the infestations spooked many growers and scientists. “We found hundreds per leaf and thousands per plant in some fields,” says John Wedberg, University of Wisconsin entomologist.
Adds Mike Gray, University of Illinois entomologist: “In some fields, densities of aphids were so impressive that the sugary liquid secretion known as honeydew soaked the clothes of scouts, producers and others who monitored the fields.”
The broad distribution suggests that the aphid has been in North America for more than a year, possibly going undetected for as long as three or four years. The aphids survive winters as eggs on several species of buckthorn as their winter host plant.
Since the aphids weren't discovered until mid-July, and not officially identified until about Aug. 1, it was too late in the season for scientists to carry out many needed studies. So there are still many questions to be answered.
The situation is further complicated by the complex life cycle of the pests … they have as many as 18 generations per year. And 2000 was an excellent year for some predator insects and diseases that can impact aphid numbers.
“The multicolored Asian ladybeetle was so numerous that I have never seen that many in Illinois,” says Kevin Steffey, University of Illinois entomologist. “And they are very good predators against such pests.”
In fact, a combination of natural enemies … lacewings, multicolored Asian ladybeetles and fungal pathogens … were active in soybean fields colonized by the soybean aphid.
These insects and diseases can cause aphid populations to crash dramatically. So it's necessary to check out that activity before deciding to spray. Remember, spraying an insecticide will likely kill those beneficial insects as well, says Wedberg.
“The sudden appearance of the aphids in such high numbers was a legitimate cause for concern,” Steffey says. “But in hindsight it probably did not wreak the economic havoc that some people suggested it might, possibly because of the impact of its natural enemies in 2000.”
If a heavy infestation should hit, with a low or moderate level of fungal diseases and insect parasites to hold it in check, this new threat to soybeans can wreak havoc if not sprayed with an insecticide.
Wedberg says that, in China, where soybean aphids originated, scientists infested soybean plants with the pests at increasingly higher levels. They ended up with over a 50% yield loss on beans not sprayed.
Aphid infestations that peak at the R1 to R2 growth stages of the soybeans may cause stunted plants with reduced pod and seed counts, thereby reducing yields.
Later in the season, heavily infested plants may have distorted and yellowed leaves. Charcoal-colored residue on stems, leaves and pods is sooty mold that grows on aphid honeydew. In some cases, dramatic yellowing symptoms appeared, says Christina DiFonzo, Michigan State University entomologist. It resembled potassium deficiency. In fact, those plants were potassium-deficient, she found.
“Late-planted soybean fields were the hardest hit,” notes Wedberg.
Yield loss is only part of the equation. In addition, aphids, like bean leaf beetles and some other insects, can serve as vectors for disease transmission.
In fact, China studies show the aphid is capable of transmitting a number of viruses present in the U.S. that naturally infect soybeans. They include alfalfa mosaic, soybean mosaic, bean yellow mosaic, peanut mottle, peanut stunt and peanut stripe.
Transmission of these viruses was not documented in the U.S. last summer, but it has since in greenhouse studies.
“Research data shows that the aphid does transmit a Midwestern strain of soybean mosaic and a Midwestern strain of alfalfa mosaic virus,” explains Craig Grau, University of Wisconsin plant pathologist. “So it can definitely transmit two and very probably more. We definitely think it can't transmit bean pod mottle virus, however.”
So what should you do to protect yields? Scout, scout and scout some more. Scouting drilled beans is a pain in the neck. But the payoff can be big.
Wedberg recommends growers buy a good-quality magnifying glass because these insects are tiny … probably why they weren't found and identified before last year.
The soybean aphid is a small, yellow aphid with distinct black cornicles (“tailpipes” on the rear of the abdomen). See “New Aphid Attack,” October 2000, pages 12-13.
Because no other aphid species colonize soybeans, it's safe to assume that colonies of tiny yellow aphids are indeed soybean aphids.
It's not yet known how early in the season soybean aphids begin their migration from the buckthorn in a Midwestern environment. But they build and peak between the late seedling and blooming stages.
Some growers in heavily hit areas sprayed with insecticides last year. But, at least in Wisconsin, Wedberg and Grau feel many sprayed too late and probably wasted their money. The damage was mostly done, and the populations were already crashing, likely from disease or predator insects.
Economic thresholds for spraying and timing of those sprays have yet to be established. But if these aphids are heavy in the bloom stage, and predator insects and/or fungal disease aren't evident, that would probably be the time to pull the trigger on spraying. If you spray too early, a new surge of aphids may arise. And growers can't afford multiple sprayings, especially with today's bean prices.
Preliminary insecticide control recommendations are likely available from your state university. In Wisconsin, Lorsban 4E, Ambush, Asana, dimethoate, Penncap-M and Warrior are suggested.
Treatment costs? Last year, they were $10.50/acre and up.
“We feel it's best to catch the aphids while they're on the upper part of the plant and on the topside of the leaves, before they move down the plant and onto the underside of the leaves,” Wedberg suggests.
New information will be coming out, so scientists urge you to check with your county extension offices frequently.
The bottom line for this new soybean threat: “We have no way of knowing what it's going to do in the future,” Steffey points out. “But my guess is that it's here to stay. It's likely it will be like other insect problems in that it will ebb and flow … a real problem some years and other years not.”