Only about half of the country’s soybeans have emerged, but some insect pests are already out and about, waiting for them.
Entomologists and agronomists have reported sightings of soybean aphids, bean leaf beetles and even the soybean gall midge — a new pest of soybeans that emerged last year.
Scout early and often, and don’t neglect those early-planted soybean fields that are further along, cautioned Jim Donnelly, a technical agronomist for DeKalb and Asgrow in Illinois. Some of them may be the only available soybean foliage in a landscape of late-planted or prevented planting fields, which makes them a magnet for soybean insects.
“Never let your guard down,” he said. “My recommendation is — regardless of the insect management strategy you have — to always be out there looking.”
Even with very few soybean fields up and growing in his region, Donnelly has already spotted some soybean aphids in northern Illinois.
Farther north, University of Minnesota IPM specialist Bruce Potter announced his team’s first sighting on June 7. Aphids overwinter on buckthorn each year, and Potter suspects volunteer soybeans may have helped populations along this year, too.
“In many fields, volunteer soybeans are numerous this spring,” he wrote in a university newsletter. “Depending on herbicide tolerance of the previous year’s soybean crop and corn herbicide selection, these volunteers can persist in corn. MN Soybean Checkoff-funded research has shown that when colonized, volunteer plants in corn can produce significant numbers of SBA to move to soybean fields later.”
Scout your emerged soybeans frequently for this pest, which is capable of population explosions when conditions are right, Donnelly noted. They like moderate temperatures and dry weather, Potter said.
Remember that it takes very large populations to cause damage, and that insecticides will also knock down the population of beneficial insect predators like lady bugs. The threshold for treatment through the R5 growth stage is more than 250 aphids per plant, with 80% of plants infested and populations increasing.
Overtreatment has helped produce some pyrethroid-resistant aphid populations in Minnesota. See more on that problem and aphid management from the University of Minnesota here. See Potter’s article here.
BEAN LEAF BEETLE
Bean leaf beetles are also active. Donnelly has spied some feeding, as have entomologists in Wisconsin and Kansas.
“We’ve seen some bean leaf beetle injury, especially in fields that were planted really early and fields that didn’t have insecticide treatments on the seed,” Donnelly said. “That’s fairly typical.”
The adult beetles have black markings overlaid on their yellow, orange or even reddish backs — and they love young, emerging soybeans. They will happily feed on cotyledons, stems and leaves, creating distinct round holes and gaps. Later in the season, pods can be a more damaging target.
The good news is that soybean seed treatments can provide good protection against their early season feeding, and soybean plants can bounce back from their defoliation, noted Kansas State Extension entomologist Jeff Whitworth. “Remember, these young plants are very resilient at overcoming up to about 50% defoliation in these early vegetative stages,” he wrote in a university pest newsletter.
However, the bean leaf beetle also serves as a vector for the damaging bean pod mottle virus. See more details on bean leaf beetle and bean pod mottle virus here. You can find information on managing the pest and treatment thresholds here. See Whitworth’s article here.
SOYBEAN GALL MIDGE
The soybean gall midge caught everyone off guard last year — scientists, farmers and industry. The midge larvae were discovered tunneling their way through soybean stems and causing economic damage on field edges in Iowa, Nebraska, Minnesota and South Dakota.
“By the way it was moving last summer, it seems like it’s only a matter of time until it’s here,” Donnelly said of Illinois. “We’re definitely watching for it.”
This year, Iowa State University scientists found the first gall midge adult flies on June 14 in northwest Iowa, in a field that had been infested with the insect last year. The scientists will continue trapping and alert growers to the best time to treat.
“At this point, just a few individuals in traps does not warrant a foliar insecticide,” wrote Iowa State University Extension entomologist Erin Hodgson. “But our plans are to make treatments when adult captures increase. We will be sure to keep you updated on subsequent detections and application recommendations in the future.”
The gall midge damage began in June last year, when farmers noticed wilting soybean plants, with swollen, sometimes broken, stems. Inside, they found the tiny midge larvae, ranging in color from pale and clear to bright orange. Scientists are still getting up to speed on the pest, but it seems to do its worst damage on field edges and at the base of soybean plants.
Growers who had gall midge infestations in their fields or state last year should watch their soybeans carefully.
“This insect, and its potential for damage, may be widespread in Minnesota,” Potter and UM Extension entomologist Bob Koch said in a recent university news alert. “During 2019, look for damage from the larvae. Look for wilting soybean plants, particularly on field edges adjacent to 2018 soybeans. Peel back the soybean stem epidermis and split stems to look for the small white to orange larvae.”
See the Minnesota news alert here.
See the Iowa State 2019 gall midge finding here.
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
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