This Midge Means Business

Researchers are gaining on this galling insect.

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Gall midge larvae shown tunneling through stems of soybean plants in infested fields. (Progressive Farmer image by Justin McMechan, University of Nebraska)

Like many in agriculture, Justin McMechan hopes 2019 was an outlier -- a freak season that won't be repeated anytime soon.


The University of Nebraska Extension entomologist is wrapping up a frustrating season battling the soybean gall midge. The insect first came into his sights in 2018, when it infested field edges in four Midwestern states. The adult flies laid their eggs on the stems of soybean plants, and their tiny larvae tunneled through those stems, weakening and killing plants in their wake.


In 2019, the gall midge returned with a challenging new twist. The adult flies emerged almost daily from fields for three weeks straight, outlasting insecticide applications and laying eggs continuously in June and July. Since the adult flies are the primary target of insecticide applications, this new pattern of behavior frustrated growers and researchers alike, McMechan says.


"With this really long emergence period, we're going to have difficulty controlling it with insecticides," he points out. "It's really hard to cover that entire window of emergence. I'm really hoping that what we saw this year was not normal. Perhaps the unusual start to the season expanded its emergence."


The good news is that the season gave scientists plenty of opportunities to study the gall midge and its habits. Here's what they've learned.


LITTLE PEST, LARGE LOSSES


The soybean gall midge appears able to survive the frigid winters of Iowa, Minnesota, Nebraska and South Dakota. Adult flies were caught emerging from formerly infested fields in these states as early as mid-June.


If you find one soybean gall midge fly in your field's traps, chances are good you have thousands. "For every adult we catch in our cages, we calculate that's about 2,000 adults per acre if we can assume the field edge is fully infested and normally distributed, which are big assumptions," McMechan says.


Once the eggs hatch and larvae feed on the inside of the stem, yield losses can rack up quickly, though the most extensive losses are mostly limited to the field borders. Growers and scientists have reported 90 to 100% yield loss in infested field edges, with total field-wide yield losses averaging out closer to 17 to 30%.

OLDER BEANS ONLY


The gall midge likes its soybeans a little on the older side. For the past two years in Nebraska, when adults emerge in a soybean field younger than the V2 stage of growth, the pest seems unable to infest the plants, McMechan says.


"But, on a plant in the V3 stage or later, you'll notice some cracking that forms at the bottom of the plant--it's part of the natural expansion of the stem," McMechan explains. "We think that's a spot where the gall midge likes to lay its eggs."


Hail damage also makes for a good entryway into the soybean stem. "We could find gall midge larvae higher in the plants' stem because of hail damage," he says. "They liked to use those damaged spots to get in."


USUAL TACTICS THWARTED


The soybean gall midge does its best work within neighboring fields in a corn/soybean rotation, McMechan says. When the infested soybean field is rotated to corn, and its neighboring corn field is rotated to beans, the emerging midge flies simply migrate to the new soybean field and infest it. When the fields switch crops again the following year, the midge flies will also switch.


Given the prevalence of this rotation in the Corn Belt, "on a landscape level, it's almost impossible to prevent," McMechan explains.


Yes, growers could simply move to continuous corn, but that certainly won't solve their insect challenges, he adds. "I would hate to see growers move to a corn-on-corn system and risk the corn pests that we see," he says.


Nor does tillage seem to discourage the midge's emergence. Nebraska scientists tracked its survival in fields with both fall or spring tillage operations. "We were really hoping it would cause some mortality," McMechan says. "But, it actually extended the emergence period by disrupting their soil distribution--we think maybe the shallow ones emerged sooner, and the more deeply buried ones emerged later."


Top Research Priorities

Scientists from both academia and industry are working hard to understand this mysterious new midge and its habits, some of which have baffled entomologists and taxonomists familiar with midge species. "It's been breaking rules left and right," entomologist Justin McMechan says.


Here are some of the top research priorities:


> What insecticides best control it? Scientists are trying different active ingredients and exploring different forms of application, he says.


> Do we have varietal resistance or tolerance out there? Nebraska scientists are hoping to test the soybean gall midge on all 700 lines of soybeans they have available, McMechan says. Companies are testing their commercial lines, too.


> Are all these infestations from the same insect? Scientists in Minnesota are finding gall midge infestations with distinctly different symptoms. It may take genetic testing to nail down if they are variants within the soybean gall midge or a different species altogether, he says.


> Is disease playing a role? Infected soybean stems turn dark black, rot and die, which suggests secondary diseases could be at play, McMechan notes. Entomologists are working with plant pathologists to see if fungicide applications could perhaps help mitigate some gall midge injury.

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Emily Unglesbee