Managing the Midge
There may be five letters in “midge,” but you can bet there were more than a few four-letter words tossed in the direction of this troublesome insect last summer by sunflower growers in certain eastern North Dakota/northwestern Minnesota locales.
Fresh on the heels of rising populations in 1996, the sunflower midge caused more consternation and damage in the Red River Valley and outlying environs in 1997 than it has for at least 15 years. While most of the region did not incur serious midge infestations, some pockets (see map) contained numerous fields with significant yield loss. A few fields even were aban-doned due to extensive midge depredation.
Why, after more than a decade of minimal activity, did the midge return in the mid-’90s? And why did the most severe infestations occur where they did in 1997?
First, the midge never did “disappear.” Along with cultivated sunflower, the midge is also found on wild sunflower throughout the Great Plains — even as far south as Texas. Since eastern North Dakota/ western Minnesota/southern Manitoba has the longest continuous span of cultivated sunflower production, it’s natural that this area runs the most risk of midge problems.
Weather played a big role in magnifying the problem the past couple years, adds North Dakota State University entomologist Gary Brewer. “In both 1996 and 1997 we had fairly wet soils during June and into early July,” he notes. “That’s when this insect is undergoing its critical develop-mental stage: still in the ground and developing from the overwintering larvae to the pupal and then adult stage.”
The emerging adult is a very small, delicate insect that struggles to get out of the ground. So if soils are hard, dry and/or crusted, the midge “doesn’t make that transition very effectively, and its popula-tion is going to be lower,” Brewer points out. The ample soil moisture conditions of the past couple years “have favored the survival and strong emergence of this insect from the soil,” he says.
While pests such as the sunflower beetle, seed weevil and banded moth can be effectively managed with timely insecticide applications, that’s not the case with the midge. The problem is not that insecticides don’t work on the midge; actually, this insect is very susceptible to a number of products. The dilemma lies in getting the chemical into actual contact with the insect.
If one is trying to control the adult midge, timing is everything. The adults are present on the plant for only one or two days, so by the time the producer becomes aware of the problem and arranges for spraying operations, it’s usually too late. Also, since the sunflower plant remains susceptible through the bud stage, residual from one or more insecticide treatments — no matter how timely — will not be around to kill the bulk of adults which may emerge over the course of that extended period.
The larvae remain on the plant much longer; but the problem there is one of location. “Midge larvae are buried down between the bracts, between the seeds,” Brewer explains. “The insecticide covers the top of the plant, but it doesn’t get down to where the larvae are located.”
What solutions exist for the control of this difficult insect? There are no “magic bullets,” Brewer emphasizes, and the unpredictability of the midge’s emergence patterns and population levels complicate its management. Here’s how various control measures presently fare:
• Insecticides — Potentially, any insecticide, including older ones like malathion, will effectively kill the midge. The problem lies in getting the chemical to the insect — or, conversely, devising a way to get the insect to the chemical. Researchers are exploring both avenues.
“Perhaps we could mix the insecticide with some sort of spreader,” Brewer ventures, “so the insecticide would move on the face of the plant head and down among the buds and bracts,” thereby contacting and killing the midge larvae. Another possibility would consist of a program where the insecticide would be applied with a substance which causes the larvae to move “out into the open.”
Under either scenario, accurate prediction of midge emergence and timely scouting would be essential, as the chemical obviously would have to be applied while the insect populations were at or near their peak. “Prediction is especially critical if we want to control the adult midge,” Brewer stresses, due to their brief lifespan and the desire to control them prior to significant egg laying.
• Cultural Practices — The midge overwinters in the soil of the prior year’s sunflower fields and then migrates to nearby current fields. It is a weak migrator, probably not traveling much more than a mile or so. If possible, Brewer suggests not planting sunflower adjacent to fields which had significant midge infestation the previous year.
Cultivation is not a viable control measure, for several reasons: unpredict-ability of midge emergence from year to year; incompatibility with some tillage systems; and the need for all fields in a given area to be cultivated for the approach to have any benefit. “Finally, we don’t know how effective cultivation is in controlling the midge,” Brewer states.
Staggering the planting dates of various sunflower fields can help spread out risk. Since midge emergence patterns are so dependent upon soil moisture and growing season weather, it’s impossible to know in advance whether early or late plantings have a better chance of escaping midge damage. By bringing different fields into bud at differing times, at least some of the fields should be able to escape infestation during that critical stage.
• Hybrid Selection — A number of commercial hybrids do possess tolerance to the sunflower midge under light to moderate infestation levels. Multiple-location tests over the past two years, conducted by Brewer and USDA entomologist Larry Charlet, indicated significant tolerance among selected hybrids from several seed companies. On a relative midge rating scale where “1” indicates an average response and a number smaller than “1” indicating tolerance, more than a dozen of the tested hybrids ranked well under “1.”
There are, as yet, no hybrids which can be classified as “resistant” to the midge. Any of the current hybrids will incur yield damage under heavy infestation levels. However, Brewer encourages concerned producers to review university trial data and talk with their seed supplier and/or extension agent regarding which hybrids possess tolerance to this insect. There are, he reiterates, some hybrids that are significantly more tolerant than others.
“Our best long-term approach to midge management is improving tolerance in our sunflower hybrids,” Brewer concludes. “Seed companies are very interested in this, and we’re working with them. I think we’ll be able to improve the tolerance level and have it show up in more of the hybrids.” — Don Lilleboe
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