Auburn University Extension entomologist Kathy Flanders is worried about the threat of Hessian fly on wheat this year, and the main source of her worry is a long-forgotten storm from 2011.
“Many growers remember we had high populations of Hessian fly in 2008 and 2009, and we had a lot of damage from this insect,” said Flanders at the recent Central Alabama Corn Production Meeting in Autaugaville.
“I’m worried because of Tropical Storm Lee,” she says. “The Hessian fly spends its summers in wheat stubble, such as in double-cropped soybeans. Along about the end of August and the first part of September, the first generation of this pest will come out. If it’s really dry at that time, it may interfere and you won’t have as many Hessian fly for the first generation.
“But Tropical Storm Lee came through Alabama last year just in time to wet the soils across the state, so if there were any pupae down there that were summering in soil, in the stubble itself, or just under the soil surface, they have a good chance of coming out.
“Then they’ll be looking for volunteer wheat, wild hosts, early planted wheat, or wildlife plots on which to reproduce. They’ll probably have time for at least a generation before they try and fly on to commercial wheat crops.”
The Hessian fly looks like a small mosquito, says Flanders, and when you’re scouting the fields, it’s hard to tell the difference between it and a gnat. “The larvae are what cause the damage. The eggs are laid on the leaves, and the maggots hatch out and make their way down the base of the plants, hiding out along the leaf sheaves and doing their feeding. We have several generations each year of this pest,” she says.
If you planted wheat early, Hessian fly might be in the field in the form of the larvae or pupae, she says. However, the first good killing frost usually kills off any adults that still might be hanging around.
(A big reason to worry about the Hessian fly in the Southeast is that varietal resistance seems to be breaking down. For an in-depth look at that situation, see http://southeastfarmpress.com/grains/hessian-fly-resistance-threat-southeast-wheat and http://southeastfarmpress.com/grains/wheat-resistance-genes-failing-against-hessian-fly.)
How do you know if you have an early season problem with Hessian fly? Flanders says when the maggots come out during the early part of the season, when the wheat is coming up, they lay eggs on the leaf. They then make their way down to the base of the plant, which is underneath the ground.
“When it starts feeding, it stunts the plants, and plants don’t emerge normally. If only half of the leaf blade is coming from the ground, it could be a sign that you have a problem. Also, if you have skips in your stand, it could be a sign,” she says.
Tillers may turn brown
Later on in the season, when plants start to tiller, you may see a thin stand. “It stunts the growth of various parts, killing tillers and plants. Some of your tillers may start turning brown.”
Recently, a pheromone was developed for these small flies so they can be trapped and monitored, she says.
“We started putting out traps last year, to see if we could see when the adults were out flying. If we can, we might be able to spray an insecticide and kill them, preventing any further generations of the pest in the springtime.”
Last year, she says, they came in from around the end of February to about March 14. “We can kill the adult Hessian fly, but once those adults get down behind the leaf sheaths, there’s nothing we can do about them.”
Growers don’t have an effective seed treatment for Hessian fly, says Flanders, and the insecticides give only 50 to 60 percent control.
“We don’t have a really good preventative, so we try and plant resistant varieties, though the Hessian fly has gained resistance to some of these varieties.
“Our lines of defense for Hessian fly are to plant a resistant variety if you can and rotate. If you’re more than a quarter mile away from a field that had Hessian fly, it probably won’t cause you much of a problem.
“The more wheat that is planted, the easier it is for the Hessian fly to spread. I think it’s worth looking at your fields this year to help prevent damage.”
Flanders advises that growers go into their fields a couple of times each week, pull up the plants that look infected or stunted, and squeeze the flaxseeds. “If they come up juicy and creamy or green-colored, it means they’re still in the larval or maggot stage. As they transform into the pupae stage, it’ll be pink as you squeeze them. That means they’re almost ready to pop out. That’s when you need to spray a residual.”
It’s always a good management practice to scout wheat fields for damaging infestations of insects, she says. At a minimum, check grain fields in the fall, in late winter before applying nitrogen, and during the boot and heading stages.
“Scouting during the first 20 to 50 days after planting is especially critical because this is when insect control with a foliar spray can provide greatest economic returns.
“Check fields as often as possible after this time, particularly before applying fertilizer, herbicides or fungicides. If insect populations exceed thresholds, it may be possible to apply an insecticide as a tank-mix with another chemical.”
More specifically, says Flanders, check five to 10 spots in the field, examining at least one row-foot at each location. Be sure to include at least two samples near the field edges and check closely because insects, particularly aphids and pupae of the Hessian fly, can sometimes be found at the base of the plant below ground level.
“For larger plants, slap the plants to jar insects to the ground for counting.
Other insects also can cause problems in wheat, she says.
“Grasshoppers sometimes invade wheat in the spring. They are particularly common in dry weather. The economic threshold for grasshoppers is three to five per square yard. Scout in the field, not just on the field borders where populations may be higher.
Chinch bugs and stink bugs may occasionally invade wheat in the spring, she adds.