Crop entomologist highlights two pests to watch for this harvest season
MANHATTAN, Kan. – As producers across Kansas gear up for wheat harvest, two pesky insects are throttling up their efforts to put a dent in this year’s crop—along with farmers’ wallets.
With wheat harvest approaching, agricultural producers across the state should start scouting crops for the wheat head armyworm and the wheat stem maggot, said Jeff Whitworth, crop entomologist for K-State Research and Extension.
Wheat head armyworm
A common pest each year, the wheat head armyworm’s infestation rates are up slightly this year compared to past years, Whitworth said.
“Every year we have a few of these, and every year it causes some concern,” he said. “They’re mostly a nuisance, but this year in the south central and the north central parts of the state, we’ve seen a few more than normal. It’s still not up to 1 percent, but it’s bordering on 1 percent infestation levels.”
Whitworth said the problem with the insect is that it can be hard to spot. Producers might not notice the armyworm’s presence this time of year.
“They’re well-camouflaged,” he said. “They’re slender, and they stretch out on the head. If you’re out looking at your crop as it just starts to turn golden, you may notice bumps on the heads. Those are the worms stretched out.”
The worm dives in and gorges on the wheat kernel, Whitworth said. The pest sometimes feeds on the awn of the wheat plant, but it most often feeds on and damages the wheat seed.
The wheat head armyworm’s larger and longer mandibles allow it to “feed a little more on the seed” than most other caterpillars, Whitworth said. And though the pest certainly leaves its mark on the kernel, damage from the armyworm does not typically lead to yield losses in the field.
Instead, it causes aesthetic loss to the wheat that could translate to financial loss at the buyer or co-op level, he said. Producers might get “Insect Damaged Kernel” or “IDK” dockage, just because armyworm damage is evident.
The pest is not a picky eater, Whitworth said, so farmers can expect to see damage from the armyworm field-wide. They’ve been feeding now for the last two to three weeks, and they’re just starting to become mature worms or mature larvae.
Now is the time to scout for the wheat head armyworm, he said, as the insect is currently actively feeding in fields. Unless producers have their eyes peeled, they might not notice the damage.
“The damage is not pronounced, so you don’t notice it unless you’re out actually looking for it,” he said. “Take a sweep net out, and sweep sample your wheat.”
Whitworth said he has already noticed several fields where larvae are starting to cause feeding damage on the kernel. Producers should look for small holes leading into the kernel.
“(Larvae) will feed right on the kernel itself, and they can actually go through a whole head,” he said. “Generally, they’ll feed on a few kernels on one head and move to another head, so they’re doing more damage than you suspect.”
If producers seek to treat their crops for the insect, Whitworth said they should be mindful of pesticide labels. The wheat head armyworm is a common pest, but producers don’t normally spray for it. Therefore, it lacks presence on many labels.
“Even if you have a 10 percent or 20 percent infestation and decide you want to treat your field, you need to read the label,” Whitworth said. “Make sure that particular insect is on the label.”
Producers should also make sure the pre-harvest interval is such that they’re going to be able to spray, and the spray will dissipate and won’t still be around at the time of harvest. Failure to follow the pre-harvest interval on a pest control label is a violation of federal law.
“Some of the products have a 24-hour interval, and some of them have 14 days,” Whitworth said. “We’re probably far enough out yet that it’s not a problem, but those worms are going to continue to be out there for another week or two.”
Wheat stem maggot
Another pest Kansas farmers should have on their radar is the wheat stem maggot. Though the maggot causes less direct damage than the wheat head armyworm, it takes a toll on the producer’s final product.
The wheat stem maggot is a fly larva that causes the wheat heads to turn noticeably white in fields across Kansas. Whitworth said that like the wheat head armyworm, infestation rates are likely less than 1 percent currently.
Against the green or golden of the wheat, the damaged white heads caused by the insect are easy to spot.
“As you look out there, they’re very distinctive,” Whitworth said. “Because the wheat stem maggot is in the stem, you don’t notice it until it has killed the head, and the head turns white.”
The pest burrows up through the stem and kills the head. Whitworth said producers can confirm the insect’s presence if they can easily remove the head from the plant.
The maggot’s damage is likely done, he said, as by now it has crawled down to the base of the plant and is in the soil pupating. Unfortunately, efforts to stop the wheat stem maggot now will likely be unsuccessful.
Whitworth said the maggot generally causes far less than 1 percent killed heads in the field. However, the practice of double cropping corn or sorghum into wheat stubble after harvest can produce a feeding ground for the wheat stem maggot.
A few years ago, he said this practice caused some producers a real headache when the eggs from the previous crop of flies hatched and began munching away at the germinating corn and sorghum plants.
“This year, nearly all the corn has an insecticide seed treatment, so that’s probably not going to be a problem,” Whitworth said. “The insecticides seem to work pretty well on the maggots themselves. However, that’s not always the case with sorghum.”
If sorghum lacks insecticide treatment, he said producers could be at risk if they notice white heads in their wheat crop. White heads mean flies, and flies mean maggots in subsequent crops.
“If it’s not treated with insecticide, (maggots) can do a number on the germinating crop following the wheat,” Whitworth said.
Wheat seed treatments utilized in the fall will not carry over into the spring for either the wheat head armyworm or the wheat stem maggot, he said.
“The maggots will be successfully controlled by a seed treatment in corn or sorghum, but not by wheat that was planted last fall,” Whitworth said. “The insecticide just does not carry over through vernalization into the spring at sufficient levels to actually control the flies. That’s why you’ll see those white heads scattered throughout your wheat crop.”
More information about the wheat head armyworm and wheat stem maggot, in addition to other crop pests, is available on K-State’s Department of Entomology website.