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Late summer brings a fresh wave of pests for late-season crops.
MANHATTAN, Kan. — As the end of the growing season approaches, the potential for insect damage is decreasing in most crops, including corn. However, a few pests are still causing concern for crop producers, said K-State Research and Extension crop entomologist Jeff Whitworth. These pests include the potato leafhopper, sorghum headworm and the webworm, a common soybean pest.
Alfalfa is particularly susceptible to the potato leafhopper this time of year, Whitworth said.
“If you’re going to swath within the next 10-14 days, that should take care of the problem,” he said. “I’ve never seen an infestation of potato leafhoppers re-infest or flare up in an alfalfa field that was swathed in a timely manner.”
Swathing removes the nymph-stage insects, and adults don’t seem to re-infest alfalfa fields once the hatched eggs have been eliminated.
“If you’re past that time, you’re still getting potato leafhoppers and they’re still pretty dense in your stubble and alfalfa as it’s trying to grow back, you might want to think about putting on an insecticide application,” Whitworth said.
The potato leafhopper is vulnerable to all tested insecticides, and alfalfa growers should consider using the lowest rate of the least expensive insecticide possible.
The pest extracts juices from the alfalfa plant, damaging the plant’s ability to transmit nutrients. The potato leafhopper’s feeding process causes “hopper burn,” a disease characterized by yellow triangles on the leaves of the plant.
“You need to take care of it if you have them,” Whitworth said. “If you’re not going to swath, put out an insecticide application. They’re easily killed, and they don’t re-infest.”
The corn earworm, or sorghum headworm as it is called in grain sorghum, is also active during the late summer and early fall. The insect prefers corn, but it looks to sorghum as a source for egg-laying and can be found in soybeans or cotton due to its wide host range.
As sorghum begins to head, the insect moves to sorghum and begins laying eggs on the heads. Between the flowering and soft dough stages, the sorghum plant is attractive and vulnerable to the sorghum headworm, and the pest can inflict damage quickly.
Small hairs protruding from the body of the worm distinguish the sorghum headworm from similar fall pests. The insect is aggressive and cannibalistic, and though three or four worms may start out feeding on a head, that number is quickly reduced to one.
“The treatment threshold is one worm equals five percent damage per head,” Whitworth said.
Sorghum plants entering the whorl stage might contain caterpillars, though Whitworth recommends not treating for those.
“They can rag up the plant pretty good,” he said. “But they get down in that whorl, and you can’t get the insecticide down into the whorl sufficient enough to kill the worm.”
Whorl-feeding could result in a negative visual to growers, but has no impact on yield.
“If you have 80 percent of your plants ragged up, there’s a lot of feeding in the whorl, and the worms are only half an inch long, then you might have to consider an application,” Whitworth said. “Other than that, we don’t consider an insecticide application to be effective or necessary on whorl-stage sorghum.”
A common soybean pest, the webworm causes significant defoliation to second and third tri-foliate stage soybean plants.
“Once the canopy grows to be knee-high, there’s enough foliage there that webworms aren’t going to cause a problem,” Whitworth said. “Normally, we’re a little bit past the time when webworms cause a problem.”
Late planting, poor growth and double planting have contributed to this season’s webworm infestation.
“The rule of thumb is 50-60 percent defoliation, there’s no problem,” Whitworth said. “On small soybean plants, if you get much over 50-60 percent defoliation, you still have small worms out there working on the plants, and you may want to treat.”
The webworm produces and inhabits a web on the underside of the soybean leaf, making it difficult to reach with insecticide. If the treatment permeates the web and reaches the pest, they are susceptible and easily controlled.
“If you get 50-60 percent defoliation, we’ve seen no impact whatsoever two weeks later,” Whitworth said. “The plants recover, especially if you get a rain or if it’s irrigated. You don’t want to let it get to 75-90 percent defoliation, because the plant will not recover. You’ll likely have a significant yield loss.”
Whitworth urges soybean farmers to check their fields for signs of webworms now.
“If you have webworms, and they’re on that borderline 40-50 percent, look to make sure they’re getting ready to quit feeding and start pupating,” he said. “If they’re not, you may want to consider an insecticide application.”
More information about the potato leafhopper, sorghum headworm and webworm, in addition to other crop pests, is available on K-State’s Department of Entomology website.