Nasty insects in wheat and sorghum

Jeff Whitworth, associate professor of entomology at Kansas State University, discusses the insects that have been the most prevalent in wheat and sorghum the last few years. (Journal photo by Lacey Vilhauer.)

Entomologist discusses troublesome insects in wheat and sorghum

Bugs are always on the brain when raising crops—it’s a never-ending battle that farmers must be prepared to fight every day of the year. Jeff Whitworth, associate professor of entomology at Kansas State University, spoke at the recent Sorghum U Wheat U event in Wichita, Kansas, about the insects he commonly deals with in sorghum and wheat crops. 

In wheat fields, Whitworth noted several pests that have been the most prevalent in Kansas and surrounding areas. They include: armyworms, army cutworms, hessian fly, wheat curl mites, winter grain mites, and aphids. 

Armyworms and army cutworms produce similar damage, but Whitworth said the biggest difference between these insects is that armyworms are gone in the fall, but army cutworms are present all winter. Whitworth said armyworms have been a problem in Kansas the last few years, especially in the south central and southeast parts of the state. 

“Armyworms will eat any grass, so they’ll get in sorghum, corn, brome, wheat, fescue,” he said. “They can cause problems in any grass the females lay their eggs in. They’re not that difficult to kill, but normally we don’t realize they are there until they are pretty large and have done quite a bit of damage.”  

Army cutworms feed any time temperatures are above 50 degrees Fahrenheit and lay eggs in whatever crop is green in the fall, which means wheat and alfalfa in the High Plains.  

Whitworth said hessian fly are the No. 1 problem in wheat worldwide, but Kansas farmers are fortunate to not struggle with this pest very often.  

“I can go in every field in Kansas and find some hessian flies,” he said. “It’s only at a 1 or 2% level, so we don’t notice it until some years when the conditions are just right the population densities can cause a real problem.”  

Females deposit eggs on upper surfaces of wheat leaves and are often mistaken for wheat rust or not even noticed. Whitworth said in three to 10 days, whitish maggots emerge and crawl to the base of wheat plants between the leaf sheath and stalk of plant. The larvae secrete a substance into the plant that causes plant damage.  

“There is not much you can do about it,” he explained. “Once you have hessian fly there’s not much you can do about it other than plow it up.”  

Winter grain mites were more prevalent this year because they are more common in dry springs, falls or winters. When crops are under moisture stress, mites can cause crop yellowing. 

“If you get some moisture the wheat will come out of it, which is why we rarely recommend treatment for winter grain mites,” Whitworth said. 

Wheat curl mites on the other hand are a different story, because they can transmit diseases. Whitworth said they can vector wheat streak mosaic virus, High Plains virus and Triticum mosaic virus.  

Another notorious bug is the aphid. Whitworth said there are more than 20 different species of aphids that can be found in wheat in Kansas. The most common are bird cherry oat, greenbugs, corn leaf aphids and English grain aphid.  

“Generally, there’s not enough of them to cause a problem,” he explained. “The biggest issue is that aphids can cause the disease barley yellow dwarf. You just need to manage it ahead of time so it doesn’t get bad.”  

Whitworth’s most crucial advice that applies to any pest in wheat is to always kill volunteer wheat so the pests and diseases from the prior crop will not continue in the next one. Additionally, he said the later you can plant, the better off you are to prevent pests. 

Pests in sorghum fields

On the sorghum side of the spectrum, the main pests he has dealt with recently are chinch bugs, sugarcane rootstock weevil, grasshoppers, ragworms, headworms, aphids, and sorghum midge. Chinch bugs were especially rampant this past year because they thrive in dry and hot conditions. 

“The nymph causes the most damage because they eat around the clock,” he said. “Anytime you have an insect that’s sucking the juice out of a plant, it’s in competition with the little moisture that’s there. They’re behind the leaf sheaths, so it’s really hard to get contact insecticides in there. If you can, go slow and use drop nozzles and spray at the base of the plant.”  

For chinch bugs, Whitworth said his best advice is to avoid planting sorghum next to wheat. The sugarcane rootstock weevil is native to Kansas, and it can attack sorghum and corn. Whitworth said it may cause lodging, especially under dry conditions. Grasshoppers, the posterchild for plant destruction, are always a concern for sorghum growers. Whitworth suggests growers scout borders in early summer to prevent migration into sorghum.  

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“About 15 to 20 nymphs per square yard in borders or 5 to 8 nymphs per square yard in a field may justify treatment,” he said.  

Ragworms refer to fall armyworms, corn earworm and armyworms. They are much more visible during the plant’s whorl stage and fortunately they do not impact yield. 

“We do not recommend treating for ragworms because once you realize you have a ragworm problem, they’re already done feeding. Besides that, your insecticide is not going to get down into the bottom of the whorl.” 

Headworms consist of the same insects in the ragworm family, but later on in their life cycle. Whitworth recommends producers check sorghum when it begins to head out. He said it is best to treat early. 

“If you see 1 to 2 worms per head, that can justify control,” he explained. “Generally, we consider 5% loss per worm per head, but the good thing is they will only damage the berries from flowering to soft dough.” 

Sugarcane aphids are another pest commonly associated with sorghum. They produce large quantities of honeydew, which can cause problems during harvest. Heavy feeding causes plants to dry down rapidly. Weakened stems can also lead to plant lodging prior to harvest.  

Sorghum midge is an occasional pest in Kansas. Whitworth said they are normally found in southeast and south central parts of the state. 

“Populations are usually too low to detect or to justify insecticide treatment in Kansas,” he said. “They are usually noticed after fly emergence because of remaining pupal cases.”  

Late-planted sorghum is most susceptible to sorghum midge. It will not cause economic damage after flowering.  

Lacey Vilhauer can be reached at 620-227-1871 or [email protected].