Wheat insect monitoring beginning to spot aphids and mites
Russian wheat aphids, brown wheat mites and greenbugs are all beginning to show up in High Plains and southern Plains wheat fields, and primarily because of the drought, according to a Texas A&M AgriLife Extension Service specialist.
But the big question by producers is when are infestation levels high enough to merit any kind of treatment, said Ed Bynum, Ph.D., AgriLife Extension entomologist in Amarillo.
So far, the reports have been spotty, Bynum said, with Yoakum, Bailey, Parmer, Deaf Smith and Lipscomb counties reporting infestations of at least one of the pests, and some counties, all three. But, as temperatures warm up, wheat fields all across the Texas High Plains should be scouted.
Brown wheat mites are primarily a pest during drought-stressed conditions, which means dryland fields often develop heavy infestations, he said.
Cold temperatures do not affect these mites, Bynum said. They actively feed on the wheat foliage on clear warm days, particularly during the mid-afternoon, and look like black specks moving on the leaves. At night they move down to the soil. A distinguishing trait for identifying these mites is the front legs, which are about twice as long as their body.
“Knowing when to treat for infestations is difficult because drought stress can severely reduce crop yields, making treatment uneconomical,” he said. “And, if you can predict when it will rain, a driving rain of at least one-third of an inch will cause mite densities to decline naturally, regardless of chemical controls.”
Russian wheat aphids are a small, about 1/16 inch long, pale green football-shaped aphid that usually infests the younger leaves, Bynum said. The body may have a powdery coating of wax, and their cornicles or “tail pipes” are not prominent. The insect has a small projection at the end of the abdomen that gives it an appearance of having two tails. The majority of the aphids are wingless, but a few will have wings.
Feeding from the aphids cause leaves to tightly curl around the colonies, he said. This protects the colony from the climate and insecticides, and makes it more difficult for predators and parasitic wasps to effectively control infestations.
The Russian wheat aphid injects a toxin while feeding that causes longitudinal white, yellow and purple streaks on leaves and tillers. Under heavy infestations, plants will be stunted and tillers will lay over, giving the plant a flattened appearance, Bynum said.
The aphids randomly aggregate across the field and as the population increases, the damage is easily seen as hot spots, he said. If infestations reach economic levels and are not controlled, yield losses can be 50 percent or more.
“Fortunately, the Russian wheat aphid is not an important vector of barley yellow dwarf or other cereal diseases,” Bynum said.
A rule-of-thumb threshold that does not consider economic costs of control and market value per bushel is available to determine spring insecticide treatment, he said. Treatment is suggested if wheat in the early boot stage has 5 to 10 percent damaged or infested plants; early boot to flowering, 10-20 percent damaged or infested plants; and after flowering, more that 20 percent damaged or infested tillers. This is explained in more detail at http://www.ext.colostate.edu/pubs/insect/05568.html.
Plants with even a single infested or damaged tiller should be considered infested, Bynum said.
Greenbug infestations also have been increasing in numbers, he said. They have a pale green colored body with a darker green stripe down their back. The cornicles are more prominent than those on the Russian wheat aphid. The tips of the cornicles and legs are usually black. They reproduce very rapidly when temperatures are between 65 and 95 degrees.
Like Russian wheat aphids, greenbugs aggregate in colonies across the field, but greenbugs feed on the underside of the lower leaves, Bynum said. The toxin injected into the plant initially cause red, yellow or orange spots on leaves. As damage progresses, there will be localized areas of yellowing and dying plants in a field.
With the increase in aphid activity, fields should be scouted at least weekly, he said.
More information about management and more precise thresholds for these pests can be found in publications on the Texas A&M AgriLife Extension Bookstore website, https://agrilifebookstore.org/publications_details.cfm?whichpublication=31. This resource has a listing of insecticide products registered for use against brown wheat mites, Russian wheat aphids and greenbugs, Bynum said.
Insecticides commonly used on the Texas High Plains for control of brown wheat mites are dimethoate and chlorpyrifos. Generally, chlorpyrifos is used for greenbug, Russian wheat aphid and other aphids, he said.
“Each will provide effective control, but the pre-harvest interval may be important depending on harvesting for grain, grazing and cutting for forage,” Bynum said. “Check the label for specific pre-harvest intervals periods for each product.”