Issue No. 2, Article 6/April 7, 2006
Do Aphids in Wheat Pose a Threat?
Kevin Black, insecticide/fungicide technical specialist with Growmark, has reported that aphids have been found in wheat fields in southern Illinois. That's not necessarily uncommon at this time of year, and we always receive questions after the aphids are found. Although aphids usually do not cause any appreciable yield losses in wheat in Illinois, some review of aphids in wheat is in order.
The bird cherry-oat aphid usually is the first species of aphid found in wheat. In fact, Kevin Black reports that most of the aphids being found in southern Illinois wheat fields are bird cherry-oat aphids. Other species can be found in wheat, but the bird cherry-oat aphid is the species most commonly found, at least early in the spring.
The bird cherry-oat aphid is olive green with a red-orange band across the rear of the abdomen; the tips of its cornicles ("tailpipes" that protrude from the rear of the abdomen) are black. The other species of aphid sometimes found in wheat are English grain aphids, which are green and have long, narrow cornicles that are entirely black, and the greenbug, the most threatening aphid species. The greenbug is bright green, with a darker stripe along the middle of its back. The tips of the cornicles are black. Comparative illustrations of these three species can be found at a North Dakota State University Web site.
Bird cherry-oat aphids. Note red-orange band across the rear of the abdomen (photo credit—Jack Kelly Clark, University of California).
Regarding direct yield losses caused by aphids in wheat, entomologists have never been able to associate economic yield losses with infestations of bird cherry-oat aphids; however, both English grain aphids and greenbugs can cause yield losses under the right circumstances. Cool temperatures sometimes slow the parasitoids that usually suppress early-season populations of aphids in wheat. If aphids begin building their colonies in the absence of natural enemies, their numbers could escalate rapidly.
Although economically important outbreaks of aphids are uncommon in wheat in Illinois, the aphids' presence and potential buildup are worth noting. Seedling wheat can be severely injured by the feeding of aphids, but wheat in the boot or heading stage is seldom damaged economically by these insects. Greenbugs generally cause more damage to wheat than the other aphids because they inject toxic enzymes into plants during feeding. Research regarding the effect of aphids on wheat yields suggests that the threshold is 12 to 15 aphids per tiller during seedling to boot stage. However, the presence of natural enemies often keeps aphid populations in wheat below economically damaging densities. In addition to lady beetles, a fungus disease and parasitoids suppress aphid populations. In cool, wet springs, a fungus disease helps to keep aphid populations in check. The presence of aphid "mummies"--swollen copper- or tan-colored aphids--reveals the activity of parasitic wasps.
Aphids also can transmit barley yellow dwarf virus. Entomologists at the University of Kentucky indicate that the bird cherry-oat aphid is the most important vector of this virus. However, management of aphids to prevent them from spreading the virus should be initiated in the fall rather than in the spring. In fact, one of the best ways to reduce the incidence of barley yellow dwarf in wheat in the fall is to plant after the Hessian "fly-free date." Many wheat growers have ignored this age-old cultural tactic, even though the benefits for pest management are significant. Insecticide application after the appearance of symptoms of barley yellow dwarf virus provides little value. Some summaries of aphids and their ability to transmit the barley yellow dwarf virus can be found at the following USDA-ARS Web site, with reference to a peer-reviewed journal article titled "Seasonal Abundance of Aphids in Wheat and Their Role as Barley Yellow Dwarf Virus Vectors in the South Carolina Coastal Plain"
One final note: It's important to diagnose the problem in wheat before making a decision to control aphids. Plant pathogens and abiotic factors (e.g., weather, fertility issues) can cause symptoms in wheat that might resemble other causes. So don't resort to an insecticide application to control aphids if aphids are not responsible for the problem.--Kevin Steffey