We have received a handful of reports of aphids in wheat fields in southern and south-central Illinois, and the numbers of aphids seem to be increasing in some fields. It's not uncommon to find aphids in wheat at this time of year, and it always causes some alarm. Although aphids usually do not cause any appreciable yield losses in wheat, some comments about this are in order.
The bird cherry-oat aphid usually is the first species of aphids found in wheat. It is olive green with a red-orange band across the rear of the abdomen; the tips of its cornicles ("tail pipes" that protrude from the rear of the abdomen) are black. Other species of aphids 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.
Bird cherry-oat aphids on a wheat leaf (photo courtesy of Matt Montgomery, Springfield).
Matt Montgomery, Extension crop systems educator in Springfield, found a few bird cherry-oat aphids in a wheat field in his area on April 22, although the numbers he observed were relatively small (12 in about 30 sweeps). People in southern Illinois have found larger numbers of these aphids in wheat fields.
Entomologists have never been able to associate economic yield losses in wheat with infestations of bird cherry-oat aphids (caused solely by their feeding); however, both English grain aphids and greenbugs are capable of causing yield losses under the right circumstances. Cool temperatures sometimes hold back 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.
Economically important outbreaks of aphids are uncommon in wheat in Illinois, but noting their presence and potential buildup is important. 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 greater 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 also 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.
Bird cherry-oat aphids, cast skins (white), and aphid “mummies" (tan and swollen parasitized aphids).
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.
One final note. It's important to diagnose the problem in wheat before making a decision to control aphids. Ron Hines, senior research specialist at the University of Illinois Dixon Springs Agricultural Center, observed some diseaselike symptoms in wheat at the center and also found a few aphids. However, Ron indicated that the symptoms of injury in the wheat probably were weather and fertility related rather than a virus transmitted by the aphids. So don't get out the insecticide to control aphids if aphids are not responsible for the problem.--Kevin Steffey