Bird cherry-oat aphids and greenbugs on wheat
click image to zoomPhoto by Phil Sloderbeck, K-State Southwest Research-Extension Center.Female bird cherry-oat aphid (right) and her nymph. Bird cherry-oat aphids and greenbugs have been found on wheat fields in Kansas recently. Should producers spray their fields to control these insects? Spraying now may be justified if numbers are high enough to prevent any further direct injury caused by these insects.
Natural enemies, both lady beetles and parasitoid wasps, are also present and growing conditions are good. So, while treating for aphids is always a possibility, it has not often been justified. It takes a pretty high population of aphids (30-50/tiller) with no lady beetles or mummies (indicating the wasp is active) and less-than-ideal growing conditions before an insecticide application to prevent damage from aphid feeding is justified. The discussions below for each species give a little more detail on the kind of direct feeding injury the aphids can cause, and economic threshold levels.
Both the bird cherry-oat aphid and greenbug can transmit a virus that causes barley yellow dwarf, but a foliar insecticide application now will not guarantee the disease has not, nor will be, transmitted to the plants.
Bird cherry-oat aphid
The bird cherry-oat aphid is one of the largest aphids to be found on wheat in Kansas and varies in color depending on the temperature and its stage of growth. Nymphs are usually pale yellowish-green, darkening as they mature to a deep olive green in the adult stage. Under very warm conditions, adults may be much paler in color. When large colonies persist on wheat plants past the boot stage they can cause the flag leaf to twist into a corkscrew shape that can trap the awns, resulting in 'fish-hooked' heads.
When the climate is sufficiently warm, asexual reproduction can continue year-round on wheat, oats, and other cereal grains. Asexual reproduction of the bird cherry-oat aphid occurs in Oklahoma and possibly in southern Kansas, and these populations are likely responsible for the migrants that colonize more northern wheat fields very early in spring, often while snow is still on the ground.
At one time, it was thought that the bird cherry-oat aphid caused very little direct yield loss to wheat except by vectoring BYDV. However, more recent research information from Oklahoma State University and the USDA-ARS suggest that the bird cherry-oat aphid is almost as damaging to wheat yield as is the greenbug. The data shows that if populations exceed 20 aphids per tiller before the boot stage, (400 aphids per foot of row) for 10 days, a 5% yield loss could be expected. If populations exceed 40 aphids per tiller for 10 days, (800 per foot of row) before boot, a 9% yield loss could be expected. Although its feeding causes no chlorosis or other visible damage to wheat plants, heavy infestations can also reduce grain quality, affect protein content and test weight, and even reduce protein assimilation by grazing cattle.