Bird cherry-oat aphids and greenbugs on wheat
Bird cherry-oat aphids and greenbugs may begin to show up in April on wheat fields in Kansas. I have sampled wheat on March 20 and 28 in Riley, Geary, Dickinson, and Saline counties and found very few aphids. All have been bird cherry oat aphids but there are a few lady beetles in all fields -- more than enough to control the few aphids that are present, at least for now. In those same counties, I checked for alfalfa weevil hatching and although I would find a pinprick-sized hole once in a while, I found no larvae yet.
If aphid populations begin to increase as the weather warms up, producers will have to decide whether fields should be sprayed to control them.
This depends on the population level, the general growing conditions, and the presence or absence of natural enemies. Both lady beetles and parasitoid wasps, can do an adequate job of controlling aphids in many cases. 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.