Bird cherry-oat aphids and greenbugs on wheat
Still, the bird cherry-oat aphid causes the most damage by vectoring plant viruses, especially BYDV. Although the hot summer weather in Kansas is usually effective in decimating aphid populations, bird cherry-oat aphid can temporarily avoid extremes of temperature by feeding on the lowest parts of the stalk, at or below ground level. It is also able to feed actively in weather too cold for other aphids, such as the greenbug, enabling the bird cherry-oat aphid to effectively colonize seedling wheat quite late into the fall.
The bird cherry-oat aphid is usually held below economic injury levels by natural enemies such as lady beetles, lacewings, hover flies, and parasitic wasps. However, conditions that favor outbreaks of greenbug or Russian wheat aphid (for example, an abrupt shift back to cold temperatures after a warm spell in spring) also benefit the bird cherry-oat aphid. The bird cherry-oat aphid will often be found forming mixed colonies with these aphids when they are abundant. In such cases, decisions to apply pesticides should be driven by the numbers of those direct-damaging species and materials applied to control them should be equally effective against bird cherry-oat aphid.
If bird cherry-oat aphid is present alone, count the number of aphids present on each of a series of 25 - 50 randomly selected tillers across a zigzag transect of the field. Treatment with an insecticide broadly labeled for aphid control on wheat can be considered if an average of 50 or more aphids per tiller is present from boot stage up until heading. However, treatment with contact insecticides will not reduce the incidence of virus transmission.
Greenbugs are pale green aphids with a dark green line down the back and antennae as long as the body. Greenbugs usually prefer to feed on the underside of lower leaves. Damage can occur in fall or spring, with tiny reddish spots on leaves signaling a beginning infestation. Later, infested leaves turn yellow, then reddish brown and eventually die. In the field, damage often appears as yellow or reddish-brown irregularly shaped patches that can spread to become almost field-wide.
The guidelines below are useful in estimating the need for greenbug control. For convenience, damaging levels are expressed as the number of greenbugs per foot of row, but in assessing the need for control, the thickness of the stand also becomes important. 50 greenbugs per foot of row in a thin stand would be more serious than in a thick stand because the number of aphids per plant would be greater. Similarly, larger plants can tolerate somewhat larger numbers of greenbugs before significant damage occurs.