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.
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.
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.
Approximate Damaging Levels of Greenbugs
Stage and development of plants No. of greenbugs per linear foot
Seedlings, thin stands less than 3 tillers 50
3- to 6-inch wheat, 3 tillers or more 100 to 300
6- to 10-inch wheat 300 to 500
Overwintering greenbugs can rapidly develop into damaging infestations during warm periods in February and March, and close surveillance of fields is necessary if greenbugs are present. Beneficial insects such as parasitic wasps and ladybeetles become increasingly effective in reducing greenbug populations around mid-April. Once parasitism levels reach between 10 and 15 percent, greenbug populations usually decline fairly rapidly.
Greenbug control on small grains is occasionally needed during periods of relatively cool weather (below 60°F, but above freezing). Experience has shown that good results are possible under these conditions with some, and perhaps most, of the recommended insecticides. Dimethoate may be an exception, however. It may not give acceptable control below 60°F.
Oklahoma State University has developed a sampling program called “Glance ‘n’ Go,” which calculates a greenbug threshold based on the cost of control, the market value of wheat and the month of the year. For more information on their greenbug pest management decision support system, see the web site at: http://entoplp.okstate.edu/gbweb/.
Consider avoiding pesticide applications when beneficial insects such as lady beetles and parasitic wasps are active, as these are often abundant enough to prevent greenbugs from reaching damaging levels. Augmenting greenbug predators such as ladybeetles or lacewings by importing and releasing is not advisable.
For more detailed information on bird cherry-oat aphids and greenbugs, see:
For specific treatment options, see K-State publication “Wheat Insect Management 2013” MF-745 at: www.ksre.ksu.edu/bookstore/pubs/mf745.pdf