In 1986 the Russian wheat aphid, Diuraphis noxia invaded the northern and western wheat-producing areas of Texas, most of the wheat-producing areas of New Mexico and Colorado, western portions of Oklahoma, and western areas of Kansas. It was also detected in extreme southeast Wyoming, and in the southwest corner of Nebraska.

Since 1986 to 1989 the Russian wheat aphid has spread to portions of Arizona, California, Idaho, Montana, Nevada, Oregon, Utah and Washington. The aphid has also been found in the provinces of Alberta and Saskatchewan. Two Russian wheat aphids were collected in a suction trap at Williston, North Dakota in May 1989. No other occurrences of the insect in North Dakota have been detected or reported to date.

This new imported post not only withdraws plant sap, but also injects a toxic substance, injuring and stunting host plants.

Damage to wheat, barley and certain other small grain and some grasses may result where area or field wide infestations develop under conditions that remain favorable for aphid population development. Permanent establishment could add to production and management costs of small grains.

Frequently asked questions concern seriousness of the insect, and its longtime potential as a pest problem. Obviously, such knowledge is lacking for the most part, although our information base is expanding, and critical answers will begin to surface as research efforts advance.

The Russian wheat aphid is native to the areas of southern Russia and surrounding Mediterranean countries. Though it has been present in these areas for many years, generally it has not been of economic concern.

In 1978, it was found in South Africa, and soon spread throughout the wheat-growing areas of that country. Two years later, it was found for the first time in the Western Hemisphere, infesting wheat at a site near Mexico City.

Again, widespread distribution occurred rapidly, and within four years the aphid was known to be within 150 miles of the Texas border. The first reported infestation in the United States was from wheat near Lubbock, Texas. This site is several hundred miles from the nearest known infestation in Mexico, and it is assumed that the wind currents that prevailed during this time transported the insect.

The RWA is a relatively small, lime-green aphid, with an elongated, spindle-shaped body. It can be distinguished from other aphids that commonly occur in wheat by the absence of prominent cornicles (tailpipes) and a projection above the caudal segment (or tail) which gives it a 'forked tailed' appearance when viewed from the side.

The RWA can be easily confused with the greenbug when trying to make a diagnosis from specimens alone. The following characteristics help to distinguish between the two.

Adults may be either winged or non-winged. The immature forms resemble the adults but are smaller. The immatures (nymphs) that are destined to be winged as adults will begin to produce wing pads, which become prominent prior to maturity. The wing pads may give these individuals more of a broad-shouldered appearance. These individuals also tend to exhibit a slight orange to pinkish coloration near the front, on the upper side of the body.

The RWA reproduces like most other aphids by giving birth to living young (perhaps between two and three offspring are produced per day during warm weather). These immatures reach maturity in approximately a week, but slower under cooler conditions.

Work is underway to investigate temperature requirements for survival and reproduction. The general range of requirements for RWA may possibly be similar to those of the greenbug, but will likely be found to vary in at least some aspects.

Wheat, barley and triticale are preferred host plants. Rye and oats are less preferred. A number of grasses appear to be suitable as alternate hosts which could allow the aphid to survive during the periods when small grains are not available. The aphid does not appear to survive on either sorghum or corn.

Feeding by RWA causes the host plant to produce an unusual set of symptoms. Aphids concentrate near the base on the upper side of the leaves. On older plants, the feeding of a small colony of aphids will cause a leaf to begin showing symptoms of discoloration within 3 to 4 days. Symptoms are usually in the form of alternating white, yellow or green longitudinal streaks. The streaks may run almost the entire length of the leaf, and may be more pronounced on some varieties than others.

At about the same time, infested leaves may also begin to roll together from the edges, giving the entire leaf a tube-like appearance. Aphids will continue to feed and reproduce within this rolled up leaf if favorable conditions prevail. If unchecked, the heavily infested leaves soon die.

In later growth stages, the aphids infest the flag leaf, and at least some of the emerging heads become trapped and turn white, exhibiting symptoms similar to frost injury. In addition to the streaking and curling of leaves, the entire plant, including portion of the leaves, sheaths and stems, may take on a purplish cast. This appearance is similar to the bluish appearance that plants assume during dry weather stress. In addition, heavily infested plants may exhibit a flattened appearance, with young tillers lying almost parallel to the ground.

On younger or seedling plants, symptoms of leaf rolling may predominate, with the streaking effect being less pronounced, at least initially. Dead spots ranging from 3 to 10 feet in diameter may begin to appear in fields where aphids become abundant early in the season.

In South Africa, growth stage 7 (appearance of the second joint) was the earliest point at which insecticide applications prevented reductions in yield. Presumably, according to these studies, an infestation averaging 10 percent of the plants infested at the jointing stage would be at treatment level (only one tiller or leaf infested is needed to consider a plant infested), assuming the infestation is field wide, yield potential is good, and satisfactory control can be expected at a reasonable cost.

One argument in favor of a low treatment threshold is that the amount of infestation can increase from 20 to 80 percent in just two weeks. In Texas, this rapid buildup seems to occur more commonly in drought-stressed fields.

Should one apply treatment in a field Where it is determined that the above threshold has been reached? No one really knows for sure. We do know this insect can be difficult to control. Much of the population is protected within rolled up leaves, and harder for beneficial insects to reach. We don't know the degree of control needed to prevent injury.

We don't know the number of applications that might have to be applied to provide good protection from jointing to flowering, nor the chances of reinfestation occurring. Low wheat prices, combined with a lack of loss/benefit information, leads us to suggest a conservative course of action.

Perhaps it might be wise to consider treatment (at growth stage 7) where random scouting reveals between 10 and 20 percent of the plants infested, and where the majority of the infested tillers consist of colonies of aphids (as opposed to individual aphids) in fields with average to above average yield potential.

Yield potential appears to be most affected from just prior to the appearance of the flag leaf until the initiation of flowering. In other words, premature loss of the flag leaf can lead to a significant loss in yield. Therefore, treatment or pretreatment appears to be advisable if field wide inspection reveals that 10 percent of the tillers are infested when the flag leaf first becomes visible.

Results to date suggest that control may be difficult to achieve since many of the aphids may be protected within the rolled up leaves. The insecticides listed are labeled for either 'aphids' or for Russian wheat aphid. Control may possibly be improved by using the upper range of labeled rates. Experience also indicates that at least two gallons of water per acre should be applied in aerial application.