Description of the Pest
The Russian wheat aphid is a small pale green insect with an elongated, spindle-shaped body that may be covered with a powdery coating of wax. It can be distinguished from all other cereal aphids by a second tail-like process (supracaudal process) located directly above the cauda, giving it a twin-tailed appearance when viewed with a hand lens. While easily seen in wingless aphids, in very small or winged forms the supracaudal process may be difficult to see. Russian wheat aphid survives the summer on a variety of grasses and migrates to cereals in late fall as summer hosts die. It is tolerant of cold weather and can survive sub-freezing temperatures.
Following its introduction into California, Russian wheat aphid spread rapidly throughout the entire state, causing serious injury and crop loss. In recent years, however, Russian wheat aphid populations have declined significantly throughout the Central Valley and the Intermountain region. The situation in the low desert (primarily Imperial County) is similar with spotty infestations appearing periodically, generally causing little or no damage. A few fields within each of these areas continue to have problems. Russian wheat aphid appears in the high desert (Antelope Valley), however, it is rarely a damaging pest because wheat heads are usually past the boot stage before damaging numbers can develop.
In most areas of California, it appears that Russian wheat aphid builds to damaging levels sporadically, much like the population cycles of bird cherry-oat aphid. The reasons probably involve natural enemies, pathogens, environmental conditions, and crop management techniques.
Damage is restricted to specific members of the grass family. Wheat and barley are the most susceptible; rye and triticale, while susceptible, are usually less damaged; and oats appear to sustain little or no injury. Russian wheat aphid does not attack corn, sorghum, or rice. While feeding, Russian wheat aphid injects a toxin into the plant. This toxin is responsible for many of the damage symptoms, the most characteristic of which are white, longitudinal streaks on the leaves and sometimes the stem. Heavily infested plants are stunted, and sometimes exhibit a flattened appearance with tillers lying almost parallel to the ground. Occasionally, particularly during cold weather, plants show a purple color. Infested leaves curl up like a soda straw and remain in a rigid upright position rather than assuming the typical drooping posture. The tightly curled, upright leaves resemble onion leaves. If the awns are trapped in the curled flag leaf, the head is usually distorted and assumes a fish hook appearance. Improperly timed applications of phenoxy herbicides may cause similar injury.
Russian wheat aphid is not a vector of barley yellow dwarf virus.
The effectiveness of biological control agents has not been fully evaluated. Russian wheat aphid is attacked by several predators and parasites commonly associated with other aphid pests of small grains. Efforts should be made to conserve these natural enemies as they are of great importance in controlling other cereal aphids and may reduce Russian wheat aphid populations as well.
Destroy and remove volunteer cereals to help reduce or delay the buildup of Russian wheat aphid populations. Plants stressed for water or nutrients are more susceptible to and suffer greater damage from Russian wheat aphid, so maintain adequate soil moisture and fertilization.
Monitoring and Management Decisions
The best management strategy in areas where Russian wheat aphid is a problem is early planting, avoiding water stress, and isolation from riparian or permanent pasture. The major problems generally occur in late-planted grains. In the high desert, most growers no longer plant highly susceptible barley. Instead, they plant oats or an oat/wheat/barley mix. Russian wheat aphid is frequently found on the barley and wheat plants in such mixes, but good forage yields can be obtained.
Check fields regularly following seedling emergence. Russian wheat aphids are often difficult to find, particularly when present in low numbers. Look for the characteristic white stripes on the leaves and stem. The earliest infestations are often found on the edge of the field, particularly the upwind side. Aphids rapidly spread across the entire field after their initial establishment. Treatment thresholds have been developed for irrigated wheat; while thresholds for irrigated barley may be similar, they are probably not the same. Thresholds for dryland wheat or barley have not been developed.
When determining aphid densities in order to apply these thresholds, take a random sample across the field. Sample a minimum of four locations (quadrants) in each field. Take the sample in such a way as to avoid sampling only plants showing symptoms (streaking) of a Russian wheat aphid infestation. One way to do this is to use a metal rod that has a ribbon attached for easy location. Toss the rod backwards over your shoulder and select the plant it lands closest to as your sample plant. Or if you prefer, with your eyes closed, reach out to select a sample plant. It is very important that the sample be taken "blindly" or the sample will be bias toward treating.
Apply chemical control if aphid numbers reach the following on the indicated growth stages:
|Plant Growth Stage||Number of Aphids per Plant or Tiller|
|two leaf||5 (per plant)|
|early tillering||5 (per tiller)|
|late tillering||10 (per tiller)|
|first node||10 (per tiller)|
|boot||20 (per tiller)|
|head exertion and later||30 (per tiller)|
|Common name||Amount per acre||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Dimethoate 4EC)||0.5–0.75 pt||48||35|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: For use on wheat and triticale only. Do not make more than two applications per year. Do not graze within 14 days. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|(Malathion 8)||1 pt||12||7|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: May be used on wheat, barley, oats, and rye. If alfalfa is in bloom, apply during the night or early in the morning when bees are not foraging in the field. Highly toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI; the longer of these two intervals is the minimum time that must elapse before harvest may take place.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|