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Adult Description: The Russian wheat aphid (Diuraphis noxia) is a small, lime-green and has a distinctive football-shaped body. The legs, antennae and cornicles are short compared to most other aphids. Viewed from the side, the terminal segment of the abdomen has a supracaudal structure that looks like a double tail. The greenbug, (Schizaphis graminum), is similar in color, but the dark green stripe, long antennae and cornicles, which are often longer than the body, make it easy to distinguish from the Russian wheat aphid. Russian wheat aphids prefer to feed in rolled leaves on the upper parts of plants. Greenbugs are typically found on the undersides of lower leaves, but do not cause leaf rolling. Damage to wheat plants is noticeable from a distance. Russian wheat aphid feeding causes purple or white longitudinal streaking on the leaves. Leaves damaged by greenbugs turn brown and appear scorched. When searching for the Russian wheat aphid in wheat, it is often useful to look for damage first, and then look for the aphids.
Larva Description: Russian wheat aphid nymphs are relatively sedentary and gregarious on the host plant, forming dense colonies. As colonies increase in size, the aphids benefit from feeding as a group, developing and reproducing at higher rates and reducing their individual risk of attack by natural enemies.There are four or five molts from first instar to the reproductive adult. Maturation occurs between 9 and 55 days depending on temperature. Low temperatures delay development and reproduction, but temperatures above 25 C (77 F) cause mortality.
Host Plant: Variety of natural grasses. Primarily wheat, barley, and rye.
The Russian wheat aphid is capable of modifying the growth of it's host plant. Longitudinal leaf rolling allows high density colonies to form in a protected location, somewhat like the galls formed by other aphids. Rolled leaves may create a favorable microclimate for the aphid colony, buffering it from temperature extremes and reducing the risk of desiccation when relative humidity is low. In addition, many larger aphid predators and parasitoids that attack Russian wheat aphids are less likely to encounter them in rolled leaves or less inclined to forage in such close quarters. Aphids hidden in rolled leaves are better protected from contact insecticides even if they are susceptible to them. Consequently, materials with systemic activity are superior for Russian wheat aphid control
The Russian wheat aphid reproduces asexually. All aphids are female, and each gives birth to live daughters carrying embryonic granddaughters. This telescoping of generations, combined with rapid, asexual reproduction is the key to the explosive population growth achieved by many aphid species. In Asia, the Russian wheat aphid may produce a sexual generation in the fall, with mated females laying eggs that overwinter. Although sexual females have been found in North America, males have not, and it appears that the aphid population reproduces without sexuality. This inability to produce overwintering eggs may have limited the northern range of the Russian wheat aphid in North America. This aphid is more cold tolerant than greenbugs, however, and easily survives Kansas winters.
The Russian wheat aphid is native to the steppe country of southern Russia. It was first detected in North America near Mexico City in 1980, and was discovered near Lubbock,Texas in March 1986. By the fall of that year, infestations were reported in New Mexico, Oklahoma, Colorado, Nebraska, Wyoming and Kansas. These aphids spread rapidly throughout wheat-producing regions of the western United States and prove to be well adapted to the arid conditions of the High Plains. Following introduction, the Russian wheat aphid caused hundreds to millions of dollars in wheat and barley production losses through reduced yields and pesticide treatment costs. Annual direct yield losses peaked at $274 million in 1988 but dropped to less than $10 million by 1993. Russian wheat aphids still appears periodically across the High Plains in southwestern Wyoming, southeastern Nebraska, eastern Colorado, New Mexico, western Kansas, and the panhandles of Oklahoma and Texas.
Russia
U.S. Habitat: Grasslands/Steppe-type environment
U.S. Present: First identified in the Texas Panhandle in 1986, the Russian Wheat aphid has since dispersed itself across the Great Plains region of the U.S. Its range now extends from Texas westward and northward to the Pacific Coast and Canadian border, respectively. All U.S. states North and West of Texas now have the Russian wheat aphid established within.
Texas: First identified near Lubbock, and only currently found in the Panhandle and west Texas regions of the state.
Greenbugs, Schizaphis graminum, another pest from the Palearctic ecozone, looks very similar to the Russian wheat aphid. There are minute differences that are easier to detect when observing the insects under a lens.
Faced with astronomical aphid populations and declining wheat production in the late 1980s, the U.S. Department of Agriculture mounted an unprecedented, far-reaching program to explore the ancestral range of the Russian wheat aphid and find natural enemies. At least 120 scientists from 20 countries participated in the program, which imported more than 12 million beneficial insects of at least 24 species. These insects were cultured, mass produced and released at numerous sites in 16 states for about five years. The majority of these insects were already represented in the United States by similar, locally adapted native species. Of the introduced insects, one species of parasitic wasp, Aphelinus albipodus, has conclusively established, but its effect on Russian wheat aphid is limited to a small portion of the aphid's current range. On the other hand, the number of native predators and parasitoids capable of preying on and parasitizing Russian wheat aphids have gradually increased, and some have extended their range to coincide with regions now infested by the Russian wheat aphid. Two factors have contributed to the gradual, area-wide decline of Russian wheat aphid: the development and distribution of resistant varieties in the most seriously affected regions, and gradual increases in the populations of native natural enemies. Because of the failure of the classical program against this aphid and the potential non-target effects of introduced biological control agents, it is generally agreed that massive, indiscriminate introduction of exotic species should not be the first response in the event of an invasion of another aphid pest of small grains. When natural enemies and host-plant resistance fail, producers must rely on insecticides to limit aphid damage. Two insecticide options are currently available. One option is to use a seed treatment, which can provide early season protection but is only cost effective where there is a high probability of significant Russian wheat aphid activity. The other option is to use insecticides to reduce aphid numbers after populations have been detected in the field.
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References
Haley, Scott D., Frank B. Peairs, Cynthia B. Walker, Jeffrey B. Rudolph, and Terri L. Randolph. 2004. Occurrence of a New Russian Wheat Aphid Biotype in Colorado. Crop Science 44: 1589-1592.
Michaud, J. P., Phillip E. Sloderbeck. 2005. Russian Wheat Aphid An introduced pest of small grains in the High Plains. Kansas State University Agricultural Experiment Station and Cooperative Extension Service.
Liu. X. M., C. M. Smith, B. S. Gill and V. Tolmay. 2001. Microsatellite markers linked to six Russian wheat aphid resistance genes in wheat. Theoretical and Applied Genetics 102(4): 504-510.
Internet Sources:
http://www.nhm.ac.uk/research-curation/research/projects/chalcidoids/database/media.dsml?IMAGENO=
http://www.ksre.ksu.edu
http://www.invasive.org
http://www.invasivespeciesinfo.gov
"Bugwood Wiki" http://wiki.bugwood.org
http://pest.ceris.purdue.edu
http://www.agric.wa.gov.au
Experts:
Mike Merchant - Texas A&M - m-merchant@tamu.edu
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