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What's bugging you

By Assefa Gebre-Amlak
CSU Regional Pest Management Specialist

Colorado

There are reports of two major pest problems, namely the Russian wheat aphids and brown wheat mite infestations, in Colorado. According to the report, all wheat fields scouted in Washington County have shown some degrees of Russian wheat aphid infestations.

Russian wheat aphids (RWA) can be found in winter wheat, usually on the younger leaves, from emergence in the fall to grain ripening. Aphid feeding prevents young leaves from unrolling. RWA colonies are found within the tubes formed by these tightly curled leaves. This not only makes it difficult to achieve good insecticide coverage, but also interferes with the ability of predaceous insects to reach and attack aphids. Leaves infested by RWA have long white, purple or yellowish streaks. Under some conditions, infested wheat tillers have a purplish color. Heavily infested plants are stunted and some may appear prostrate or flattened.

In 2003, a new biotype of the Russian wheat aphid was observed. It is virulent to all of the resistant varieties used in its management earlier. Russian wheat aphid resistant varieties may still be used, if they perform well in a given area; however, they likely will not provide any useful resistance.

There are also important differences among the small grain crop species. Oats are resistant to RWA. Although heavy infestations have been observed, little economic damage has been detected.

Chemical control of Russian wheat aphids is justified if you detect 5 to 10 percent damaged and infested tillers (during regrowth and early boot) and 10 to 20 damaged and infested tillers during early boot to flowering stage. For effective insecticide products, you may refer to the High Plains IPM website, http://highplainsipm.org/.

Brown wheat mite is a pest of small grains in most small grain-producing areas of the world. In Colorado, it is most common on drought-stressed winter wheat on the eastern plains.

They feed on plant sap during the day and spend the night in the soil. Their activity peaks at about mid-afternoon on warm, calm days (the best time to scout). This mite is not affected by cold temperatures, but populations are quickly reduced by driving rains of 1/3 inch or more.

Management of volunteer wheat is an important preventive measure for brown wheat mite, as it is with several winter wheat pests. Any management practices that serve to minimize drought stress are also important. Once an outbreak occurs, however, chemical control is the only effective management.

The economic threshold for this pest is not well defined, but it is at least several hundred mites per row-foot in the early spring. This figure will increase with lower wheat prices and decrease with higher wheat prices. It often is difficult to justify a chemical treatment, since brown wheat mite infestations are associated with drought stress. If a driving rain of at least 1/3 inch occurs, mite levels will be significantly reduced regardless of chemical treatment.

If it does not rain, crop yield may be so reduced by drought as to make chemical treatment uneconomical. Also, if white eggs are present and red eggs are mostly hatched, the population is in natural decline and treatment is not economically justifiable.

Look for stunted wheat that has a white to silver flecking on the leaves. Also, look for the mite on the leaves. For more information on brown wheat mite, go to www.ext.colostate.edu/Pubs/insect/05578.html.

Other pests on the look-out are army cutworms and alfalfa weevil, and it is time to include these insect pests in your field scouting program for early detection and management.



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