By D. Bruce Bosley
Extension Agent/Cropping Systems
Colorado State University Cooperative Extension
Nearly all fall germinated winter Wheat is now jointing. Therefore, Wheat grazing should be discontinued unless it is decided that this forage is more valued than it's grain production protection. If grain production is a farmer's goal for winter wheat, the following insect pests are or may be present in Northeast Colorado this spring.
Army cutworm can significantly reduce Wheat yields when it's population is high and especially when Wheat growth is already stressed by drought conditions. Army cutworm spends the winter as a partially grown caterpillar. In the spring it feeds frequently and development proceeds rapidly. Army cutworm is found under soil clods and other debris during the day. This is a climbing cutworm that always feeds above ground. Consider treatment if counts are more than four to five per square foot.
Brown Wheat mite (BWM) is a tiny Wheat pest but it can significantly reduce drought stressed Wheat yields when enough are present. BWM feeds 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. In Colorado this mite is considered to be a pest primarily of fall-seeded small grains that are drought-stressed. Brown Wheat mites are a sporadic problem.
The economic threshold for brown Wheat mite is not well defined, but it is near a hundred mites per row-foot in the early spring. The decision to treat is difficult since the mite is associated with drought stress. If it rains, mite levels will be significantly reduced regardless of the use of insecticides, while if it does not rain the crop yield may be so reduced by drought that it may not be worth treating. Also, if the dormant over-summering white eggs are present and normal cool season red eggs are mostly hatched, the population is in natural decline, and treatment is not economically sustainable.
Russian Wheat aphid (RWA) are found in the High Plains on winter Wheat and other small grains. The most severe spring infestations of winter grains are caused by wingless aphids that over-wintered in the crop. 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 developed by CSU and other Wheat breeders. The original aphid is still the most common of the two types, so resistant varieties still have some value and are still recommend in areas with consistent Russian Wheat aphid problems.
Chemical control of Russian Wheat aphid will probably not be necessary on resistant Wheat varieties, but may still be necessary on susceptible types and on barley. If one tiller shows damage, then the plant should be considered damaged. Aphids can be very difficult to find during cold weather, so base treatment decisions on damage alone under such conditions. At this time the treatment threshold is reached when five to ten percent of tillers are damaged and infested.