Figure 1. Army cutworm (Photos by Jim Kalisch) Figure 2. Pale western cutworm

May 1, 2009

Watch for Army Cutworms, Hessian Flies, Russian Wheat Aphids

Wheat growers in the Nebraska Panhandle are reporting scattered areas of army cutworms and Colorado growers are reporting higher-than-normal numbers of Russian wheat aphids. In western Kansas there have been a few reports of Hessian flies.

Nebraska wheat growers across the state should be scouting their wheat fields and continue scouting them weekly to determine pest levels and whether they've reached the treatment threshold.

Army and Pale Western Cutworms

Army cutworms (Figure 1), usually the first insect pest to encounter in spring in winter wheat, can prevent green-up after dormancy. Initial damage usually shows up by early April, and if wheat is able to green up, army cutworms probably will not be a problem. The treatment threshold is 4 or more larvae per row-foot.

In western Nebraska, especially in the southern Panhandle, the pale western cutworm (Figure 2) also may be present in significant numbers. Survival of this insect is enhanced during dry years. Pale western cutworm eggs hatch in the spring (late March-early April) and the larvae feed from April to May. These cutworms are quite destructive as they feed below the ground cutting tillers off as they feed. Look for cut tillers laying on the soil between the rows and dig to find the larvae. The treatment threshold is 1-2 larvae per foot of row.

Figure 2a. Hessian fly larva Figure 2b. Hessian fly adult

Hessian Fly

A spring generation of Hessian fly (Figure 3) can occur beginning in March or April, however, no control measures are available for spring infestations. Damage will appear early as dark green plants, and later during the heading stages, as stems broken off near the base of the plant. Hessian fly problems usually develop when:

• early planting the previous fall allows a Hessian fly generation to occur on the wheat crop in the fall
• volunteer wheat grows from late summer through the spring and carries the fall generation of Hessian flies until the spring when the next generation will establish on the current wheat crop.

Cereal Aphids, Including Russian Wheat Aphids

Figure 4. Russian wheat aphid adult wingless female

Cereal aphids can be another spring insect pest in wheat beginning in the late jointing to early boot stages. We have observed greenbugs and other cereal aphids much earlier than normal when they were able to overwinter through a mild winter. For more information on aphid biology and identification, see the NebGuide, Cereal Aphids (G1284).

In western Nebraska, the Russian wheat aphid (Figure 4) is the primary aphid threat. Population increases can be explosive, especially during warm dry weather. If conditions are optimum, infestations can double in a week, resulting in a rapid onset of economic infestations and damage.

Aphid damage will be most dramatic when the flag leaf and head are emerging. Damage (discoloration or striping) to the flag leaf will result in reduced ability of the wheat plant to produce photosynthate to supply to the filling head. Even more dramatic can be the impact of the tightly curled flag leaf as the head is trying to emerge.

During regrowth to early jointing stages, look for leaf curling associated with purpling and yellow or white striping.

Economic thresholds for the Russian wheat aphid vary with the potential return for the crop and treatment costs and range from when 5% to 25% of tillers are infested. The lower thresholds are used for wheat with high yield potential and higher thresholds are for wheat with lower yield potential. The most critical time for control of the Russian wheat aphid is just before the flag leaf emerges. Treatments should eliminate aphids, allowing the flag leaf to emerge normally without trapping the heads.

Further information on scouting and thresholds for the Russian wheat aphid and other cereal aphids can be found on the Web site for the High Plains Integrated Pest Management Guide for Colorado, Western Nebraska, Wyoming, and Montana.

Gary Hein, Extension Entomologist
Panhandle REC, Scottsbluff
Robert Wright
Extension Entomologist, Lincoln