Wheat Insects

There are several insects which damage wheat in eastern Colorado and western Kansas.  Below is a summary of the insects most likely to affect you.

You may click below on the insect name to jump to that section, or simply scroll down.

Bird Cherry-Oat Aphid
Brown Wheat Mite
Cutworms: Army and Pale Western
Hessian Fly
Grasshoppers
Greenbug
Russian Wheat Aphid

Bird Cherry-Oat Aphid is relatively large, a dark green color with a reddish-brown patch toward the rear of the abdomen. The antennae and cornicles are black. To see a comparison photo of aphids, click here.

Infestations may occur in fall or spring, and in our area it usually accompanies warmer-than-noirmal weather. KSU says that populations of 50 or more per tiller at the boot to heading stage may be damaging. The aphid may roll up the flag leaf into a corkscrew shape that can trap the awns, resulting in “fish-hooked” heads. This aphid is also a vector of barley yellow dwarf virus.

Brown Wheat Mite usually occurs in the spring in dry, dusty conditions and more commonly near volunteer or in continuous wheat. The mite is very small, with a  dark brown, oval body. The first pair of forelegs are longer than the others. Look for mottled leaves that appear bronzed at a distance. KSU says that because all adults are female, and since each can produce 70 eggs in a three-week period, there is potential for rapidly increasing populations. 

Economic thresholds are debated, but several hundred per foot of row would require treatment, and lower numbers in stressed wheat. Heavy rains can reduce the BWM populations.

Cutworms normally infest our wheat in the spring. Dry years and sandy soils are the most likely to show symptoms, and south facing hillsides on a warm afternoon are good for scouting.  Scratch the soil carefully, looking for larvae at the 1-2" depth.

The Pale Western Cutworm often causes more damage than the Army Cutworm, because it clips the plant at the soil level instead of consuming the leaves. Whitish larvae hatch underground in the spring from eggs laid by moths in the fall. Control can be more difficult than the Army Cutworm, because the Pale Western stays below the surface at all times.

The Army Cutworm  has a similar life cycle to the Pale Western, but it feeds above ground during the day.  An adult female may lay 1,000 eggs.  Lush wheat with luxuriant growth can tolerate more worms than a thin stand.  Fields slow to green up may have a cutworm infestation: scout on warm days in sandier portions.

Economic levels can be as low as one cutworm per square foot, or as high as 4-5 per square foot, depending upon the wheat's stand and vigor.

Comparison photos of the two cutworms are here.

Hessian Fly damages wheat in the spring and in the fall, and is worse in wet years. No remedial measures are available: planting resistant varieties after September 15 and controlling volunteer minimize the problem.

Using treated seed may help, especially against fall infestations.  Lodging can occur in the spring, especially if a significant fall infestation exists.

Grasshoppers typically cause problems in the fall, after wheat drilling. They feed on warm afternoons, then hide in adjoining cover during the cooler evenings. Bait can be used for control before wheat emerges. Spraying can be cost-effective: with $4 and 40 bushel per acre wheat, a ten foot-wide loss equates to nearly $200 per mile of lost yield.

Greenbugs seldom reach economic levels as far north as our region, although our increasingly mild winters make them more likely than in the past. They are pale green aphids with a dark green line down the back. Click here to see an image.

Greenbugs usually feed on the undersides of leaves, but their sucking activity discolors the leaf surface.  Small red spots are followed by leaf yellowing and  ultimately leaf death, if left untreated. Early infestations are spotty, often spreading to cover the field.
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Parasitic insects, including wasps and ladybug larvae, can be effective in reducing greenbug populations.  However, chemical control is often required because parasites arrive too late to prevent damaging levels.  

There are resistant wheat varieties which tolerate higher levels of some greenbug biotypes, but the insect has a history of rapidly evolving into new biotypes which defeat the resistance traits.

Economic levels are difficult to establish, because plant size, vigor, and resistance, as well as the levels of parasitism affect the calculation. Despite this, on very small plants a few greenbugs per plant may be economic, while ten times that number may be well tolerated in healthy, larger, resistant varieties.
    
Russian Wheat Aphids can do significant damage to wheat in our area. First found in Kansas in 1986, populations can range from non-existent to severe in any given year. Untreated fields may have yield losses exceeding 50%.

Their appearance is similar to greenbugs, with several notable differences: RWA have a slightly shorter body, with very short cornicles and antennae.  Also, the supracaudal structure on the rear has a double-tailed appearance when viewed from the side--twin "tailpipes"..

Aphids feed on new growth, injecting the plant with a toxin which inhibits chlorosis, thereby causing the distinctive whitish and purplish longitudinal leaf striping.  The leaves are also tightly rolled around the insect, which makes it harder to find and difficult for parasitic insects to attack.  Severely infested plants take on a flattened appearance.

Economic levels are as low as 10 percent of the tillers infested, especially in smaller plants.  After soft dough stage, 30-40 percent of infested tillers is economic. The RWA often appears in spots which worsen quickly, with the infestation spreading across the field.  Typically, early infestations are mostly wingless, with more winged insects appearing later in the infestation. Strong southerly spring winds can bring in large numbers of RWA in a short time frame.

Resistant varieties exist, but as with greenbugs, RWA can evolve into biotypes that defeat the host plant's resistance.

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Another source for wheat insects is the KSU 2008 Wheat Insect Guide. Download it here.  (PDF, 290 Kb)