By: John Van Duyn, Entomology Extension Specialist, NCSU and Randy Weisz, Crop Science Extension Specialist, NCSU

Wheat fields are susceptible to many kinds of insects. Only a few species may become pests and even when they do, they usually do not reach damaging, “above threshold”, numbers. However, in some seasons, or under certain circumstances, insect pests of wheat can be very damaging. The following insects may become abundant enough to significantly damage the wheat crop in North Carolina: aphids (several species), cereal leaf beetle, Hessian fly, and armyworm (sometimes called true armyworm). Other plant-feeding insects may occasionally damage wheat, such as grasshoppers, chinch bug, or fall armyworm. Descriptions of these insects may be found at the following sites: http://www.ces.ncsu.edu/plymouth/pubs/ent/index4.html, and http://www.smallgrains.ncsu.edu/Insects/Insects.html, or in the 2004-05 Small Grains Production Guide or local county Extension offices.

Wheat feeding aphids. There are three primary species of aphids in NC wheat; the English grain aphid, the bird-oat cherry aphid, and the corn leaf aphid. All aphids in wheat have a short life cycle, and may quickly multiply to large populations under favorable food and temperature conditions. Aphids suck sap from leaves, stems, and heads. Feeding may reduce yield potential but usually not dramatically. Aphids are also vectors of barley yellow dwarf virus (BYDV) and this disease can be a serious concern.

A complex of biological control agents accompanies aphids, including parasites, predators, and pathogens (chiefly fungi). Biological control agents ordinarily exert a powerful controlling influence on aphid populations, especially in the spring, although it usually requires some time before the aphid populations “crash”, due to the combined influence of these agents.

Cultural practices that help reduce the chance of aphid damage and BYDV infection are: (1) avoiding early planting, (2) avoiding excessively high soil nitrogen levels, and (3) using wheat varieties that are resistant to BYDV.

Avoid Early Planting: Ideally, wheat should be planted within two weeks of the first frost. A good freeze can reduce the aphid population, and mark the beginning of cool weather that will slow down aphid feeding and activity in the newly emerged crop. Planting after the end of warm weather not only reduces the chances of crop damage due to aphid feeding, but also makes it less likely that aphid-transmitted BYDV infections will occur. The aphid population may build up again in the spring, but these populations are less damaging because plants are larger and growing more rapidly, and are more tolerant to feeding and BYDV infection. While late planting can be effective for reducing aphid feeding and BYDV infection, this is not an ideal practice because late planted wheat is at increased risk to weed problems, cereal leaf beetle attack, lower yield, and reduced test weight.

Avoid Excessively High Soil Nitrogen Levels: Aphids reproduce most rapidly on high-nitrogen content plants. Maintaining nitrogen levels within prescribed agronomic levels helps to avoid high aphid populations.

Plant Wheat Varieties With BYDV Resistance: If BYDV is a regular problem on a producer's farm, or on neighboring farms, a highly effective strategy is to select wheat varieties that are resistant to this disease. Every year, wheat varieties are evaluated for resistance to BYDV by the NC Small Grains Extension Program. A list of wheat varieties and their resistance to BYDV can be found at: http://www.smallgrains.ncsu.edu/Varieties/Varieties.html.

Armyworm. Armyworm moths are one of a few moths active in late winter/early spring. Moths prefer to lay eggs on various grasses, and wheat can be very attractive. Thick planting, narrow row spacing, and high nitrogen rates promote dense and lush growth, which is conducive to high armyworm infestation. The armyworm is described, with illustrations, in the “Keys to Small Grains Insects”.

Armyworm is the only caterpillar found in large numbers in small grains. They are active at night, hiding under plant litter (such as old corn stalks) and at the base of wheat plants during the day. After dark, they feed on foliage from the bottom of the plant upward. Armyworm caterpillars may cause serious defoliation and substantial head drop. They are most prevalent in the northeast part of NC.

Organic Pesticides There are a number of insecticides approved for use in certified organic production systems—mainly non-synthetic compounds or biocontrols. These include: neem (and its derivatives), Bacillus thuringiensis, Beauvaria spp., diatomaceous earth, Pseudomonas, pyrethrum, spinosad, and Trichoderma. While these products do have potential for controlling insect pests in corn, no research has been done on them in field corn in North Carolina and, therefore, we can give no recommendations for their use in the state. The cost of organically approved pesticides may be prohibitively expensive for field crop production. Conditions for use of a pesticide must be documented in the organic system plan (NOP 2000). See Pest Management Web Resources for more information or check out the OMRI site.

Management options :There are few cultural management options available for armyworm. Organic growers have the choices of accepting the feeding of armyworms or using an organically approved insecticide in emergency situations. Research on cereal leaf beetle has shown that 100% defoliation, occurring early in the heading period (Feeks 10 – 10.5), has a maximum yield reduction of approximately 45%. Later defoliation, for example in the dough-stage (Feeks stage 11.2), has much less impact and typical armyworm populations, with large larvae, occur during this period, or later. Accepting the feeding of armyworm would likely not result in large yield losses, unless plants were defoliated early (before or during the heading period). For scouting information on army worm, check the 2004-05 Small Grain Production Guide.

Cereal Leaf Beetle. The cereal leaf beetle (CLB) has one generation each year and both the adult and larval stages eat leaf tissue on wheat and oats, but not barley, triticale or rye. Leaf feeding, by larvae, during April and May can reduce yields. Although adults will feed on small grains, their feeding does not affect the plant's performance. However, CLB larvae can reach very high numbers and larger larvae can defoliate wheat plants. Often CLB populations will be somewhat localized and damage may be confined to spots or sections of fields or farms. Damage may build-up quickly, often in as little as five days, after larvae molt from a smaller, less damaging larval stage, and become larger, more voracious feeders. If accompanied by warm temperatures, larger CLB larvae can damage wheat in a few days when populations are high and the wheat plants will not replace these damaged leaves. Leaf feeding reduces the plant's ability to photosynthesize and limits reproductive growth, particularly if the upper leaves are destroyed. Yield reduction to 45% has been observed when defoliation was near 100% and the damage occurred early in the heading period. Later damage, late in the head-fill period, does not have a great impact. Yield reductions of 10% to 20% are typical in infested commercial fields. Click here for a full description of cereal leaf beetle.

Cereal leaf beetle is an introduced pest and few native biological control agents affect populations of adult beetles, or its eggs or larvae. A few generalist predators, like lady beetles, appear to consume cereal leaf beetle eggs and, perhaps, young larvae in early spring. The North Carolina Department of Agriculture has released several species of exotic parasites throughout the state. These parasites develop within cereal leaf beetle eggs or larvae and have the potential to keep populations below an economic level. Parasite release programs have worked well in several other states but have had limited success in our state up to the current time.

Cultural Practices: Cereal leaf beetle like thin wheat fields. Given the choice of attacking a thin field full of little plants, or a thick lush field full of large healthy wheat, the beetles will choose the former. This gives organic producers a potential advantage over the pest. To minimize the chances of beetle invasion, the organic producer needs to do everything possible to assure a thick, well tillered, healthy crop. That means planting on time, good seed bed preparation, using high quality seed, correct drill calibration, and being sure to get good soil-seed contact at the proper seeding depth. These steps will also increase the crop’s tolerance to CLB feeding.

Insecticide application may be performed in emergency situations, with organically approved insecticides labeled for cereal leaf beetle. For scouting information for cereal leaf beetle check the 2004-05 Small Grain Production Guide.

Excellent cereal leaf beetle information can be found in the North Carolina Small Grain Production Guide 2004-05 which is also available from North Carolina county Extension agents. Additionally, a special publication on this pest can be found here.

Hessian Fly. In recent years, numerous North Carolina fields have suffered extensive losses because of Hessian fly infestations. Historically a wheat pest in the Midwest , changes in field-crop production including early planted wheat, increased adoption of no-tillage double-cropped soybeans, and the use of wheat as a cover crop for strip-tillage cotton and peanut production have permitted the Hessian fly to reach major pest status in North Carolina . Here are several methods organic producers should use to minimize their risk to this pest.

Rotation: Because the Hessian fly life cycle is largely dependent upon the presence of wheat stubble, rotations which prevent new wheat from being planted into or near a previous wheat crop’s stubble will be the most effective way to prevent infestations. Growers should avoid planting wheat into last season’s wheat stubble! Continuous no-tillage wheat-double-cropped-soybeans may result in severe problems and should be avoided in Hessian fly problem areas. Additionally, since the Hessian fly is a weak flier, putting at least one field distance (about 1/4 mile) between the location of new wheat plantings and the previous season’s wheat fields can be a successful method of preventing new infestations.

Tillage: Disking wheat stubble after harvest effectively kills Hessian fly. Planting soybean no-till into wheat stubble enhances Hessian fly survival by preserving the site where they spend the summer. Burning wheat straw will reduce populations but many will survive below the soil surface. Therefore, burning is not as effective as disking.

Careful use of cover crops: Serious Hessian fly infestations have occurred in areas where wheat for grain was planted near early-planted wheat for cover, or early planted wheat for dove hunting purposes. In cropping systems where cover crops are used, such as in strip-till cotton or peanut production, the use of other small grains besides wheat will reduce Hessian fly populations. Oats, rye, and triticale are not favorable for Hessian fly reproduction and do not serve as a nursery making them preferable over wheat for cover cropping in areas where wheat for grain is also produced. If triticale is used for cover cropping, varieties such as “Arcia,” that are adapted to NC, should be planted.

Delayed Planting (Not A Good Idea!): Because Hessian fly adults are killed by freezing temperatures, a traditional method for preventing Hessian fly infestation is to delay planting until after the first freeze (often called the fly-free date). This concept has not worked well in North Carolina because an early freeze is not a dependable event. Often a “killing freeze” does not occur until December in many areas of North Carolina , after most growers need to have wheat planted for agronomic purposes.

Avoid Planting Early: Like powdery mildew, Hessian fly like warm fall weather. Research in North Carolina and Virginia has shown that planting wheat ahead of the suggested starting dates (see the 2004-05 Small Grain Production Guide) puts the crop at risk to Hessian fly attack unless an insecticidal seed treatment or an early foliar insecticide is used. Organic producers throughout North Carolina should avoid planting ahead of the recommended planting dates.

Resistant Varieties: Many wheat varieties are advertised as having Hessian fly resistance. Unfortunately, in most cases this resistance is to a “biotype” or kind of Hessian fly that we do not have in North Carolina . To be effective in North Carolina , wheat varieties must be specifically resistant to “Hessian fly biotype-L”. Wheat varieties with biotype-L resistance can be found on the web here.

Want more information on Hessian Fly? See The Hessian Fly: A Pest of Wheat in North Carolina (AG-368) (or ordered through your county Extension office).

 

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