Managing Insect Pests In Organically Certified Wheat
John Van Duyn, North Carolina State University, Entomology Extension SpecialistPrintable Version
Wheat fields are infested with many kinds of insects. Some feed upon the living wheat plants, others prey upon the plant feeding insects, while yet others may feed on dead organic matter or weeds, or merely be passing thorough. A few species may become pests but, although present, most often 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 http://www.cropsci.ncsu.edu/smallgrains/pubs.htm
Wheat Feeding Aphids and Their Management:
There are three primary species of aphids found in NC wheat; the English grain aphid, bird-oat cherry aphid, and corn leaf aphid. All aphids in wheat have a short life cycle, under favorable food and temperature conditions, and may quickly multiply to large populations. Aphids suck sap from leaves, stems, and heads. Feeding may reduce yield potential but does not have a dramatic effect on the crop. Aphids are also vectors of barley yellow dwarf virus (BYDV).
A complex of biological control agents accompanies aphids, including insect parasites, insect predators, and insect 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 conditions that help reduce the chance of damaging aphid population are: (1) avoiding early planting and (2) avoiding soil nitrogen levels above the recommended range. By planting after early fall conditions are past, e.g. in November, wheat seedlings are not available when the aphids are most active, thus decreasing the incidence of aphids moving into and within wheat fields. Cool temperatures limit aphid reproduction and movement, thereby reducing the chance of high populations and aphid-transmitted BYDV. Aphids remain less active in cool weather, not feeding, moving or reproducing. Populations decline in freezing weather. In the spring, aphid populations are less damaging than fall population because plants are larger, and more tolerant to aphid feeding and BYDV. Also, aphids reproduce most rapidly in warm weather and on high-nitrogen content plants. Maintaining nitrogen levels within prescribed agronomic level also helps to avoid high aphid populations. In instances of extremely high aphid populations, a rare occurrence, yields will decline somewhat, but not be dramatically reduced.
The Cereal Leaf Beetle:
The cereal leaf beetle has one generation each year and both the adult and larval stages eat leaf tissue on wheat and oats, but not barley and 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, cereal leaf beetle larvae can reach very high numbers and larger larvae can defoliate wheat plants. The larvae eat long strips of green tissue from between leaf veins and may skeletonize entire leaves, leaving only the transparent lower leaf surface tissue. Severely defoliated fields can take-on a white "frosted" cast after green leaf tissue is consumed. Often cereal leaf beetle 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 feeder. If accompanied by warm temperatures, larger CLB larvae can damage wheat a few days when populations are high. 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. The plant will not replace damaged leaves. See the following web page for a full description of cereal leaf beetle: http://www.ces.ncsu.edu/depts/ent/notes/Grain/smg_3.html
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.
Management options for cereal leaf beetle:
Cereal leaf beetle shows a preference for very late-planted fields and thin stands of small grains. Consequently, planting on time and establishing a uniform, full plant population makes fields less attractive to egg laying adults and increases the tolerance of the wheat plant population. Managing wheat to achieve high yields will automatically make those fields in the best possible circumstance to ride-out cereal leaf beetle populations. However, in some years or locals, this insect may cause significant yield loss. Insecticide application, with organically approved Entrust® (from Dow AgroSciences), which is labeled for cereal leaf beetle, is an option in emergency situations. Entrust® is a formulation of natural spinosad, a toxin derived from bacteria, formulated to meet organic specifications.
Scouting for cereal leaf beetle:
Scouting should be done after peak egg-laying has occurred and the majority of eggs have hatched, usually in early- to late-April. Development will occur earlier in the Piedmont vs. the Mountains and Coastal Plain; earlier in south, later in north. On warm springs scouting should be done earlier than on cooler springs. Scouting should be done when both eggs and mostly small larvae are in the field (counts should include both forms). If the population is mainly made-up of eggs, then scouting should be at a later date, when a minimum of 50% is in the larval stage.
Eggs are elliptical, about 1/32 of an inch long, and colored yellow to burnt brownish yellow. Most often the eggs are laid singly or in small scattered groups (often end-on-end) on the upper leaf surface between, and aligned with, the leaf veins.
Samples should be taken at a minimum of 10 random sites in the interior of each field (avoid the edges). At each site, 10 tillers (stems) should be examined for cereal leaf beetle eggs and larvae. This will result in 100 tillers (stems) per field being examined. Eggs may be on the leaves near the ground. Record the number of eggs and larvae counted at each sample site. After leaving the field, calculate the total number of eggs + larvae found. Alternatively, stems can be examined at random while walking through the major portion of the field; again 100 stems per field should be examined.
Because cereal leaf beetle is often unevenly distributed in the field, it is necessary to determine if a portion of a field is above threshold. If the random sampling indicates an uneven distribution (lots in some samples but few in others), it may be necessary to subdivide the field into two or more parts and sample each part as an individual field.
In instances of very high counts the sampling can be abbreviated after the samples have exceeded the threshold. For instance, after examining 30 tillers the scout has found 35 eggs + larva, which exceeds the threshold for 100 stems. However, if this is done the scout should realize that the portion of the field not scouted may not have high populations.
Threshold for Egg/larval Counts:25 eggs and/or larva total per 100 tillers (this is an average of one per each four tillers or 0.25 eggs and/or larva per tiller)
Scouting Frequency:Once egg-laying has reached a peak, many fields will need only a single scouting for eggs and larvae. If the proportion of eggs in the sample is 50% or greater then sample again in 5-7 days.
Hessian fly is a small, two winged insect resembling a mosquito (see pictures and information at http://www.ces.ncsu.edu/plymouth/pubs/ent/HFLYupdate03.htm. Flies lay small, orange, elongated eggs on the upper leaf surfaces, and between leaf veins, in the fall and spring; it has two to three generations in a wheat growing season. Larvae hatch and quickly move deep into the whorls of seedling plants or young tillers or, on larger plants, locate under the leaf sheath at the lower joints on the stem. Larvae (maggots) tear into tender plant tissue with mouth-hooks and eat the liquid cell content. In the fall, and on small plants, a concentration of larvae can destroy meristem tissue and cause tillers or plants to be stunted, unproductive, or die. On larger plants, in the spring, larval feeding at the joints reduce plant vigor and weakens the stem, causing fewer heads, smaller heads, and lodging. High concentrations of Hessian fly larvae infesting small plants in the fall can be very damaging. High spring infestations usually result from a recycling of fall populations in the same field and may also be very damaging. Hessian fly can be successfully managed by not allowing wheat fields to become significantly infested in the fall and by using resistant varieties.
Avoiding the Hessian fly:
Hessian fly can be avoided if the production system is properly planned. Hessian fly aestivates (warm temperature hibernation) during the summer, as pupae, in wheat residue. Flies emerge in the early-to-mid-fall. Early emerging flies (e.g. September) may lay eggs on volunteer wheat plants in the fields of the previous season’s wheat crop. They may also infest a few species of wild grasses or very early-planted wheat, planted for cover crop or dove hunting fields. New flies, from these early fall sources, may infest newly planted wheat later in the fall (e.g. in October). Also, late emerging flies from the previous wheat crop may contribute to these infestations.
Summer carry-over of Hessian fly is largely dependent on the presence of wheat stubble. Rotations which prevent new wheat from being planted into, or near, a previous wheat crop’s stubble is an effective tactic for reducing damaging infestations. Growers should avoid planting wheat into last season’s wheat stubble! Continuous no-tillage wheat-double-cropped-soybeans is a recipe for making a Hessian fly problem. Since Hessian fly is a weak flier, putting distance between the location of new wheat plantings and the previous season’s wheat fields is also important in avoiding economically damaging fly populations. Although Hessian fly can become serious under other situations, most serious infestations occur when wheat is early planted into wheat stubble or into fields next to wheat stubble.
Tillage effects on Hessian fly:
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 pupa spend the summer. Burning wheat straw will reduce pupa but many are found below the soil surface. Therefore, burning is not as effective as disking.
Choosing 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.
Hessian fly adults are killed by freezing temperatures and 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 been entirely satisfactory in North Carolina because an early freeze is not a dependable event, however, it does work much of the time by avoiding active populations of flies or reducing the time flies are active within fields of wheat seedlings.
Many wheat varieties are advertised as having Hessian fly resistance. Unfortunately, in most cases this resistance is to a “biotypes”, or kind of Hessian flies, that do not occur in North Carolina. “Hessian fly biotype-L” is most common in NC and wheat varieties with biotype-L resistance must be used to be successful. A list of biotype L resistant varieties may be found at http://www.smallgrains.ncsu.edu/Varieties/Varieties.html. This list is updated every year. In most cases, varieties rated as “good” in Table 1 should provide enough protection to avoid economic loses due to Hessian fly biotype-L if coupled with the management tactics presented above.
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. Young armyworm larvae are pale green, yellowish, or brown and have a habit of looping as they crawl. When they become larger (1 to 1-1/2 inches), they are greenish-brown with pale white and orange stripes running down their bodies; the head is honeycombed with faint dark lines. The armyworm is described, with illustrations, in the “Keys to Small Grains Insects” at http://www.ces.ncsu.edu/plymouth/pubs/ent/index4.html. 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. As they eat the lower foliage, or as it is destroyed by leaf pathogens, the armyworm larvae feed higher, eventually reaching the fag-leaf. If populations are high, large caterpillars may also feed on the stem just below the head. Armyworm caterpillars may cause serious defoliation and substantial head drop. They are most prevalent in the northeast part of NC.
There are few cultural management options available for armyworm. Organic growers have the choices of accepting the feeding of armyworms or using the organically approved insecticide Entrust® 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), had 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.
Scouting should be done for armyworm to determine the intensity and timing of infestations. The threshold used in conventional agriculture is two caterpillars, 3/8 inch or larger, per square foot (ca. 87020 armyworms per acre), and can be used in organic production. Above threshold populations pose the greatest threat if the majority of the populations is composed of large larvae (e.g. ¾ inch or larger) and are feeding at the early head-set stage (Feeks stages 9 through 11).
Scouting for armyworm:
Infestations of armyworms are not easily detected by casual observation because caterpillars hide during the day. Fortunately, there are several signs of armyworm infestation, and caterpillars can also be monitored if the correct technique is used. Blackbirds (grackles and red-winged blackbirds) commonly search for armyworms in small grain. Scout any feld with signifcant bird activity. Signs of armyworm leaf feeding and caterpillar droppings can also be good indicators. Feeding is sometimes inconspicuous because small caterpillars do not eat much and feeding signs are often concentrated on the lower part of the plant. When caterpillar populations are high, droppings may be seen on the soil surface, but should not be confused with weed seed.
Fields should be scouted for armyworms during Late April and during May. April and early-May caterpillars are usually small. Thorough scouting should not be done until the average caterpillar size is approximately 3/8 inch, because populations of little caterpillars are difficult to accurately detect and they often die-out. Early scouting can be used to determine if the caterpillars have reached 3/8 inch size. Once caterpillars reach 3/8 inch or more in length (on average), take at least five samples per field (10 samples in larger felds of 20 acres and more). For each sample, examine 3 square feet of area (e.g. two, 3-foot-long strip containing one 7 inch drill-row). Look for and count the caterpillars in litter, around the base of plants, and under old crop residue. Pay special attention to felds in which blackbirds and grackles are actively landing into the field. Fields should be scouted weekly until a management decision is made. Re-infestation of caterpillars in May does not occur.
Corn and Soybean Insect Note (ENT/cs-04)
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Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
CAUTION: The information and recommendations in these Notes were developed for North Carolina conditions and may not apply elsewhere.