Cutworms are the immature stages of drab, brownish moths that are most active at night. Several species can damage small grains.
Cutworm moths lay eggs in grassy, weedy fields; army cutworm moths lay eggs even on bare soil. Newly hatched cutworms are brown to black and feed on small grain seedlings. The older larvae look shiny or greasy (Fig. 7). The larvae clipthe aboveground part of the plant from the root system at or below the soil surface (Fig. 8). Infested fields look as if they have been closely grazed, and the damage may be “clumped” in spots in the field.
In Texas, the most common cutworm species attacking small grains is the army cutworm (Euxoa auxiliarias). Although it is a true cutworm, it feeds much like an armyworm. The caterpillars are surface feeders and move to above ground at night and on cloudy days to feed. They cut off small plants at or near the soil surface. During the day, the larvae hide under soil clods and debris close to the base of the plant. The army cutworm has one generation per year. During late summer and early fall, the female moth lays 1,000 to 2,000 eggs as it migrates through an area. The eggs hatch in a few days, and the larvae feed periodically through the fall and winter on warmer days. By mid- to late winter, the larvae in small grains can be ⅜ to 1½ inches long. Large populations can cause considerable damage by defoliating plants and reducing stands, especially in February and March as the grains begin to green up. Particularly vulnerable to the army cutworm are
thin, late-planted, or poorly tillering stands.
In outbreak years, fields can have 10 to 20 cutworms per square foot. On sunny days, they will be under debris or slightly below the soil surface. Consider applying insecticide when infestations reach four to five cutworms per square foot. The larvae pupate in the soil in early spring and emerge as moths about 3 to 4 weeks later. These moths are attracted to lights and can become a nuisance around homes and buildings. The moths migrate from the Great Plains to the Rocky Mountains to escape the summer heat, returning in late summer and early fall to begin the cycle again. To minimize cutworm numbers, reduce weeds and crop residues in fallowed fields, and delay planting until the fields have been clean-plowed.
Caterpillars of the fall armyworm (Spodoptera frugiperda (J. E. Smith)) feed on seedlings in the fall, thinning stands and reducing the production of early-season forage. The larvae are most consistently shades of brown but may also be greenish to nearly black (Figs. 9 and 10). They have a white inverted “Y” between the eyes (Fig. 11) and four distinct black spots on top of the eighth abdominal segment. Mature larvae are 1½ inches long. In addition to small grains, fall armyworms feed on alfalfa, corn, cotton, cowpeas, peanuts, grain sorghum, and sorghum-grass hybrids.
Early planting of small grains greatly increases the risk of fall armyworm infestations. The moths may deposit egg masses (Fig. 12) on the leaves of seedling small grains. Small larvae feed on the leaf tissue, creating tiny “windowpanes” in the leaves. Larger larvae consume entire leaves and are more difficult to control. On seedlings, consider applying insecticide if fall armyworms are present and reducing the stand.
Once plants are established, control is suggested when there are 4 or more larvae 1 inch or longer per square foot and when their damage threatens the stand. Delaying planting can reduce damage when other host crops have large infestations or when dry conditions limit the attractiveness of other hosts. Under these conditions, large numbers of armyworm may infest newly planted wheat.
The larvae of the true armyworm (Pseudaletia unipuncta (Haworth)) can attack small grains in large numbers, devouring all material in their path. Outbreaks are favored by cool, damp weather from late March through June. When fully grown, the larvae are 1½ inches long and green to brown with light stripes on their sides and back (Fig.13). A brown or dark band is on the outer side of each proleg (small, fleshy leg on the abdomen). The head, which has a pattern of narrow lines that looks like a net, lacks the white inverted “Y” of the fall armyworm.
Armyworm larvae do not develop well once daytime highs average 88°F. The heat causes their numbers to decrease dramatically. Infestations often begin in areas where the small grains are the tallest and thickest or near the edge of fields, where weeds provide a favorable environment. During the day, armyworms hide at the bases of the plants; they move up the plants to feed late in the afternoon, at night, and during cloudy weather. They can cause extensive damage below the crop canopy before they are detected. Early armyworm detection is important because small larvae are easier to control. Also, the larger the larvae, the more they consume. Signs of damage include defoliation and beard and head clipping. It is important to protect the flag leaf and grain head from armyworm damage. Control measures are suggested when four to five larvae per square foot are found in combination with evidence of extensive feeding on lower leaves.
Wheat Head Armyworm
Although wheat head armyworms are a minor pest of wheat, they appear each year as a late- season wheat pest in the Texas High Plains. There are 13 known species of wheat head armyworms in the genus Dargida (synonym Faronta) and all of the larvae and moths look similar. The moths have a wingspan of about 1¼ to 1½ inches. Moths are yellow-brown with a lengthwise brown stripe on each forewing. They emerge to lay eggs in the spring, and the first generation of larvae emerges in late May and June. Infestations often occur along field margins.
The larvae are slender and greenish to light tan, depending on the maturity of the grain (Fig. 14). They can be up to 1½ inches long. Yellow, white, and brown stripes run lengthwise on each side of the body. The larvae feed in the wheat heads and damage the kernels, primarily those in the soft dough stage. Infestations are heavier in dryland fields and on the edges of irrigated fields. Feeding damage is most often evident after harvest, when hollowed out kernels become apparent. No thresh- olds have been established for treatment. Plus, the pre-harvest intervals (PHIs) for most available insecticides make it difficult to treat when the larvae are causing damage. Most products labeled for other armyworms have a 14- to 35-day PHI for grain. Insecticides with the active ingredients malathion and chlorantraniliprole have PHIs of 7 days and 1 day, respectively.
Greenbugs (Schizaphis graminum (Rondani)) are pale green aphids that usually have a dark green stripe on the back (Fig. 15). They are about 1⁄16 inch long. Greenbugs suck plant juices and inject toxins into wheat plants, causing the leaves to yellow and die. They also are an important vector of the barley yellow dwarf virus and possibly more plant diseases.
Under favorable conditions—temperatures between 55 and 95°F—greenbugs reproduce rapidly, develop in large numbers (Fig. 16), and can cause economic losses. Their natural enemies, however, reproduce slowly at temperatures below 65°F. Consequently, greenbugs may increase to enormous numbers in cool weather, while their natural enemies are multiplying slowly.
In winter, 99 percent of the greenbugs can be killed when temperatures average below 20°F for at least 1 week. The population also must be without protection from snow cover. Greenbugs can infest spots in fields or entire fields. As the populations increase, areas in the field may turn yellow from aphid feeding. Heavy, uncontrolled infestations may kill the plants. Greenbugs cause more damage when small grains lack moisture during a mild winter and cool spring. The damage may be con- fused with moisture stress, nitrogen deficiency, or dryland root rot (foot rot).
Estimating greenbug infestations: The Glance ‘n Go greenbug sampling system calculates treat- ment thresholds based on the potential crop value, cost of control, and time of year. This method is quicker and simpler than counting greenbugs.
The system was developed by wheat researchers of Oklahoma State University and USDA–ARS at Stillwater, Oklahoma. For Glance ‘n Go information and scouting forms, visit http://entoplp.okstate. edu/gbweb/index3.htm.
Producers still wanting to count greenbugs can walk diagonally across the field, making at least five random counts per 20 acres of field area. Each count should consist of 1 linear foot of row. Green- bugs can be counted while they are on small plants. For larger plants, slap the plant against the ground or a clipboard to jar the insects loose for counting. If the greenbugs are numerous, estimate the number of insects present. Sample greenbugs during the warmest part of the day, when they are most likely to be exposed on the aboveground parts of the
plants. During cool, dry weather, the insects may congregate in loose soil at the bases of plants, which makes detection and chemical control difficult.
When to treat greenbugs: Table 2 provides a general guide for determining the need for treat- ment when counting greenbugs. It is impractical to specify all the conditions under which to apply insecticides for greenbug control. Some of the factors are the number of greenbugs present, the size and vigor of the plants, air temperature, time of year, moisture conditions, plant growth stage, and effectiveness of parasites and predators. However, low temperatures slow the activity and effectiveness of most insecticides. It may take twice as long for an insecticide to kill at 45°F as it would at 70°F. For best results, apply insecticides when temperatures are above 50°F. If you must spray at lower temperatures, use the highest rate recommended.
Irrigated small grains can withstand larger greenbug populations. The presence of yellow or brown plants caused by greenbug feeding in spots in the field may indicate a need to estimate infestation levels. Occasionally, treating very young plants may be warranted when greenbug populations average 25 to 50 aphids per foot of drill row.
Heavy, rapidly increasing greenbug infestations can cause excessive damage. However, when the weather is warm, lady beetles and parasitic wasps can reduce greenbug populations. Where there are one to two lady beetles (adults and larvae) per foot of row, or 15 to 20 percent of the greenbugs are mum- mies from being parasitized, delay control measures until you can determine whether the greenbug population is continuing to increase. Other impor- tant predators include spiders, damsel bugs, lace- wing larvae, and syrphid fly larvae. When weather conditions are favorable for predators and parasites, they will significantly reduce greenbug populations within a week. More information is available in the Texas A&M AgriLife Extension publication Biological Control of Insect Pests of Wheat, which is available online at agrilifebookstore.org.
Insecticide-resistant greenbugs: In 1990, surveys conducted in sorghum fields in counties north of Amarillo found greenbugs that were resistant to some registered insecticides. And, a resistant population was again found in the western Panhandle in 2014. Resistant greenbugs may be mixed with susceptible greenbugs and be detected only after an insecticide application for greenbugs or Russian wheat aphids. To delay greenbug resistance to pesticides, apply insecticide only to fields where economic thresholds have been exceeded.
Host plant resistance: In 1996, TAM-110 was the first wheat variety to carry resistance to all current greenbug biotypes (E, I, and K). In 2005, TAM-112, which had the same greenbug resistance genes as TAM 110, was released with improved yield and milling characteristics. In some years, planting wheat varieties with resistance to disease may be equally or more important than planting varieties with resistance to insects.
Russian Wheat Aphids
The Russian wheat aphid (RWA) first appeared in the United States in March 1986 in the Texas High Plains. It has since spread throughout the Great Plains, into Canada, and to the West Coast. This aphid (Diuraphis noxia (Mordviko)) is lime green, spindle shaped, and about 1⁄16 inch long.
It has short antennae and a projection above the cauda, or tail. The projection gives the insect a “double tailed” appearance (Figs. 17 and 18).
Russian wheat aphids lack prominent cornicles. While feeding, Russian wheat aphids inject a toxin that causes white and purple streaks run- ning lengthwise on the leaves. Tillers of heavily infested plants appear flattened, and leaf edges roll inward, giving the entire leaf a tubelike appear- ance (Fig. 19). Russian wheat aphids prefer feeding on the younger, uppermost leaves of a plant. Unlike the greenbug and bird cherry-oat aphid, the Russian wheat aphid does not spread the barley yellow dwarf virus.
Because these aphids cause the most damage when small grains are moisture stressed, use cul- tural practices that reduce crop stress. Destroying volunteer wheat and planting later will delay the initial infestation.
Predators and parasites are also important in reducing Russian wheat aphid populations. Many of the natural enemies that attack greenbugs also attack Russian wheat aphids. To help conserve these natural enemies, use insecticide judiciously.
For hosts, the Russian wheat aphid prefers wheat and barley to oats, rye, and triticale. They are occasionally observed on corn and sorghum but are not known to cause any damage.
In 2003, a Russian wheat aphid biotype was detected that had developed resistance to all previ- ously resistant Russian wheat varieties with the Dn4 resistant gene. This aphid biotype, RWA2, was first found in Colorado. The formerly resistant wheat varieties are Ankor, Halt, Prairie Red, Prow- ers, and Stanton. Although adapted to the Texas High Plains, they were used primarily in Colorado and Western Kansas. A survey across the Great Plains in 2005 found that 88 percent of the Russian wheat aphids sampled from the Texas High Plains were the resistant RWA2 biotype. However, surveys in 2010 to 2013 indicated that the aphid had shifted to biotypes that were not resistant (90 percent)
to the Dn4 resistant gene. These changes in RWA biotypes indicate that to help protect against RWA damage, producers need to plant wheat varieties with the latest resistance to RWA. Also, scout the fields routinely for unexpected infestations and damage. RWA populations are largest and, consequently, the most damaging in the High Plains (Fig. 20).
Although the aphid occurs in the Rolling Plains, it is not an important pest because it cannot survive the summer (over-summer) there. In the High Plains, RWA can over-summer on warm-season grasses such as buffalo grass, green sprangletop, and several species of grama grass. Cool-season grasses that are wild hosts of the Russian wheat aphid include various brome grasses, jointed goat grass, and several species of wheat grasses. Insec- ticides applied to the seed can control early-season infestations of RWA. Once the crop has reached soft dough, an insecticide application may not be justified.
Sampling and economic thresholds for RWA: Sampling involves walking across a field and randomly selecting 100 tillers, each from a dif- ferent site. To prevent bias, reach down and grab the tillers without looking at them. Then carefully examine each tiller and record the number of tillers that are infested. Consider any tiller with one or more Russian wheat aphids as infested. Determine the percentage of tillers that are infested. Then use Table 3 to decide whether treatment is justi- fied. For example, if the market value of the crop is projected to be $50 per acre and control costs are $9 per acre, the treatment threshold is 36 percent infested tillers. The thresholds in Table 3 are for Russian wheat aphids infesting wheat in late winter and spring. The thresholds are based on the cost of control and the market value of wheat. For every
1 percent of the tillers infested, yield drops by 0.5 percent.
Table 3. Russian wheat aphid economic threshold using percent infested wheat tillers as the sampling unit
|Control cost per acre $||Market value of crop ($) per acre|
|Percent infested tillers|
A formula for calculating when to treat can be used instead of the table. The formula for deter- mining the economic threshold level is based on the percentage of tillers infested before flowering:
To calculate the threshold during and after flowering, substitute 500 for 200 in the formula.
Bird cherry-oat aphid
Bird cherry-oat aphids feed on various grains and grasses and are particularly abundant on small grains. These aphids (Rhopalosiphum padi (L.)) are yellowish green, dark green, or black and have a reddish-orange area around the base of the cornicles (Fig. 21). Because bird cherry- oat aphids do not inject a toxin while feeding (unlike the greenbug), they are less damaging. Control measures for the bird cherry-oat aphid are rarely needed to prevent damage from direct feeding. However, an insecticide treatment may be neces- sary when this aphid is very abundant and the crop is under moisture stress. Like the greenbug, the bird cherry-oat aphid is an important vector of the barley yellow dwarf virus and possibly other diseases. Seed treatments to control early-season infestations of greenbugs and bird cherry-oat aphids may reduce the potential spread of barley yellow dwarf virus by these aphids.
English Grain Aphid
English grain aphids (Macrosiphum (Sitobion) avenae (F.)) are about 1⁄10 inch long and larger than other cereal aphids. They are light green to brown and have long, black antennae, cornicles, and legs (Fig. 22). These aphids can be a concern in the spring, when they feed on stems during flowering and on developing kernels in the wheat heads. Their feed- ing can result in shrunken grain and lower test weight. Because many of the same predators and parasites that help control the greenbug also control English grain aphids, they seldom cause yield losses. They are a vectors of barley yel- low dwarf virus. Treatment thresholds for English grain aphid have not been developed for Texas.
Until more information is available, the thresholds used on winter wheat in Nebraska can serve as a guide for managing English grain aphid in Texas.
To use these thresholds:
- Count the English grain aphids on each stem and each head at several sites across the
- Calculate the average number of aphids per stem (including those on the head).
- Consider applying insecticide if infestations across the field average
- 5 or more aphids per stem during the flowering stage
- 10 or more per stem during the milk stage of grain development
- More than 10 aphids per stem during the milk to medium dough stage
Rice Root Aphid
The rice root aphid (Rhopalosiphum rufiabdominalis (Sasaki)) feeds on plant roots in spots within wheat fields. Stunted plants may be the first indica- tion of its presence, with only a small percentage of plants infested. These aphids are olive or dark green and up to 1⁄10 inch long (Fig. 23). They usually have a reddish area at the rear between and
around the base of the cornicles. Rice root aphids vector barley yellow dwarf virus. Insecticidal control measures have not been developed for this insect. Its primary hosts are peaches and plums. Secondary host plants include rice, wheat, and other small grains.
Winter Grain Mite
The winter grain mite (Penthaleus major (Dugés)) feeds on the leaves of barley, oats, and wheat. Feeding turns the leaf tips brown, stunts the plants, and causes them to appear silvery gray. These mites range from 1⁄32 inch to 1⁄16 inch long. The adult has four pairs of reddish-orange legs, and the body is dark brown to black (Fig. 24). Winter grain mites feed primarily at night and remain around the base of the plant during the day. They are less active in hot, dry weather. The most significant damage occurs in winter and early spring. It is usually more severe in fields where small grains were planted in previous years. To reduce infestations, rotate with other kinds of crops. The appearance of feeding symptoms and the presence of mites indicate the need for control.
Brown Wheat Mite
The brown wheat mite (Petrobia latens (Müler)) is about the size of the period at the end of this sentence and is considerably smaller than the winter grain mite. Its rounded body is metal- lic dark brown with a few short hairs on the back. The front legs are about twice as long as the other three pairs of legs (Fig. 25). This mite occurs throughout the High Plains and Roll- ing Plains. Brown wheat mites are most prevalent in dry weather (Fig. 26), and populations increase when wheat suffers from deficient moisture.
Miticides are often not warranted if the crop is so drought stressed that it cannot respond.
Wheat curl mite
The wheat curl mite (Aceria tosichella Keifer) is white, sausage shaped, and about 1⁄100 inch long (Fig. 27). It has four small legs on the front. This mite vectors wheat streak mosaic, triticum mosaic, and wheat mosaic virus (formerly called High Plains virus) but causes very little damage otherwise. Mite feeding alone causes the leaves to roll and take on an onion leaf appear- ance. If the virus is present, the leaves become mottled and streaked with yellow. Wheat curl mites reproduce fastest at 75 to 80°F. They crawl very slowly and depend almost entirely on wind for dispersal. The mite is most active during warm weather and moves mostly on warm, southwesterly winds.
Consequently, most symptoms of the wheat streak mosaic virus develop from southwest to northeast across a field. The mites over-summer on grass- type hosts and volunteer wheat. Volunteer wheat is the most important host for the mite as well as for the wheat mosaic virus, wheat streak mosaic virus, and possibly the triticum mosaic virus.
Populations of wheat curl mites are likely to be highest when:
- Mites migrate to early volunteer wheat after hail damages wheat that is nearing
- July rains produce good stands of volunteer wheat.
- Volunteer wheat is not destroyed, or not de- stroyed until after the planted wheat is
- Wheat is planted
- Summers are
- Autumn is warm and dry, the optimum con- ditions for mite reproduction and
There are no remedial control options once a wheat plant is infected with the wheat streak mosaic virus or wheat mosaic viruses. Chemi- cals do not control the mites or prevent disease infections. Therefore, the most effective control strategies are avoiding infection and choosing
plant resistant varieties. Research has shown that TAM112 has resistance but not immunity to both the mite and the viral diseases.
Prevent the wheat curl mite from transmitting viruses by breaking the “green bridge” from one wheat crop to the next by using these management practices:
- Eliminate grass weeds and volunteer wheat around your fields and neighboring proper-
- Delay plantings near properties under the Conservation Reserve Program (CRP) or native stands of grasses until the grasses have
- Destroy grass weeds and volunteer wheat by tillage or a burn-down herbicide at least 21 days before planting wheat.
The Hessian fly (Mayetiola destructor (Say)) infests wheat in the Central Rolling Plains and central and southwest Texas (Fig. 28). The mos- quito-like Hessian fly adult is 1⁄10 inch long and has dark wings, a black thorax, and a dark red abdo- men. Females deposit an average of 200 eggs in clusters of 5 to 12 glossy red eggs in the grooves on the upper leaf surface. They prefer to lay eggs on younger plants and leaves. After hatching, the lar- vae move down the leaf grooves and under the leaf sheath, coming to rest just above the plant crown or just above a node. As they develop, the larvae suck plant juices and form a shallow depression in the stem. Newly hatched larvae are red but turn lighter in a few days. Fully developed larvae are white with a semitransparent green stripe down the middle of the back (Fig. 29). At maturity, the larva forms rigid, dark brown, outer case, or puparium. This period is known as the “flaxseed” stage because the pupar- ium resembles a flax seed (Fig. 30). The Hessian fly survives the summer as a dormant, fully devel- oped larva inside the puparium. The adult fly emerges from the puparium (Fig. 31). It will live no more than 3 days.
The larvae injure wheat by feeding on stem tissue at the crown of young plants or just above the nodes on jointed wheat. They cause more damage to newly emerged and younger seedlings than to older, estab- lished plants. In the fall and early winter, feeding stunts infested tillers, and the leaves become somewhat broader and darker green. Stunted tillers, particularly in younger plants, usually wither and die. Consequently, stands are thin in the fall, less forage is produced, and more plants succumb to winterkill. If the infested tillers survive, their growth and yield will decrease. Hessian fly infestations in the spring also stunt tiller growth and cause uneven plant height. Larval feed- ing at the nodes weakens the stem at the feeding site and may cause significant lodging or stem breakage, making harvest more difficult. During kernel for- mation, feeding can also interfere with nutrient flow to the head, reducing grain quantity and quality.
Expect significant grain losses when fall infestations exceed 5 to 8 percent or when spring infestations exceed 20 percent of the stems.
Although the preferred host is wheat, infesta- tions have been seen on barley, emmer, rye, spelt, and triticale. Oats are not a host for the Hessian fly. It has occasionally been found on wild grasses such as little barley, goatgrass, quackgrass, timothy, and western wheatgrass. There are likely other grass hosts in Texas.
To reduce economic losses, adopt the following cultural practices:
- Grow adapted wheat varieties with resis- tance to Hessian Information about these varieties for your area is available from your county Extension agent and wheat seed dealer.
- Plant later in the fall to reduce the potential for a fall generation.
- Destroy volunteer wheat, which serves as an early-season host.
- Bury crop residue 4 to 6 inches deep.
- Rotate to crops other than wheat or barley to suppress the fly population.
- Avoid moving infested straw to a non-infest- ed area.
Resistant varieties: Some wheat varieties are resistant to certain populations of the Hessian fly but susceptible to other populations of this pest. These unique populations of Hessian flies are called biotypes. They result from genetic changes that allow the flies to feed and survive on different varieties of wheat.
Some biotypes cannot survive on wheat varieties that have specific genes for resistance. This is why planting Hessian fly-resistant varieties usually works well to prevent losses.
However, Hessian flies can overcome resistance in wheat just as rust fungi develop new races. Over time, the widespread planting of one or two resis- tant varieties can favor biotypes that survive on the resistant varieties. This new, virulent biotype even- tually can become so common that the formerly resistant varieties begin to suffer damage.
The Texas Wheat Variety Trial Results lists the resistance of hard red winter varieties to Hessian fly each year. The report is posted under “Wheat Vari- ety Results – State Wide, at http://varietytesting. tamu.edu/wheat/#varietytrials. It includes a table, “Hard Red Winter Wheat Characteristics,” that lists the susceptibility or resistance of these varieties.
Delayed planting: Postponing planting mini- mizes the damage from the Hessian fly and reduces the number of fall generations.
A date in late fall after which flies do not emerge is called the fly-free date. In central Oklahoma and farther north in the wheat belt, planting after this date has effectively reduced or prevented Hessian fly infestations and damage. This practice has proved to be of limited value in Texas, where inter- mittent periods of warm weather allow the adults to emerge, mate, and lay eggs well into December.
If you must plant wheat early for grazing live- stock, minimize the risk of Hessian fly infestation by planting a variety that is resistant to the fly, or consider treating the seed with insecticide for fields with a history of Hessian fly damage.
Destroying volunteer wheat: Controlling vol- unteer wheat is a useful management tool for many wheat pests, including aphids, Hessian flies, and wheat curl mites. The lack of wheat deprives the first- generation adults of a place to deposit their eggs.
Reducing crop residue: Plowing under old wheat stubble 4 to 6 inches deep in August greatly reduces adult emergence from buried plant residue. However, soil erosion and moisture-retention prob- lems in some areas can dictate that residue burial be limited to conform to conservation practices.
Rotating crops: Although crop rotation helps reduce Hessian flies in a given field, they can remain in the old wheat residue for 2 years, and wind can carry the adults to nearby fields. Burning the straw kills the exposed pupae and larvae in the stems but not the pupae at the soil surface or below the soil line.
Containing infested straw: Avoid moving infested straw or hay to a non-infested area. When buying or selling wheat hay or straw, look for brown pupae behind the leaf sheaths at the nodes to make sure the material is not infested with live Hessian flies.
Applying pesticide: Insecticide seed treatments labeled for control of Hessian fly can suppress light infestations in seedling wheat in the fall. These treatments may not protect the seedling wheat from damage when large numbers of Hessian fly infest the crop.