Wheat Insect Management

Wheat in Arkansas is considered a winter crop. As a result, the insects that attack wheat are oriented primarily to the fall and spring time periods. In the fall, the primary insects of concern are aphids, fall armyworms and the initial infestation of the Hessian fly. During the winter months, the Hessian fly will develop during periods of warmer weather and can develop into a serious problem in areas where wheat is produced every year and on the more susceptible varieties. The true armyworm and aphids are of concern in the spring. Armyworms are a problem in the state in late May and early June. Infestations of armyworms can reach high levels in some years and fields should be scouted during this time period. The consequences of the Hessian fly infestation will be evident by the presence of shorter stems, lodging and reduced yields when infestations are significant.

Armyworms

In the fall, fall armyworms may be a problem in seedling plants. Fall armyworms feed on the young plants and eat the plants to the ground, causing a loss of stand. Damage of this type may occur from emergence until a bad frost or freeze eliminates the threat of armyworms. Wheat will usually recover from moderate fall armyworm damage. An infestation level of five to six worms per square foot will justify treatment. Treat only during the warmer part of the day; most of the recommended materials have limited activity below 60 degrees F.

In the spring, true armyworms are a threat about the time heading starts to occur. Wheat is very attractive to the armyworm and thick, vigorously growing fields can attract high infestations. Occasionally, when wheat starts to mature, armyworms will move up from leaf feeding and cut the wheat heads from the plant stem. Since this type of damage can have serious consequences on yield, close field observations are required. Treatment should be made if head cutting is beginning to occur and armyworms are present. Scouting of fields for armyworm infestations should occur in the spring when wheat starts to grow vigorously.

Armyworm Control Recommendations

The armyworm may be controlled using registered insecticides. Refer to the University of Arkansas Insecticide Recommendations Guide (MP144) for the currently labeled insecticides. The best time to apply an insecticide would be late afternoon since the armyworm feeds primarily at night.

Aphids

Wheat in the mid-south is attacked by four species of aphids. These include the greenbug, bird cherry-oat aphid and the corn leaf aphid. All of these aphid species may occur in wheat fields at any time during the production season. In addition to wheat, all the aphids attack a wide range of grass hosts including all the grain crops. Most aphids that occur on wheat will transmit the Barley Yellow Dwarf Virus (BYDV), however, the bird cherry-oat aphid is the most common vector of this disease in Arkansas . Preventing transmission of BYDV by controlling aphids is difficult to achieve using insecticides. Damaging BYDV outbreaks occur approximately 1 out of 10 years. Therefore, in any given year, controlling the aphids that year will only give you at best a 10 percent chance of preventing BYDV outbreaks with a significant potential to reduce yield. The most effective method of controlling BYDV is avoidance of early-planted wheat allowing infected summer hosts to die before wheat emerges, thus, reducing chances of aphid transmission.

Greenbugs are a serious threat to wheat from planting through early spring. Greenbugs can damage wheat during this period of time and potentially cause yield reduction. Feeding on wheat causes distinct damage that is easily recognized by yellowing of leaves and the occurrence of chlorotic spots. This is the result of the greenbug injecting a toxin into the plant leaves as it feeds. When infestations are high, plants may turn brown and die. It is quite typical for greenbugs to be higher in number on sandy knolls or higher places in the field. The greenbug damage symptoms are usually not uniform and usually occur in spots across the field. The composite damage from greenbug populations generally results in yellow areas in fields. Heavily infested plants observed from a distance may appear similar to plants under drought or water stress. Many of the yellow-looking areas within typical wheat fields in the Arkansas delta may be the result of "wet-feet," poor nutrition or other conditions. When yellow spots occur in wheat fields, the field should be scouted to determine the cause of the problem.

The greenbug is a pale green aphid. When the young aphids become about half grown, they have a dark line down the middle of the back. This is an internal marking, not a surface stripe. The dark green marking or line on the back is a reliable identification characteristic for non-winged forms. In the winged forms the head is brownish-yellow and there are blackish lobes on the back of the thorax. The medial vein of the forewing has one branch, whereas it has two branches in the forewing of the apple grain aphid, English grain aphid and the corn leaf aphid.

During warm weather, beneficial insects, such as lady beetles and parasitic wasps and flies, have some effect in maintaining greenbug populations in check. However, the activity of beneficial insects is slowed or may cease when temperatures cool below 60 degrees F, while greenbug activity may continue until temperatures reach 45 degrees F. For this reason, periodic scouting of wheat fields is necessary in order to determine the need for control of the greenbug.

Field scouting for greenbug infestations should occur weekly from emergence until cold weather (temperatures below 45 degrees F) is occurring regularly. As spring temperatures start to go above 45 degrees F, weekly scouting should resume. It is quite common for greenbug infestations to first occur on the undersides of the lowest leaves, so whole plant examination should be used to determine infestations. Table 1 lists the guidelines on suggested treatment levels for greenbugs.

When treatment is required, treat during warmer periods (above 50 degrees F) to receive the greatest activity from the insecticide.

Table 1. Treatment levels for greenbugs in wheat.
Number Per Linear Foot	Plant Height	Time of Season
50	4 - 6 inches	Fall and early springs
200	6 - 10 inches	Mid-March
300	18 - 20 inches	Mid- April
800	30 + inches	Mid-May

Bird Cherry-Oat Aphid

The bird cherry-oat aphid is probably one of the most common aphids found in Arkansas wheat. This aphid is relatively easy to identify in the field. The bird cherry-oat aphid is about 1/16 inch long. The body is usually olive green but may vary from nearly black to a pale green. A reddish-orange patch occurs at the tail of the insect between and at the base of the cornicles. This dark spot, reddish in color, is probably the most distinguishing field character that can be used for identification purposes. The legs and cornicles are pale green with black tips.

The aphid gives birth to living young and the life cycle is typical of aphids. The bird cherry-oat aphid does not inject a toxin into the plant and thus does not cause injury to the plant. Damage seldom occurs from this insect and plants tolerate high populations without losses. Treatment is seldom required.

Corn Leaf Aphid

The corn leaf aphid is about 1/16 inch long. The body is greenish-blue with darker spots surrounding the base of the cornicles. The cornicles are short and broad with a dark spot at the base. The legs and cornicles are black.

Growers are often concerned about this aphid but it seldom requires any treatment. The corn leaf aphid reproduces rapidly and large numbers are common in some years. Sometimes they cover entire leaves. Both winged and wingless forms are found, especially when high populations occur. The insects feed until they are killed by a heavy frost or until their food plants dry up. Large numbers may be tolerated on small grains and grain sorghum without loss in yield.

Evaluation of Insecticides to Control Aphids

Field tests on the greenbug and bird cherry-oat aphid demonstrate that most insecticides currently recommended give fair to excellent control. Refer to the University of Arkansas Insecticide Recommendations Guide (MP144) for the recommended insecticides. The greenbug was controlled by most insecticides tested but the bird cherry-oat aphid was more difficult to control. Fortunately, the bird cherry-oat aphid doesn't damage wheat unless extremely high numbers are present. On the other hand, the greenbug can damage wheat severely if populations are present but control may be accomplished using most insecticides.

Insecticide applications should be made when aphids reach treatment levels. Heavy rainfall and natural parasitism will significantly reduce aphid populations so these factors should be considered before making insecticide applications.

Grasshoppers

Grasshoppers may occasionally feed upon the borders of wheat fields. Damage to fields is usually restricted to small areas, so spot spraying may control populations. Refer to the University of Arkansas Insecticide Recommendations Guide (MP144) for the recommended insecticides.

The Hessian fly, Mayetiola destructor , recently has become a major factor limiting wheat production throughout the southern United States . Wheat is the primary host of the Hessian fly but it also will infest triticale, barley and rye.Although rye generally is not damaged by this insect. Hessian fly does not attack oats but can develop on some grasses such as little barley and wild rye grasses.

Adult Hessian flies are small black flies about the size of a mosquito. Adults live for about two days, during which time they mate. Females lay about 200 eggs in the grooves of the upper side of wheat leaves. Eggs are orange-red, 1/32-inch long and hatch in three to five days. Newly hatched larvae move down the leaf groove beyond the leaf sheath to the stem where they begin to feed at the base of the leaf. Maggots become white after the first molt and appear greenish-white when fully grown. After approximately 14 days in the larval stage, maggots molt into a resting (puparial) stage. The pupa is often referred to as the "flaxseed" stage because it resembles seeds of flax. The entire life cycle requires about 35 days at 70 ° F. Newly hatched larvae are exposed on the leaf surface and are susceptible to disease and adverse weather conditions, but once larvae move to the stem base they are protected from natural enemies and the environment.

Maggots suck sap and stunt tillers, possibly by injecting a toxin into the plant. Feeding by a single larva for several days is sufficient to completely stunt or kill a vegetative tiller. Stunted vegetative tillers are dark green, do not elongate or produce new leaves, and usually die after the maggots pupate. Infested jointed stems are shorter and are weakened at the joint where feeding has occurred. Grain filling of infested stems is reduced and damaged stems often lodge before harvest.

The Hessian fly is a cool season insect that can function normally at temperatures as low as 38 ° F. The insect spends the summer months as puparia (flaxseed) in wheat stubble; therefore, burying stubble can reduce fall populations. The number of generations during the year is governed largely by temperature. Three to four generations occur per season in Arkansas . Adults emerge from flaxseed with the first cool rains of fall, often before wheat has been planted. Consequently, the first generation often develops entirely on volunteer small grains and weed hosts. A second and sometimes a third generation occurs in late fall and winter and one to two generations develop in the spring. The fall and winter generations may stunt and kill seedlings and vegetative tillers. The spring generation infests jointed stems during or after head emergence.

The most effective method for controlling the Hessian fly is use of a resistant variety. Unfortunately, few wheat varieties grown in the South are resistant to the biotype (Biotype L) of Hessian fly that commonly occurs in Arkansas . Crop rotation, destruction of volunteer wheat and tillage that buries wheat stubble will help reduce Hessian fly infestations in susceptible varieties.

The best method to reduce injury and damage by the Hessian fly is to delay planting as late as is reasonably possible. In Hessian fly research, early-planted wheat had a 21 percent infestation compared to less than 1 percent in the later planting date. Dissulfoton granules also reduced Hessian fly infestations. Generally, wheat planted after October 1 in north Arkansas and October 10 in the south, may substantially lower infestations of Hessian fly. If the Hessian fly is a serious threat, the use of in-furrow insecticides at planting is effective in protecting the wheat against the first and perhaps the second field generation of the Hessian fly. Wheat that is planted in mid- to late September is very susceptible to Hessian fly infestation. The later in the fall that wheat is planted the safer the crop is from threat of Hessian fly infestation. A fly-free or low infestation risk planting date probably does not exist in southern Arkansas where winter temperatures do not limit Hessian fly activity. However, later plantings in the northern areas of Arkansas have reduced infestations of the Hessian fly. The risk of the Hessian fly infestation must be weighed against the threat of wet weather that can prevent the planting of the crop.

The Hessian fly may be controlled in susceptible wheat during the fall by using a systemic granular or liquid insecticide applied in-furrow at planting. The rates and insecticides for control of Hessian fly can be found in the University of Arkansas Insecticide Recommendations Guide (MP144). This treatment, as effective as it is, will not prevent re-infestation by subsequent generations during the winter and spring. However, research conducted on in-furrow treatments has shown significant yield increases when Hessian fly is a significant factor in the production of wheat. Studies have shown that foliar applications of insecticides in the winter and spring for Hessian fly control are not effective.

The cereal leaf beetle, which is native to Europe , was first found in the United States in 1962 in southern Michigan and first found in Arkansas in 1995. The adult beetle is about 3/16-inch long, metallic blue-black in color with red legs and a red pronotum (neck). The female is generally larger than the male. Adults begin to emerge in late March to early April from overwintering habitats, which include crop remnants, field trash, grain stubble and corn plants. The females mate and then deposit eggs on the upper surface of wheat leaves. The eggs are long, yellow, cylindrical and turn darker as they mature. The eggs hatch in 3 to 7 days. The newly hatched larvae are pale yellow with a brownish-black head and legs. The larvae characteristically cover their body with fecal material and appear black and slug-like in the field. The larval stage lasts 3 to 4 weeks and the pupal stage from 7 to 14 days depending on the temperature.

Adults and larvae feed on the leaves of the plant, removing long narrow strips of tissue between the veins similar to the rice water weevil except the strip is slightly larger. Fields with heavy damage have a white frosted appearance. The adults and larvae damage oats, wheat, barley, rye, corn and grasses. Oats are the preferred host with plant maturity playing an important part in attractiveness and susceptibility.

Control of Cereal Leaf Beetle

Adults and larvae are easily detected in the field by visual examination. The population of cereal leaf beetles may be determined by randomly selecting 20 stems at five locations in the field. At each location, examine the flag leaves for the beetle adults and larvae and count the number of each. The economic threshold for cereal leaf beetle is one larvae or adult per flag leaf.

Pictures of chickens, flowers, wheat, a boy looking through a magnifying glass, irrigation pipe, soybean pods, and fruits and vegetables.	Cooperative Extension Service





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Insect Management
Wheat Insect Management