Insects
Part 2, Section 5: Small Grains Pest Management
Small Grains Pest Management
INSECTS
Few insects are of economic importance to Pennsylvania small-grain production. Insecticide applications seldom are needed. Other methods of insect management, such as rotating crops, plowing under crop residues, adjusting planting dates to avoid damage, planting resistant varieties, and using biological control, are alternatives to chemical control. Chemical control, however, is necessary under some conditions. As is true of management in other crops, the pest problem should be identified properly before a control action is instigated. Figure 2.5-2 shows key periods of small-grain insect occurrence in Pennsylvania. Tables 2.5-7 and 2.5-8 describe insecticides for use in small grains.
Aphids: Corn leaf aphid and English grain aphid. In Pennsylvania, these species of aphids are common on small grains and vectors of barley yellow dwarf virus. Corn leaf aphids occur in small numbers throughout the season, but during warm periods in early autumn, large numbers can develop on grain seedlings. The English grain aphid is most abundant in late spring and feeds primarily around the grain heads.
The principal damage from aphids is the transmission of virus disease into the plants. This damage is most likely to be a problem in years with extended warm periods during autumn. Barley yellow dwarf virus causes barley to turn yellowish and stunts the plants. In wheat and oats, the leaves can turn reddish. Fall aphid problems on winter wheat can be avoided or at least reduced by not planting until or after October 1. The lower temperatures and lessening daylight during October slow the rate of aphid buildup. Except for years with unusually warm weather during October and November, chemical control of aphids on winter wheat is not suggested.
In addition to transmitting viruses, these aphids can injure plants by sucking out sap and injecting toxins, causing a yellowish-brown spot to appear on the plant. The appearance of small deadened areas and the presence of aphids indicate a need for treatment. During autumn, when plants are relatively small, populations of 25 aphids per foot of row may warrant treatment. During the spring, populations exceeding 100 per foot of row are needed before treatment is warranted.
Corn leaf aphids are blue-green with darker spots surrounding the base of the cornicles (two antenna-like extensions on the posterior end of the body). The cornicles on the corn leaf aphid are short, broad, and have dark basal spots. Legs and cornicles also are black.
English grain aphids have a green or yellow-green (occasionally orange or brown) body with black antennae and cornicles. The legs have areas of green and black. The cornicles are long, narrow, and black.
Armyworm. In some years, armyworms (also called “true” armyworms to differentiate them from fall armyworms) cause high losses to small grains by cutting off the seed heads of the plants. Such damage is frequent in areas of the fields where lodging is present. Damage from armyworms normally occurs during mid- to late June. Rescue chemical sprays are possible but not practical unless applied by aircraft. Armyworm populations of three to five larvae per square foot are necessary before control is economical.
Because the adults are moths, only the larvae damage small grains. Larvae are pale green to tan and have a narrow broken stripe down each side. Each abdominal proleg has a diagonal dark stripe around it. (Prolegs are the legs at the posterior end of the larva.) Fully grown larvae are 0.75 to 1.5 inches long.
Armyworm overwinters in southern states and migrates into Pennsylvania each spring, often on storm fronts. Arriving females will mate and lay eggs on small-grain leaves. Young larvae feed on the small-grain foliage, giving the leaves a ragged appearance. As the grain begins to head out, larvae begin feeding on the stalk just below the head, making the head fall off.
Cereal leaf beetle. During the past few years, damage from cereal leaf beetle has become more common. It is not clear why outbreaks are occurring though it is likely that a lack of scouting is catching growers off guard. Cereal leaf beetle is most easily control when growing populations are detected early and killed. Although some feeding by the adult beetles and larvae may be seen on fall-sown small grains, particularly wheat, the heaviest infestations and damage have been occurring on oats. Control measures are not necessary until late May or early June, and only when the infestation averages one or more larvae over 0.13 inch long per stem at boot stage, or one larva per flag leaf as oat heads begin to form. Parasites released in Pennsylvania during 1976–78 appear to be established and are helping to hold down the cereal leaf beetle population and it is possible that overuse of insecticides is disrupting natural control.
Adults are about 0.19 inch long, elongate, and have metallic bluish-black wing covers and a reddish-orange thorax. When full grown, larvae are 0.19 inch long with a brownish-black head and legs. The body is yellowish and covered with fecal matter. Large larvae may be mistaken for slugs, but slugs are nocturnal and rarely active during the day.
Overwintering adults appear in early April. After the eggs are deposited, larvae hatch and begin to feed on the leaves for about 3 to 4 weeks. Both adult and larval feeding appear as a long narrow strip along the leaf.
Hessian fly. The Hessian fly usually can be controlled by planting resistant varieties and/or planting after the fly-free date, which varies from early September to early October in Pennsylvania. Plowing under infested fields immediately after harvest also can help. Volunteer wheat can be disked under to prevent later summer infestations. One of the newer seed treatments, Cruiser, has been labeled for Hessian fly control; however, we recommend using cultural practices to manage this insect, especially using fly-free planting dates.
The Hessian fly is similar in appearance and size to a small mosquito. The life span of the adult is generally no more than 3 days. The Hessian fly completes two generations per year. The spring generation emerges from the flaxseed stage when the average daily temperature is 45 to 50°F. Emergence usually occurs over several weeks, during which time females lay eggs on wheat leaves (and in some cases rye or barley). The newly hatched maggots work their way under the leaf sheath near the nodes. Maggots change to puparium about the time wheat heads out, and remain in stubble as “flaxseeds” until fall. Adults of the fall generation begin emerging around early September and live about three days, during which time the females lay eggs. Larvae hatch and begin feeding on young wheat seedlings.
Plant injury caused by larval feeding stunts the plant. Stunted plants usually wither and die; if they survive, their growth and yield are reduced. Significant grain yield reduction can be expected when 20 percent or more of the tillers are infested. Infested tillers are dark green.
Slugs. Slugs can be a challenging problem in small grains, and they can show up under extended periods of cool and wet conditions, which have been common in recent autumns.
Heavy residue and dense leaf canopies can provide ideal habitats for slugs. The cause of slug outbreaks in a field is not totally understood, but wheat planted no-till following an old stand of alfalfa, hay, or heavy crop residue appears to be more susceptible. Fields near wooded areas also may provide habitat and food for slugs.
