Insects of Small Grains

Small grains-wheat, oats, barley, and rye-are usually fairly free of severe insect pest problems in New York. Three species have been of concern to farmers in recent years: the armyworm, the cereal leaf beetle, and the Hessian fly. Infestations of these insects are variable, occurring at irregular intervals or in localized areas of the state.


Armyworm outbreaks are sporadic with this insect occurring in very low numbers in some years and very numerous over a wide geographic area in other years. This insect does not overwinter in NYS or the Northeastern US but migrates into the area on the spring storm fronts from its southern overwintering area. Arriving migrant moths lay their eggs on available grassy areas which include small grains. While there are 2-3 generations of armyworm per season, it is the first generation which can cause economic losses in NY small grains.

The larvae are smooth cylindrical caterpillars, 1-1/2 to 2 inches long when fully grown. Their color ranges from tan to dark olive green, with a lighter stripe running along each side. They feed at night on the leaves of grasses and small grains but may "march" from badly infested fields during the day to other grain and corn. Look for chewed leaves, pellet-like frass on the ground, and larvae hidden during the day in the soil surface at the base of the plants. Armyworms may feed on the stem when most of the leaves are consumed and cause the grain head to drop off.

Larvae are often parasitized (look for small white parasite eggs on the neck area of the armyworm), and these parasites should be conserved by avoiding unnecessary spraying. In most years parasites and other natural factors keep armyworm numbers low. Little will be gained from spraying wheat fields that are fully headed out and in which the leaves are already starting to turn yellow, unless there is a very real danger of head-clipping by the armyworms.

It is important to detect problem areas early, while the larvae are still small, because the large larvae do most of the feeding and may quickly destroy whole stands. Check fields frequently throughout June for signs of damage. Pay special attention to areas of heavy growth, lodging, and grassy weeds. An insecticide should be applied only when armyworms are numerous (three or four per square foot). Only the infested area and a surrounding border of 20 to 40 feet need be treated. Oat and corn fields adjacent to heavily damaged grass or wheat should be protected from larvae marching out of these areas when their food supply is exhausted. A 20- to 40-foot treated border around these fields should prevent the armyworms from entering and minimizes the use of chemicals. Insecticide treatments for armyworms are most effective if applied late in the day.

Cereal Leaf Beetle

The cereal leaf beetle is primarily a pest of spring-planted small grains such as oats. Much lower numbers are found in winter wheat, and feeding damage to that crop is usually insignificant. Eggs are laid on the upper surface of leaves, near the midrib, in April through June. They are elongate, yellow to brown, about 1/16 inch long, and are laid singly or end to end in chains of two or three. The larvae are most abundant in May and June. They are about 1/4 inch long when fully grown, rounded, and covered with a slimy black fecal coat. They feed on the leaf surface, leaving long narrow white strips between the veins.

Plant spring grains as early as possible. Avoid planting spring grains adjacent to winter grains. Apply an insecticide only if there are three or more eggs and larvae per stem before the boot stage of oats or one or more larvae per flag leaf after the boot stage. Check 30 stems distributed throughout a field to determine whether numbers exceed these levels. Do not treat winter wheat or other winter grains for the cereal leaf beetle, because the population in these crops serves as a reservoir for parasites and is not likely to cause economic losses.

Hessian Fly

The recent localized recurrence of Hessian fly infestations in central New York after many years' absence should serve as a reminder that this potentially destructive insect is still with us and that winter wheat must be managed carefully to minimize losses to this pest. The larvae of this small insect feed between the stem and leaf sheath near the base of the plant in newly established wheat in the fall and again in the spring. Damage during the fall causes stunting of the new plants; the spring and early summer damage results in unfilled heads and fallen straw. Look for the small white maggots and brown puparia (the resting stage, commonly called "flaxseeds" for their resemblance to the flat spindle- shaped seeds of flax) deep within the sheaths of the lower leaves in the weeks just before wheat harvest.

No insecticides are recommended for control of the Hessian fly. Plow under stubble of infested grain at least 6 inches immediately after harvest. Destroy all volunteer wheat by disking when the plants are small. Plant wheat only after the fly-free date for your area but as soon after that date as possible (see Figure 5.8.1 for Hessian Fly free dates predicted for NY). Ask your seed dealer about the availability of Hessian fly-resistant varieties.