The most important insect pests of field corn in New York are currently the western and northern corn rootworm, seed corn maggot, and European corn borer. These pests are generally a problem over large parts of the state every year. Other insects such as cutworms, armyworms, wireworms, the potato stem borer, and the hopvine borer occasionally cause losses in isolated areas. Their numbers vary considerably from year to year and field to field. Nematodes and slugs, which are not insects, also are occasionally important.
EUROPEAN CORN BORER
Broken leaf midribs, tassels with saw-dust-like frass at the breaks, and holes in the stalks and ears with bunches of frass at the tunnel openings are common signs of the European corn borer. This damage can lead to reduced yield, lodging, and dropped ears, and the borings may provide an entryway for stalk and ear rots.
The European corn borer over-winters as a fully grown larva in cornstalks and stubble in fields and in ears stored in cribs. Moths emerge from late May to early July and again in late July to early September in those areas where two generations occur.
The moths lay their eggs on the underside of corn leaves. The eggs are flat, about the size of a pinhead, and are arranged in masses of 5 to 50, overlapping like shingles. They are white when first laid but have a black spot in the center of each just before hatching.
The eggs hatch in a few days, and the resulting larvae damage the crop. When small, the larvae feed on the leaf surface and in the whorl of the plant. As they mature, they tunnel in all parts of the plant and, at this point, are out of the reach of insecticides. Proper timing of treatments, when called for, is therefore essential; the insecticide must reach the young larvae before they bore into the plant. The fully grown larvae are 3/4 to 1 inch in length, flesh colored, and marked with small, round brown spots. When mature, these larvae either cease activity and overwinter in the stalk or pupate in the tunnel to produce the moths of the second generation.
Select a variety that is well adapted to your area, is rated highly for standability, and has done well for your neighbors. Avoid early-maturing varieties and planting very early if the corn borer has been a problem in the past. When harvesting, cut stalks as close to the ground and as early as possible.
Chopping the crop for silage or fodder will kill any borers in the stalks. If soil erosion is not a problem on your farm, clean plowing (leaving no crop residue on the soil surface) in the fall or before May 1 is effective in reducing the corn borer population overwintering in that field. This practice should be avoided where soil erosion is likely to occur. Consult your local office of the USDA’s Natural Resources Conservation Service for advice.
Conserve natural enemies by avoiding unnecessary use of insecticides. Planting-time treatments with soil insecticides are not recommended for European corn borer control. Chemical treatments must be timed properly to be effective and are not generally recommended in NYS. Fields with chronic corn borer problems can be planted to the BT-corn borer corn varieties for control. However, if the BT-corn varieties are planted, growers must plant a BT-free refuge according to the label requirements.
NORTHERN AND WESTERN CORN ROOTWORMS
The northern and western corn rootworms are strictly pests of corn. The eggs, which are the overwintering stage, are laid in the soil in cornfields in late summer. The eggs hatch late the next spring, and the larvae feed on the roots of corn plants if corn has been planted in that field again. Because the larvae can feed only on corn and cannot move to another field, rotation to another crop will effectively reduce the rootworm population. A quick check for corn rootworm larvae can be made by examining the roots of a few corn plants selected at random throughout a field in late June and early July. Look for roots that are dead and broken on the tips. The larvae are small, elongate, and white, with a brown head and well-developed mandibles. They are found in the soil at the root zone as well as tunneling in the roots. Corn rootworm larvae pupate in the soil.
Root damage by corn rootworm (CRW) larvae weakens plants and may cause lodging or “goose-necking.” Severe lodging greatly complicates harvest. CRW damage can also result in stunted ears and reduced yields. During rainy seasons, corn can lodge without rootworms. Adult CRW beetles, which are about 1/4 inch long, emerge from the soil about the time of corn pollination. CRW beetles are very active, feed on pollen and corn silks, and may cause some stripping of leaves resulting in a windowpane appearance in the affected area. Northern corn rootworm (NCRW) beetles are tan to pale green. Newly emerged beetles are light in color, gradually becoming more green with age. Females have slightly longer abdomens than males. Western corn rootworm (WCRW) beetles are yellow with a black stripe along the sides of their wing covers. Male WCRW beetles usually have a broad, dark band across their wing covers compared to female beetles of this species, which tend to have black stripes. Females have slightly longer abdomens than males. Beetles of both species survive until the first hard frost, laying eggs for the next year’s population. There is a single generation per year.
Corn rootworm may be a problem in the second or later crop year where extensive acreages of corn are grown. Few problems exist where only one year of corn is grown in rotation. Rotation, therefore, is recommended as the best control measure. Routine “insurance” applications of soil insecticides at planting time are strongly discouraged. The number of adult beetles in a cornfield this year provides an indication of the potential larval population in that field next year. Check fields for CRW beetles on the leaves, tassels, and silks of corn. This check should be made beyond the margins of the field. If 55 or more CRW beetles are found on 55 plants, rotate the field to another crop the next year if at all possible. Note: WCRW beetles pose a higher risk to yields than NCRW. Threshold for NCRW is 110 or more per 55 plants. In fields with mixed populations of CRW, divide number of NCRW observed per 55 plants by 2, add this number to number of WCRW observed per 55 plants to calculate total number of CRW observed. Treat with an insecticide (Table 3.6.1) or use one of the available BT-corn rootworm corn varieties if corn must be planted in that field again. Remember that an insecticide or a CRW resistant variety may cost $15 or more per acre. If the producer decides to plant a BT-CRW resistant corn variety, close attention must be paid to the label requirements regarding the required planting of a BT-CRW free refuge.
In New York, the feeding of the adult beetles on silks can occasionally be a problem. Clipping of the silks can prevent pollination, resulting in poorly filled ears. If 10 or more adults are found per plant at silking, less than 50 percent of corn silks are brown, and silks <0.5 inch long, treatments to control adults may be warranted, and pollination has not yet occurred, apply an insecticide (see table below). Pollination generally occurs within three to four days after silk emergence. The silks quickly dry up after pollination.
Planting a corn variety that has a high standability rating may help reduce lodging from rootworm larval feeding in untreated fields. Planting early may help to avoid silk feeding by the adults. Planting fields last that are to be rotated the following year will attract CRW beetles to this last source of fresh corn pollen on the farm and may reduce their populations from other cornfields.
A sequential sampling method is available and greatly reduces sampling time when CRW populations are very low or are above economic numbers. The sequential sampling plan is available in the Cornell Cooperative Extension, NYS IPM Program publication, IPM: Field Corn Pocket Guide, available through NRAES (Natural Resource, Agriculture, and Engineering Service), Cooperative Extension, PO Box 4557, Ithaca, New York 14852-4557.
SEED CORN MAGGOT
Adult seed corn maggots are medium-sized flies that are very similar in appearance to the common house fly. Adult flies are present throughout the growing season and locate egg-laying sites by flying alternately close to the ground’s surface or searching the moist soil cracks on the soil surface. The adult female flies search for egg-laying sites close to decaying plant material or germinating seeds to provide a food source for the newly hatched larvae. Adult flies lay eggs in moist soil cracks near these potential food sources, and typical-looking fly larvae (maggots) hatch from the eggs within a few days. After hatching, larvae move through the soil searching for decaying plant matter or germinating seeds to feed on.
Large-seeded crops like corn or soybeans are very susceptible to seed corn maggot attack, resulting in stand losses. Germinating corn seeds are often killed or severely injured, thereby reducing plant populations within an area of the field or throughout the entire field. Losses typically range from 3,000 to 8,000 plants per acre. Fields in which animal or green manure crops have been used have a greater potential for seed corn maggot attack than fields not using these manures. However, non-manured fields are also at risk from seed corn maggot damage.
Seed corn maggot is easily and inexpensively controlled with the regular use of insecticide added to the planter box as a seed treatment or seed which has been pretreated with a seed corn maggot active insecticide.
For maximum effectiveness with a planter box seed treatment, corn seed needs to be evenly coated with the seed treatment. If using a plate type or plateless planter, pour one-half bag of seed corn into the seed hopper, then sprinkle one-half of the seed treatment packet on the surface of the seed corn and mix thoroughly with a wood stick. Pour the remainder of the seed corn into the hopper, sprinkle the remainder of the seed treatment on the surface of the seed, and repeat mixing. In air planters, seed corn and seed treater must be mixed in a bucket before dumping seed into the seed drum. If seed corn and seed treater are dumped separately into the seed drum, the drum action will not coat the seed adequately with seed treater and protect the seed from seed corn maggot injury.
Several species of cutworms are found in New York; the black cutworm is most commonly found in corn. The adults (moths) migrate into the state from the southern overwintering sites on the spring storms and are attracted to weeds on which they lay their eggs. One or more generations may occur per year, but it is the first generation which causes economic loss in NY corn. Cutworm larvae are large (1 to 2 inches long when fully grown), smooth, dull-colored caterpillars, which curl tightly when handled. They hide in the soil during the day and feed at night at thebase of small corn plants during May and June. Symptoms include missing, cut, or wilted plants. The large, nearly mature larvae do most of the feeding damage. Each one is capable of destroying several plants, and damage may appear very suddenly as the larvae grow larger.
The key to cutworm control is to monitor emerging plants closely, particularly in fields with conditions favoring cutworm outbreaks. These conditions include late planting; weed infestations; low, wet areas; and fields previously in pasture or sod. Cutworm problems may be worse in fields planted with minimum or no tillage. Plowing, good weed control, and early planting should help reduce cutworm problems. Check fields every two or three days until plants are well established for signs of missing, cut, or wilted plants. Search for the larvae in the soil near damaged plants. Treatment is suggested (Table 3.6.1) if 5 percent or more of the plants have been cut. Cutworm larvae should be controlled while small – 1/2 inch long or less. Since the larvae are active at night, chemicals should be applied late in the day. When the soil is dry and crusted, larvae remain beneath the soil surface and will be difficult to control. Only the infested area and a 20- to 40-foot surrounding border need be treated. Direct the spray at the base of the plants. Portions of the field may need to be disked and replanted if damage has gone beyond the point of control.
Application of soil insecticides at planting does not provide effective control of cutworms despite claims by the insecticide manufacturer.
Western Bean Cutworm identification card – including larval stages.
Western Bean Cutworm Pest Alert, North Central IPM (PDF)
Cornell Sweet Corn Monitoring Network
Penn State Pest Watch (Includes WBC data from NY, New England and other states)
Ontario WBC Trap Network
Western Bean Cutworm – Corn scouting videos:
Armyworms are occasionally a problem in corn, especially in weedy fields, in fields near severely infested small grain, and in no-tillage corn established in grain stubble or on grassy land. Armyworm moths are long-range migrants which arrive on the spring storms from their southern overwintering locations. While there are more than one generation per season of armyworms once they arrive, it is the first generation which causes economic losses in NY. Check fields regularly for ragged holes chewed from the leaf margins and pellet-like droppings (frass) in the whorls and scattered on the ground. The larvae will be found in the leaf whorls or at the surface of the soil.
For whorl-stage corn, apply an insecticide (see table below) only if most plants show damage and about three larvae per plant are found. Tall corn will seldom need to be treated unless the leaves above the ear are also damaged. Only the infested portion of the field and a 20- to 40-foot border around it need be treated. A border 20 to 40 feet wide treated with insecticide will prevent armyworms from invading from an adjacent infested field. Because the larvae are active at night, apply treatments late in the day.
WIREWORMS AND WHITE GRUBS
Wireworms are the larvae of insects commonly known as click beetles. The larvae are long, smooth, very hard-bodied, and yellowish to reddish brown. White grubs are the larvae of scarab beetles such as June beetles (May beetles). They are thick, white, soft-bodied larvae, which curl into a C-shape when disturbed. Either insect may occasionally be a problem when corn follows sod, pasture, or grassy hay. The larvae feed on the roots of grasses and germinating crop seeds and at the base of young plants. Symptoms are missing, wilted, or stunted seedlings and young plants. Check the soil around damaged plants for the larvae.
If losses due to wireworms are severe, about the only thing that can be done is to disk up the affected portion of the field, treat with a soil insecticide (see table below), and replant.
POTATO STEM BORER AND HOPVINE BORER
Potato stem borers and hopvine borers are occasional pests of field corn in New York, especially in weedy fields. Symptoms of injury are dead whorls in corn plants from 2 to 12 inches in height and a borer hole at or below ground level. An active whitish larva with a brick red color on its upper side will be found at the base of the plant. These symptoms should not be confused with those of the common stalk borer, which attacks the corn plant a few inches above the ground level. The potato stem borer also attacks a variety of other crop plants and weeds. In cornfields it is particularly associated with quackgrass and curled dock.
A related insect, the hopvine borer, has been found infesting corn in several central New York counties. It is very similar to the potato stem borer in appearance, life cycle, and injury, but occurs about a month later.
Both species are relatively poor flyers and tend to be localized problems. Improved grassy weed control in cornfields is the best control recommendation known.
Slugs are shell-less mollusks related to snails. They occasionally damage field corn, especially in wet, weedy areas of fields in cool, moist springs. Problems are likely to be worse in no-tillage corn or in fields previously in sod. Slugs feed at night, chewing irregular holes through the leaves and leaving slimy trails. As the corn gets taller and fields dry out, damage should become minimal. No chemicals are currently available for the control of slugs.
How to Recognize Nematode Injury
Several species of nematodes are known to attack corn. Field diagnosis of a corn nematode problem is difficult, however, because injury to roots caused by insects, fungi, or other factors may result in aboveground symptoms that are similar in appearance.