Corn Insect Pest Descriptions
Part 2, Section 2: Corn Pest Management
Corn Pest Management
Corn Insect Pest Descriptions
Following are descriptions of common insect pests of corn. For more information on these and other insect pests, contact Penn State’s Department of Entomology at 814-865-1895.
Armyworm. Armyworms (also known as true armyworm) do not appear to overwinter in Pennsylvania. They overwinter in southern states and upon emergence move northward on storm fronts from early May to early June. Eggs are deposited on the leaves of grasses and corn plants. Larvae hatch about a week later and develop over a period of approximately 3 weeks. Damage is observed most frequently in no-till fields that were sod the previous year or in fields where small grains cover crops (rye or wheat) were planted.
The first symptom of damage is ragged feeding on the top leaves, with wet, brown pellets in the area. True armyworm caterpillars hide inside the whorl during the day and are easily found by unrolling the innermost leaves. At night, they emerge to feed on leaf tissue. As feeding progresses, it becomes evident that caterpillars are avoiding the midrib. Heavily infested fields can have a caterpillar in the whorl of every plant, and heavily damaged plants have little leaf tissue left beyond the midribs. Corn fields likely to be attacked should be checked every few days during the first 2 weeks of June. In some years, like 2012, such heavy populations migrate into Pennsylvania that nearly every corn field is at risk. It is important to note that at this time, no Bt varieties offer protection against true armyworm caterpillars. Field treatments are not usually profitable unless 7 percent or more of plants are infested or showing damage symptoms. For more information on armyworms, consult the fact sheet “Armyworm as a Pest of Field Corn” available online at ento.psu.edu/extension/factsheets/armyworm or from your county extension office.
Corn rootworm. Two species of corn rootworm (northern and western) are present in Pennsylvania. These species have similar life cycles, except that the western corn rootworm tends to hatch about 3 to 5 days earlier in the spring. Corn rootworm beetles emerge during late summer (mid-July to mid-August) in most areas of the state and can remain until the first killing frost in the fall. In some southern areas of the state, beetles emerge as early as early July. It is at this time that corn fields should be scouted and the need for control determined for the next season.
Rootworm beetles begin depositing eggs in corn fields approximately 2 weeks after they emerge. They deposit the eggs in soil around the base of corn plants, and the eggs remain there until the following spring. Larvae begin hatching about mid-June in most areas of the state but may begin hatching earlier in southern areas. The larval stage inflicts the most severe damage to corn plants, so most chemical control is aimed at this stage.
Research data collected from numerous sites over the past 7 years indicate that soil insecticides applied at planting for corn rootworm control have not increased yields significantly except in a small percentage of the fields (about 25 percent). There is no need to treat for rootworm if the problem is not in the field. Rootworms will not be a problem in the field if any crop other than corn was grown the previous year. Fields in continuous corn may develop problems, but these can be predicted by what happens to the plants in July and August of the previous year. Plants that fell over in July should be checked for damage to the root system. Badly damaged roots at this time indicate a problem in the field. In mid-August, examine the ear tips for beetles. For the publication “Western and Northern Corn Rootworm Management,” which provides economic thresholds and other information, visit Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or your county extension office.
Populations of western corn rootworms that have developed resistance to some Bt varieties have become quite problematic in Illinois, Iowa, Minnesota, Nebraska, and South Dakota. A variant of western corn rootworm has also become a significant problem in many Midwestern states, including Iowa, Illinois, Indiana, and western Ohio. This variant lays its eggs in soybean fields and can cause significant root injury in first-year corn. Currently, this variant is not found in Pennsylvania and is unlikely to arrive here due to the more complex crop rotations used by Pennsylvania growers.
Billbugs. The only billbug species of concern in Pennsylvania is the clay-colored billbug. Billbug larvae cannot develop in corn; thus, damage is Corn Insect Control limited to fields where nutsedge was a problem the previous year. Billbugs overwinter as adults in the soil, emerg-ing during May and feeding on various kinds of grasses. Most feeding and subsequent corn plant damage occurs during May. Upon emerging from the soil, adults chew small cavities in the stems. Numerous stem punctures can severely retard or even kill small plants. Eliminating nutsedge from the field is the best pest management option. See the “Billbugs as Pests of Field Corn” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Black Cutworm. Black cutworms are among the most difficult insect pests of corn to control. The insects overwinter as pupae in the soil and also as adults. In early Spring adults migrate into Pennsylvania from states to our south. Black cutworms deposit their eggs in April and May, usually before the crop has been planted. Adults tend to be attracted to trashy fields or those where green weeds are present, particularly winter annuals such as common chickweed. Population sizes and risk associated with black cutworm can be gauged by trapping adult moths using pheromone traps which are available from many suppliers. Once a “significant” flight of moths (eight moths over the course of two nights) is detected, degree-day accumulations (base 50° F) can be used to predict when larval cutting damage should occur and when scouting should occur (300 degree days). Due to the patchy distribution of black cutworm populations, rescue treatments based on scouting, and not preventative insecticide applications, are the most economic approach for controlling black cutworm damage.
Newly hatched larvae feed on the weeds until new corn seedlings emerge. When larvae are small, their feeding may go unnoticed. Damage to plants becomes noticeable when the larvae are about one-half to two-thirds grown. If the soil is moist, cutworms cut the plants off just above ground level. Under dry conditions, however, larvae stay below ground level and cut the plants off (plants appear wilted).
Black cutworm develops from egg to adult in approximately 2 months. If the majority of larvae are 1.5 to 2 inches long, control does not help because most of their feeding is complete. For more information, see the “Black Cutworm” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Brown marmorated stink bug. Brown marmorated stink bug is a new pest that has only been problematic in field corn since about 2010. The damage caused by this pest species has yet to be documented well enough to understand its economic impact, but it is clear that stink bugs accumulate on corn ears while ears are growing and filling in July and August. Typically, these populations accumulate in the border rows of fields and have rarely been reported from entire fields. Populations are often heaviest adjacent to woodlots. Stink bugs feed directly through the husk on individual developing kernels, removing moisture. Stink bugs may also cause early season damage by feeding on growing seedlings. Both these types of feeding would seem to cause yield reductions, but their cost and the benefits of controlling stink bugs remain to be determined.
Corn leaf aphid. The corn leaf aphid is a minor pest of field corn in Pennsylvania. From early July until fall, colonies of the pest can be found on or near tassels or whorl leaves in most corn fields. Unless the insect is widespread across the field, there is no need to instigate control measures. If 50 percent or more of the plants are infested with colonies of over 75 aphids each during late-whorl and pollen shed, treatment is warranted. If left uncontrolled, the honeydew produced from their feeding falls onto the silks and prevents pollination, resulting in some barren plants. For more information, see the “Corn Leaf Aphid on Field Corn” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
European corn borer. Historically, European corn borer has been a very significant pest of field corn; in recent years, however, Penn State researchers have documented a significant drop in their population. The exact cause of this drop is unknown, but it may be related to widespread adoption of Bt varieties. Bt hybrids are very effective for controlling this pest species. Treatment with an insecticide is suggested only when more than 75 percent of the plants show whorl feeding in mid-June for the first brood, and when 50 percent of the plants have an egg mass by peak flight in the second brood. Corn planted before May 20 seldom is hurt by the second generation. If corn is planted late and is to be harvested for silage, borer damage is of little significance. The big loss occurs with corn varieties that have a tendency to drop ears.
The European corn borer completes two generations per year in most areas of Pennsylvania; however, in the mountain-valley region a single-generation European corn borer type co-occurs with the two-generation type. First-generation eggs are laid between late May and late June. Larvae hatching from the eggs can be found feeding in the whorl from early June to mid-July. Second-generation eggs are deposited on corn leaves from late July to late August. Larvae can be found feeding in the corn stalk during August and September. For more information on this pest, see European Corn Borer, Ecology and Management, available through Penn State Cooperative Extension from your local office.
Fall armyworm. The fall armyworm is a pest late in the growing season. Larvae can be found feeding in the whorl of late-planted corn in late July and during August. Fall armyworms overwinter along the Gulf of Mexico, and the moths migrate to Pennsylvania. On arrival, the moths deposit egg masses onto green plants. The eggs hatch in about 5 to 7 days, and the young larvae feed in the whorl. Infested plants appear ragged, and wet, brown, sawdustlike pellets are scattered throughout the whorl. See the “Fall Armyworm as a Pest of Field Crops” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Flea beetle. Survival of overwintering flea beetle adults is closely tied to temperatures during the winter months. For this reason, they seldom are a problem in the state, except for a few of the southeastern counties. Flea beetles complete many generations during the growing season, but only those generations attacking seedling corn are considered of economic importance. Flea beetles causing problems occur from early May to late June. Damage is caused by adults feeding on the leaves and transmitting bacterial wilt disease (Stewart’s wilt).
Garden centipede (symphylan). The garden symphylan is an occasional pest of field crops, but it is difficult to diagnose because of the pests’ mobility within the soil. Most problems occur in the eastern half of Pennsylvania and some researchers believe that symphylan infestations are a symptom of other problems in a field rather than the main cause of a problem, though this hypothesis remains to be established with certainty. These creatures are difficult to study given their ghostlike tendencies. Symphylans overwinter in the soil as adults. In the spring, they move up into the top 6 inches of soil when the soil temperature rises to about 45°F. Symphylans deposit eggs in late April, May, and June. The eggs hatch 2 to 3 weeks later into tiny, white nymphs that resemble the adults except that they have only six pairs of legs. Control of garden symphylans involves preventive treatment before or at the time of planting. No rescue treatment is effective while the crop is growing. For more specific information, see the “Garden Symphylan as a Pest of Field Crops” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Japanese beetle (adults). The Japanese beetle is the adult of one species of white grub. Its larvae are soil-feeding insects. The adults begin emerging around July 4 and move to many species of plants to feed on the foliage. Corn fields that are silking can be extremely attractive to the beetles. When populations of the pest are extremely high, the number of beetles feeding on corn silks can reduce pollination, resulting in poor kernel set. In many cases, however, damage is restricted to the field margin. Consider control only if the feeding is spread throughout the field. See the White Grubs in Home Lawns fact sheet available from Penn State’s Department of Entomology at 814-865-1895.
Seedcorn maggot. The adult flies of the seedcorn maggot emerge during late April and early May. Eggs are deposited on or near the soil surface and hatch in a few days. Problems can be especially severe when a green, living cover crop such as a rye or what crop or perhaps an old alfalfa stand is incorporaated into the soil, and planting occurs within 2 weeks. The maggots work their way into the soil in search of food and complete their development in 7 to 10 days. Three to five generations emerge each year. Any cultural practice that speeds up germination and plant emergence helps to reduce crop losses from maggots. For more information, see the “Seedcorn Maggot as a Pest of Field Corn” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Slug. Several slug species produce one, and possibly a partial second generation per year. Slugs overwinter in either the egg or the adult stage, depending on species. Juvenile slugs hatch from eggs in the spring, soon after the soil warms, and remain in the field all summer, though they burrow deep into the soil during the hot, dry portion of the summer. Slugs prefer environments with high humidity, relatively cool temperatures, and debris such as crop litter or manure for shelter from the sun. For more information, consult the “Slugs as Pests of Field Corn” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Sod webworms. Several species of sod webworms occasionally may be serious pests of field corn in Pennsylvania. The corn root webworm and bluegrass webworm are the two most common species. Damage usually occurs in corn fields previously in sod and is seldom uniform throughout the field.
Sod webworms overwinter as partly grown larvae. They start feeding in April and complete their development in early June. Young corn plants are damaged near ground level. Some plants may be cut off and partly dragged into the silk-lined tunnels made by the larvae. The damage is similar to that caused by the black cutworm. . See the “Sod Webworms as Occasional Pests of Field Corn” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Stalk borer. The importance of the stalk borer has grown with the increase in no-till corn production in Pennsylvania. One generation occurs per year. Moths are present from late August to mid-October, and most of the eggs are deposited from mid-September to early October. Eggs are laid singly or in groups in folded, dead grass and weed leaves. They also may be deposited on corn plants. Moths are very attracted to orchard-grass and rye for egg deposition.
The stalk borer overwinters in the egg stage, and then hatches over a relatively long period of 4 to 5 weeks, from mid-May to mid-June. The larval stage lasts approximately 9 to 12 weeks, and most corn damage occurs from early to late June. For more information, see the “Stalk Borer” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Western bean cutworm is a pest first detected in Pennsylvania in 2009. It has one generation per year and is unlike a typical cutworm because adults are active in late June, July, and August. Larvae (i.e., caterpillars) do not cut plants; rather, they feed on reproductive tissue of corn, including pollen, tassel, silk, and, particularly, developing kernels. Complicating matters is that its feeding on ear facilitates fungal infections. It can be difficult to control once caterpillars get inside the ear husk, so timing of insecticide application is crucial. An accepted economic threshold is 8 percent of plants having egg masses. See (ento.psu.edu/extension/field-crops-old/corn/western-bean-cutworm) for more information on this pest and details of its current distribution.
White grubs. This complex of insect species is a problem in home lawns and in corn planted into an old grass sod; particularly in bluegrass sod fields. In heavily infested sod fields, grubs can be found feeding on the roots of grasses about 1 to 4 inches deep in the soil. If the soil is dry, the insects migrate deep into the soil profile. When corn is planted into a grass sod, the food source (grass roots) is eliminated, leaving only the corn roots as food.
Injury symptoms include small stunted plants, dead plants, and plants with a purple coloration (caused by the roots’ inability to acquire phosphorus in the soil). Grubs are dirty white, soft bodied, and robust, with a brown head and six well-developed legs.For more information, see the “White Grubs in Home Lawns” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.
Wireworm. Wireworms overwinter mainly in the soil as partially grown larvae and adult beetles. The larvae feed on underground plant parts for 1–3 years before reaching maturity. This insect is a pest for only 2–3 years after a field has been in a grass sod. Preventing wireworm damage requires treatment before or at planting. There are no practical or effective ways to control the pest after the crop has been planted. See the “Wireworms as Pests of Field Crops” fact sheet available from Penn State’s Department of Entomology Web site (ento.psu.edu/extension) or through your county extension office.