Corn plant health and yield is determined by genetic capabilities of the corn plant, soil factors, management decisions, and natural events. Among the natural events are pests, including insects, weeds, plant pathogens and others. The amount of pest damage to a corn crop is determined by the severity of pest attack and the plant’s ability to resist attack and compensate for injury. Management to prevent economic damage from pests in corn therefore emphasizes:
- Keeping pest populations below threshold levels.
- Making the crop unattractive as possible to invading pests.
- Keeping the crop as competitive and tolerant as possible.
Since corn is often used as a rotational crop, pest management decisions in corn may also be influenced by pest management needs of the rotational crop (e.g. soil pathogen suppression in peanut, nematode or weed suppression in cotton and tobacco).
Seedlings are the most easily damaged corn plant stage. On many field crops, insect feeding on seedlings is often not considered very important, however, since corn is a plant population sensitive crop seedling insects are a potential threat. To obtain profitable corn yields, the farmer must achieve an adequate plant population in the seedling stage.
Billbug, wireworms, cutworm, seed corn maggot, and true armyworm are the most commonly encountered insect pests of seedling corn in North Carolina. However, many other early-season insect pests may be found (e.g. flea beetle, greenbug and other aphids, webworms, stalk borers, white grubs, southern corn rootworm, stinkbugs, sugarcane beetle, etc.). The severity of damage from insects depends on three factors:
- The existing number of plants.
- The ability of these plants to tolerate attack.
- The number and kind of insects pests present.
Management of seedling insects uses tactics that impact one or more of these areas. Cultural practices are very important in establishing a vigorous, full stand of seedlings. Also Cultural practices can greatly influence pest populations, as well as crop competitiveness and tolerance. Producing a vigorous and competitive corn crop requires planning and site preparation.
Perhaps the most effective, and often the lowest cost, cultural tactic for reducing many pest problems is crop rotation. Rotation of corn with a non-grass crop reduces the levels of many pests through starvation, elimination of insect reproduction, or by allowing management alternatives (such as certain pesticides) that cannot be used in corn. Rotation may be yearly or in multiples of years depending on the pest problems of a particular locale and crop combination. For best results, at least a yearly rotation is needed, especially on organic soils or where billbugs and other yield decline factors are common. Rotational patterns are also important to the management of moderately mobile pests, such as billbugs. In this case, rotation in large units with maximum isolation from last year’s corn is most effective. Corn is a superior rotational crop for peanuts, tobacco, cotton, soybeans, vegetables and other crops and, therefore it can be an important part of an integrated pest management program for all crops on a farm.
Soil and Agronomic Considerations
Optimal soil pH and fertility are essential to vigorous plant growth and high yield capacity, which can be critical to the crop’s ability to perform in the presence of pests. Likewise, fields with drainage limitations, soil pans and other barriers can suffer from low plant vigor and, therefore, increased susceptibility to pests. Site selection is critical in no-tillage situations since options to correct drainage, pH, and other soil problems are limited, and without tillage certain pests can be more abundant. The corn farmer should avoid planting fields with problems that severely limit crop growth and health as this allows poor plant performance and low tolerance to pests. The use of starter or banded fertilizers to promote seedling growth are frequently helpful in reducing damage for seedling pests, especially on cool, wet-natured soils and in no-tillage, where early growth is often slow.
Rapid Seed Germination and Grow-Off
Corn plants are most vulnerable to insect injury when seedlings are small. Insects can kill seedlings and injury to a seedling may make it a less productive plant. Rapid germination and seedling growth reduces the time corn seedlings spend in the most sensitive stage, from germination to the six leaf stage, and helps the crop gain a size advantage. Therefore, attention to factors that promote early germination (such as planter operation, hybrid selection, starter fertilizer, etc.) can reduce the loss to seedling insects and other pests.
Avoiding Serious Insect Infestation
Corn should not be planted in fields where pests are at very high levels if suitably effective and economical pest management options are not available. This is especially true in the case of severe billbug infestations.
Avoiding certain pests is often a cheap method of pest management. In corn, timely maturity of the crop almost always assures lower insect damage. Certain pest insects and pathogens reach high levels in late July and August and may severely infest late maturing corn. For example, late season corn borers and fall armyworm, that are often very abundant, are attracted by green, immature plants but will not infest mature plants. Timely planting and avoiding late maturing hybrids (such as 128 day maturity or more) can make a big difference, both in the level of pests attracted to the crop in late season and in yield loss. This represents another form of pest avoidance
Hybrids vary in their ability to withstand insect pests. Rapid germination, early vigor, strong ear shanks, tight husks, resistance to stalk rots, strong stalks, and uniform performance over a wide population range are all factors influenced by hybrid genetics that may influence losses to insects. Seedling insects, stalk borers, and ear feeding insects are most influenced by hybrid traits. Bt transgenic corn can have a great impact on many caterpillar pests.
A farmer’s goal is to end the season within the recommended plant population for each hybrid planted. The level of plant loss that can be tolerated by pests without economic impact is related to the population level initially established. Plants lost to insects are not important to yield but plants remaining are. Typically, a field with minimal pests and other problems will have no more than 90% as many plants as the initial seeding rate. Modern hybrids can compensate for some plant loss before yield begins to decline under favorable growing conditions. A 10% plant loss from a full seedling stand will usually not noticeably affect yield. When planting the grower should assure that the full recommended seeding rate is used if any level of seedling insects is anticipated. In the event that greater insect loss is anticipated, using a seeding rate of 10% above the maximum recommended seeding rate will increase the tolerance to seedling loss.