Managing Insect Pests in Organically Certified Corn
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Organic production of field corn must rely upon three basic strategies for insect management:
- Keeping pest populations below threshold levels.
- Making the crop unattractive as possible to invading pests.
- Keeping the crop optimally competitive and tolerant of insects as possible.
Management of seed and seedling feeding insects uses tactics that impact one or more of the above mentioned 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 and elimination of pest insect reproduction. Also, rotation gives the option of isolating this year’s corn from last year’s 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 if corn follows corn. 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 several 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.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, where early growth is often slow.
Control of Insects with Tillage
Seed and small seeding-feeding insect pests are typically found in the soil or at the soil surface. Wireworms, cutworms, grubs, seed corn beetle, and other pests are affected by winter or early spring disking, the accompanying bird feeding, and exposure. Residue on the soil surface provides protection for these pests. Damage to corn seed and small seedlings is greater in no-tillage or minimum tillage culture versus conventional tillage. Typically, early spring disking destroys this cover, brings many pests to the surface, and attracts large numbers of sea gulls, grackles, cow birds, blackbirds, and/or other species. The combined action of these factors kills and disrupts populations of potential pests and can give meaningful protection to planted seed and small seedlings. In organic corn production, no-tillage culture should be avoided, especially in areas where southern corn billbug, wireworms, and black cutworm are commonly occurring pests.
Rapid Seed Germination and Seedling Grow-off
Corn plants are most vulnerable to insect injury when seed are swelling and sprouting and when seedlings are small. Insects can easily kill germinating seed and seedlings. Those seedlings suffering early insect injury may fail to have an ear or otherwise be a less productive plant. Rapid germination and seedling grow-off reduces the time corn seed / seedlings spend in the most sensitive stage, from germination to the six leaf stage, and helps the crop gain a size advantage over weeds. Therefore, attention to factors that promote early germination (such as row-bedding, seeding at the recommended depth, and hybrid selection for performance under cool conditions) can reduce the loss to seedling insects and other pests.
Avoiding Serious Insect Infestations
Corn should not be planted in fields where pests are at high levels if suitably effective and economical pest management options are not available. This is especially true in billbug infested areas (chiefly in the lower coastal plain), where current-season fields should be isolated away from corn fields of the previous season.
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 120 + day maturity hybrids, can make a big difference, both in the level of pests attracted to the crop in late season and in yield loss. Hybrids maturing in 112 days, or less, will usually avoid late-season caterpillar attack if planted early.
Hybrids vary in their ability to withstand insect pests, as well as their attractiveness to some pests, such as European corn borer. Rapid germination, early vigor, strong ear shanks, tight husks, resistance to stalk rots and other pests, 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. Some hybrids have European corn borer resistance traits that will reduce hybrid susceptibility to this important insect.
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 the remaining plants 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 seed / seedling insects are anticipated. In the event that greater insect loss is anticipated, using a seeding rate of at least 10% above the maximum recommended seeding rate will increase the crop tolerance to seedling loss.
Southern Corn Billbug Management
Billbugs can be serious pests of corn seedlings. In conventionally grown corn, management of the southern corn billbug is realized through an integrated system of cultural and insecticidal tactics. No organically approved insecticide is available that has activity against billbugs. The combined effects of the cultural tactics are designed to avoid concentrations of adult billbugs (rotation with isolation), and to promote rapid accumulation of tolerance, through rapid seedling emergence and grow-off. Tolerance is influenced on both the individual plant and plant population basis. Most of the cultural methods, mention above, apply to billbug management. Three additional tactics are: 1.) avoiding areas with abundant nutsedge infestation, since this group of plants are alternate hosts of billbug; 2.) not planting in no-tillage conditions, since soils warm more slowly; and 3.) planting at the earliest possible date, to allow seedling growth prior to billbug adult emergence.
Wireworm and Black Cutworm
These insects are addressed with cultural tactics and insecticides in conventional corn production systems. In organic systems, the major insect reducing tactic will be to disc-cultivate, as described above, and to avoid no-tillage situations. Also, cultural tactics that promote rapid seedling growth and seeding at adequately high populations to allow some seedling loss can be important.
European Corn Borer and Southern Cornstalk Borer
Borers occur in all NC corn fields. Their populations fluctuate greatly between years and sometimes within a single growing season. The wide planting of Bt corn has greatly reduced the occurence of these insects, but non-Bt fields suffer losses from these pests each year. European corn borer (ECB) has three generations per season on corn and southern cornstalk borer has two. First generation ECB may nursery in white potato fields, wheat fields and certain early weeds (e.g. dock). Infestations of moths can later move to near-by corn. The second generation develops on corn and weeds and can build-up, leading to very high numbers of third generation caterpillars. The organic producer can influence the abundance of these borers through rotation, site selection (away from nursery crops), early planting, using short season corn hybrids, and selecting corn hybrids with ECB tolerance. These actions will help avoid high populations, in space and time, and tolerate those borers that do occur. Organically approved Entrust® insecticide is labeled for use against ECB on corn, but this will likely not be effective when sprayed to tall corn and may be prohibitively expensive.