Part 2, Section 2: Corn Pest Management

Section 2 Table of Contents

Corn Pest Management

A well-planned pest management program for corn involves using multiple strategies. These should include preventive techniques such as monitoring, cultural controls, mechanical or physical controls, biological controls, host plant resistance, and chemical control tactics. Preventive techniques may start with planting weed-free crop seed or choosing an alternative field or planting date. Regularly monitoring for pests is an important predictive tool. Rotating crops to disrupt pest life cycles and planting adapted varieties are good examples of cultural controls. Mechanical or physical controls may include tillage and mowing to disrupt certain pests. Biological controls may include using insect or disease organisms or even grazing animals in pasture production systems. New hybrids are available which have built in protection from numerous pests. Finally, chemical controls are an important component of many IPM systems, but their use should be based on sound management decisions. See Part 2, Section 1 of this guide for more information about designing an integrated pest management program.

This guide provides chemical control suggestions based on university research and manufacturer recommendations. Management information for common corn pest problems is outlined in the following pages under “Weeds,” “Insects,” and “Diseases.” For more information on corn management and hybrid selection, see the Part 1, Section 4. This publication strives for accuracy; however, omissions, inaccuracies, or dated information can occur because of the dynamics of pests and pest management. Seek out additional information from the manufacturer or other reliable sources when making important management decisions. Remember, this guide is not a substitute for the manufacturer's product label.

Herbicide- and insect-resistant corn hybrids

Herbicide- and insect-resistance traits are available in several corn hybrids. Some of these traits are the result of transgenic events involving the introduction or transfer of one or more genes from a different plant or organism. Consumer concern about these transgenic crops has affected some export markets. Other herbicide- or insect-resistance traits result from more traditional plant breeding techniques; the marketability of these crops has not been affected by consumer concern.

Information about herbicide or insect resistance, other agronomic traits, yield potential, and marketability is available for many of these hybrids. Others may be relatively new with little public exposure or performance information. Consult university variety testing results, other public variety tests, and local seedsmen for performance information about newer hybrids.

Herbicide-resistant corn varieties

Following is a brief summary of the herbicide-tolerant corn hybrids currently available. See detailed information on herbicide use for these crops later in this section. Refer to section 1 for additional information and for concerns associated with herbicide-tolerant crops.

IMI (IR/IT) or Clearfield (CL) corn was developed by tolerance selection to be resistant/tolerant to imidazolinone herbicides (e.g., Pursuit, Scepter). Although these hybrids were initially introduced to help manage herbicide carryover, Pursuit and Pursuit-containing products such as Lightning can be applied directly to the IMI corn hybrids as part of the weed management program. Some IMI varieties (IR) also are tolerant to some sulfonylurea (e.g., Accent, Exceed) and sulfonamide (Broadstrike products, Python) herbicides and are used to reduce the injury potential of these products when applied alone or in combination with organophosphate (OP) insecticides.

LibertyLink/GR corn is genetically engineered to allow over-the-top applications of Liberty (glufosinate) herbicide. This program should provide broad-spectrum control of annual broadleafs and grasses of low to moderate pressure. Sequential applications or tank mixtures may be required for new weed flushes and perennials.

Roundup Ready corn was developed using genetic engineering techniques. It allows postemergence applications of Roundup and some other glyphosate-type products directly to corn. This system should provide broad-spectrum annual and perennial weed control in corn.

Corn Borer- and Corn Rootworm-resistant (Bt) corn varieties

Many seed companies now offer corn hybrids that include a gene from a soil-bacterium called Bacillus thuringiensis (Bt). Depending on the subspecies of the bacterium and the protein toxin it produces, the hybrid will either provide protection against European corn borer or corn rootworm. Companies have also put both genes into the same hybrid to protect against both pests, calling these stacked gene hybrids. They have also combined the same genes with the gene for Roundup Ready resistance. The Bt subspecies that produce toxins against European corn borer and several other Lepidoptera (moths and butterfly larvae) is Kurstaki. Several types of crystalline protein toxins are produced by strains of this subspecies, which include Cry 1Ab (YieldGard Corn Borer), Knockout, and Cry 1F (Herculex I), and recently Agrisure CB, which also has the Cry1Ab protein. With the exception of the Herculex material, these hybrids provide protection against first and second generation European corn borer and suppression against fall armyworm and stalk borer. Herculex also provides protection against western bean cutworm, fall armyworm, and cutworm. Other subspecies of Bt produces toxins that are active against some beetles including the Cry 3Bb1 and the Cry34/35Ab1 that protect against the corn rootworm in YieldGard Rootworm and Herculex RW, respectively. It is expected that there will be an Agrisure RW hybrid efective against the rootworm shortly. The stacked gene hybrids (both corn rootworm and European corn borer protection) for the two family of products are know as YieldGard Plus and Herculex Xtra. There will also be an Agrisure hybrid with both genes at a later date.

Research conducted in Pennsylvania and Maryland suggests that, on average, European corn borer feeding causes about a 5.5 percent reduction in yield. However, when infestation levels are high these losses can reach as high as 30 percent (although, this is a rare situation).

Although the cost for Bt-corn for European corn borer protection is approaching the cost for non-Bt seed, not all fields experience a yield loss from the ECB. Thus a key issue is to identify fields that will be most responsive to the Bt technology. The following criteria might help increase the chances of achieving an economic benefit from a Bt-corn hybrid:

A key aspect of assessing the need for Bt corn is to assess the level of corn borer damage through a scouting program in the fall. Bt corn hybrids vary in other agronomic traits such as yield, moisture, drought tolerance, and disease resistance, so you should consider these factors as well in the hybrid selection process. As is the case with normal hybrids, the use of Bt hybrids also should be based on performance data.

A concern with the use of Bt hybrids is the potential development of resistance by the corn borer. Because of this, seed companies have been mandated to develop resistance management plans for the use of Bt hybrids. Resistance management plans vary among companies. The main component of a resistance management plan involves farmers’ leaving some areas in a Bt-planted field, at least 20 percent of the field, or in a nearby field, planted to non-Bt corn hybrids to allow susceptible genetic types of the ECB to survive and mate with resistant individuals that survive on Bt corn. Growers who use this technology should check with their seed dealers on the correct insect resistance management (IRM) program to use.

The corn rootworm Bt hybrids provide protection of the root system against the pest’s feeding. Field evaluations suggest that under moderate to heavy corn rootworm pressure corn plant height is significantly reduced over the protected Bt-hybrids and protection of the rootworm system significantly reduced the frequency of stalk lodging. The Corn rootworm hybrids only provide protection of the root system from larval feeding and do not reduce adult feeding activity in the field. Therefore, the Bt corn rootworm hybrids should be considered as alternatives to at-planting time insecticide applications for protection against larval feeding and not as an adult control alternative. All hybrids that the Bt rootworm gene will have their seeds treated with one of the newer seed treatments, either Cruiser or Poncho, for control of secondary soil pests.

As with the Bt hybrids for European corn borer protection, all fields will not necessarily benefit from the technology. Determining the likelihood of gaining economically from the technology can be accomplished using an integrated pest management approach. During late July and August fields should be scouted to determine whether adult populations exceed the economic threshold. Those fields that exceed the economic threshold of 1.0 beetle per plants are good candidate fields for the technology. See the Entomology Department’s website for specifics on field scouting (http://www.ento.psu.edu/extension/field_crops/PDFs/Ex8.pdf).