Herbicide- and Insect-resistant Corn Hybrids

Part 2, Section 2: Corn Pest Management

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.

Transgenic Corn Borer- and Corn Rootworm-Resistant Corn Varieties

Most seed companies offer transgenic corn hybrids that include a gene or genes from a soil bacterium called Bacillus thuringiensis (Bt). Depending on the subspecies of the bacterium and the protein toxin it produces, the hybrid will provide protection against either European corn borer (ECB) and other caterpillar species or corn rootworms. Companies have also put both genes into the same hybrid to protect against both pests, calling these stacked-gene hybrids. They have also combined these 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, including Cry 1Ab (YieldGard and Agrisure Corn Borer) and Cry 1F (Herculex I ). With the exception of the Herculex material, these hybrids provide protection against first- and second-generation European corn borer and suppression against fall armyworm, stalk borer, and corn earworm. Herculex also provides protection against western bean cutworm, fall armyworm, and black cutworm. Other subspecies of Bt produces toxins that are active against some beetle larvae, particularly corn rootworm larvae, include Cry 3Bb1, the Cry34/35Ab1, and MCry3A in YieldGard Rootworm, Herculex RW, and Agrisure RW, respectively. The stacked-gene hybrids (both corn rootworm and European corn borer resistant) for the two families of products include YieldGard Plus and VT Triple PRO, Herculex XTRA, Agrisure 3000GT or CB/LL/RW, and SmartStax. In 2011, growers began to have access to products containing “Vip” genes, insecticidal genes that were also isolated from Bacillus thuringiensis; however Vip genes produce a toxin different from the Cry proteins. The Vip3A protein is claimed to control a complex of caterpillars, including corn earworm, black cutworm, western bean cutworm, dingy cutworm and stalk borers, and is likely to be stacked with Agrisure traits to provide a spectrum of control that includes ECB and rootworms. A listing of the various Bt traits and trait packages is available in a summary compiled by Dr. Christina Difonzo (Michigan State University) and Dr. Eileen Cullen (University of Wisconsin). The the most recent version available at the time of printing this guide (April 2012) is included as Table 2.2-24 at the end of this chapter.

Prior to widespread adoption of Bt varieties, research conducted in Pennsylvania and Maryland suggests that, on average, European corn borer feeding caused about a 5.5 percent reduction in yield. When infestation levels are high, yield losses can reach as high as 30 percent, but this situation is rare.

In 2009, 63% of field corn planted in the United States was transgenic for Bt traits. Adoption rates for individual states have been even higher and these high adoption rates have suppressed populations of ECB, the pest against which Bt varieties were first deployed. In fact, in many parts of the country ECB populations have reached historic lows. Moreover, similar trends in populations of corn rootworms may also be developing. Therefore, growers are advised to try to understand their local pest populations to determine whether the Bt technology is necessary and vital for their profitability. We strongly suggest that growers consider evaluating the pest populations annually in their non-Bt refuge acreage or plant a specific non-Bt test strip to track ECB or rootworm populations.

Because not all fields experience a yield loss from ECB, a key issue is identifying fields that will be most responsive to Bt technology. The following criteria might help increase the chances of achieving an economic benefit from a Bt-corn hybrid:

• A history of significant ECB damage in a field usually is a good indicator that the conditions in the field are conducive to the pest.
• Early-planted long-season hybrids that pollinate during early- to mid-August will be attractive to both first- and second-generation ECB.
• Fields planted in low areas along streams and rivers are more susceptible to ECB.
• Fields managed for high yield or at higher population levels will benefit more from the technology than those with a low yield potential (for example, 3 percent of 150- bushel-per-acre corn is greater than the same percentage of 100- bushel-per-acre corn).
• Corn fields grown for grain will benefit more than those grown for silage. Silage tends to be harvested before ECB causes ear droppage and stalk breakage, which are a significant component of losses caused by the pest.

As mentioned above, a key aspect of assessing the need for Bt corn is to assess the level of ECB and corn rootworm 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 5-20 percent of the field, depending on the hybrid—or in a nearby field planted to non-Bt corn hybrids to allow susceptible genetic types of ECB or corn rootworms 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,though the evidence that these seed-applied insecticides are vital for productivity remains thin.

As with the Bt hybrids for European corn borer protection, all fields will not necessarily benefit from the rootworm resistant technology. In Pennsylvania, rootworm damage can easily be managed with crop rotation. In the Midwest, several states (Iowa, Illinois, Minnesota, South Dakota, and Nebraska) have been experiencing heavier-than-expected damage from western corn rootworm larvae in their continuous corn acreage despite using Bt varieties targeting this pest species. These populations appear to be resistant to some of the rootworm traits, particularly the YieldGard trait (the Cry3Bb1 protein); therefore, continued use of these products in continuous corn acreage may threaten their viability. Because we do not want similar troubles to develop in Pennsylvania, we strongly recommend rotating corn with other crops to avoid long-term selection pressures for resistant rootworm populations. If continuous corn is necessary for particular growers, consider using Bt varieties with different Cry proteins to avoid continual pressure with a single trait. In potentially problematic fields, continuous corn growers also might consider the option of foregoing Bt varieties targeting rootworms and plant with a soil insecticide. In the Midwest there is also a variant of western corn rootworm has developed that lays its eggs in soybean fields so that larvae emerge in corn fields in the typical two-crop corn-soybean rotation. Moreover, a variant of northern corn rootworm has developed an extended two-year diapause allowing it to similarly overcome the corn-soybean rotation. These two variants do not occur in Pennsylvania and are unlikely to develop here given the more complex rotations typically employed by Pennsylvania farmers. If growers are committed to growing continuous corn, determining the likelihood of economic gain from the Bt technology against corn rootworms can be accomplished by using an integrated pest management (IPM) approach. During late July and August, fields should be scouted to determine whether rootworm adult populations exceed the economic threshold. Fields exceeding the economic threshold of 1.0 beetle per plants are good candidates for the technology; however, keep in mind that extended use of the same tactic year after year may favor localized resistance to particular tactics. For specifics on field scouting, see the entomology department’s Web site at ento.psu.edu/extension/factsheets/corn-rootworm.