Growers In some areas have observed yield benefits from using both corn rootworm (CRW)-resistant hybrids and soil insecticides. But are these combos a good idea or are the CRW hybrids enough?

“Today's CRW hybrids possess the best genetics available,” says Tim Maloney, Agri-Tech Consulting, Janesville, WI, a production research agronomist. “The Bt trait has been a tremendous advancement in crop production. Add herbicide tolerance and you have a winner.”

Expression of the trait for control against the European corn borer is 100% effective but is less perfect for corn rootworm, Maloney says. This is because trait expression is concentrated in the roots and is effective against corn rootworm larvae only. Adult beetles that feed on corn silks are not controlled.

Under most circumstances, the CRW Bt trait is very effective, but it requires larval feeding, which can create other problems, Maloney says, explaining that larval feeding holes can open the plant to fungal disease that can result in stalk rot and standability problems. Under extreme larvae pressure, larvae will still overwhelm plant roots even though feeding — and subsequent larval development and emergence as adults — slows.

By removing soil insecticides (with the growing use of CRW hybrids), growers have reduced nematode control, Maloney says. “Nematode populations have steadily increased since we've removed soil insecticides,” he says. He adds that seed suppliers have added seed-applied insecticides and often seed-applied fungicides to make up for shortcomings in CRW hybrids.

Do growers need to bring soil insecticides back? “The answer might be yes,” Maloney says, noting that the addition of soil insecticides may help solve some other yield-robbing problems.

Crop rotation

Robert Wright, University of Nebraska-Lincoln research and extension entomologist, believes that crop rotation may be a better answer. Soil insecticides may help protect roots from injury and increase yield in some situations, but they do not provide a high level of control of adult rootworms, especially compared with crop rotation, Wright says. He also points to concerns that rootworms may develop resistance to insecticides. “Total reliance on chemical controls is not the best long-term choice in areas where crop rotation is effective,” Wright says, noting that crop rotation has other benefits, including disease control.

Michael Gray, University of Illinois entomologist, notes that the use of soil insecticides applied in a band or in furrow has not led to insecticide resistance. Because not all of the larvae are exposed to the soil insecticide (larvae in between the rows), producers have unwittingly created refuges by using banded or in-furrow treatments of granular products through the decades. The resistance that Wright mentions is related to the use of broadcast applications of insecticides targeted at corn rootworm adult females to prevent them from laying eggs, Gray explains. This adult control strategy has been used for decades in western Corn Belt states, such as Kansas and Nebraska.

Gray adds that if corn rootworm develops resistance to the Bt protein in CRW-resistant hybrids as well as the two currently available modes of chemical action, growers will have serious problems. A variant of the western corn rootworm, popularly referred to as the rotation-resistant corn rootworm, has spread across several states (primarily Illinois and Indiana) in the eastern Corn Belt. This variant developed “resistance” to crop rotation in a span of about two decades due to the intensive rotation of only two crops, corn and soybeans. “I understand the economics of protecting the crop, but growers also must look at insect control long term, including the longevity of tools [such as CRW traits and insecticides],” Gray says.

“A hallmark of pest management is the identification of target pests or the informed assessment of a high level of risk of pests before pesticide application occurs,” says Larry Bledsoe, Purdue University entomologist. “The probability of achieving consistent economic benefit from randomly applying soil insecticides to CRW-resistant hybrids within the Midwest to manage secondary pests has not been calculated. Cost/benefit analyses for specific local areas are possible and useful. However, the determination of benefit on a large regional basis would be exceptionally difficult.”

High pressure

However, AMVAC Chemical Corporation, which markets several corn rootworm insecticides, including Counter and Fortress, says that using soil insecticides along with traits can have a substantial yield payoff when corn rootworm pressure is high.

AMVAC has sponsored trials at several Midwestern universities at the request of growers who have noted yield benefits from applying insecticides to CRW-resistant hybrids. In 17 trials, the combination of trait and insecticide improved yields an average of 12 bu./acre. A combination of factors was thought to be responsible for the yield boost. The soil insecticides in the trials controlled corn rootworm larvae as well as wireworms and grubs. The insecticides tested included Counter 15G, Fortress 5G, Aztec 4.67G and Force 3G applied with the SmartBox closed handling and application system.

Paul Vaculin, AMVAC's marketing manager of granular insecticides and closed delivery systems, notes that because the active ingredient in Counter insecticide also has strong activity on nematodes, and is the only corn soil insecticide labeled for nematode control, suppression of nematodes could have been a factor. In most of the trials last year, soil insecticides were applied at three-quarters of the full rate recommended to control corn rootworm.

Secondary pests

Studies conducted by Syngenta indicate that, when heavy corn rootworm populations and/or moderate to high levels of secondary pest populations are present, an application of Force insecticide in conjunction with a CRW trait will provide an economic return on investment, says Jeff Cecil, Syngenta insecticide brand manager. Trials to date have focused on heavily infested areas in central Illinois. Syngenta says that the 2007 trial data show that Force, when applied in conjunction with a CRW trait, increased yield an average of 12.9 bu./acre over 22 trials.

The increase demonstrated by the insecticide application over a CRW trait had a positive yield response on 95% of the trials and demonstrated an economic return on 70% of these tests, Cecil says. “This advantage is based first on choosing a high-performing genetic package, then following with additional pest control methods and combinations based on the demands and history of each field.

“There seems to be growing concern across the Corn Belt that the area affected by corn rootworm is expanding and that populations are intensifying,” Cecil says. The increase in pest pressure and the adaptability of corn rootworm are beginning to challenge the low-dose CRW events, he says. The CRW trait does not control secondary pests, and without a broad-spectrum soil insecticide, like Force, secondary pest populations grow more rapidly, Cecil says. This often leads to more crop damage.

Yield benefits

Over the last four years, Agri-Tech Consulting's Maloney has tested CRW hybrids alone as well as a combination of CRW hybrids and soil insecticides in both continuous corn and in corn-soybean rotations. He has found that the best yields generally result when CRW hybrids are planted after soybeans. Overall plant health, standability and yield still improve when soil insecticides (Force, Aztec and Counter) are applied at one-half the full rate, he says. Maloney has observed an average yield benefit of 4.3 bu./acre when using a combination of CRW hybrid and soil insecticide.

CRW-resistant hybrids are relatively new and the amount of data about how they yield when combined with insecticides is small, Purdue's Bledsoe says. Where other soil insects are present in economically significant numbers, growers could expect yield benefits where they have used both insecticides and transgenic protection. However, Bledsoe has seen inconsistent results. “I feel these results are based on random occurrence of secondary pests,” he says.

Bledsoe has planted rootworm product comparison trials since 1983 at up to five locations per year across Indiana. He has seen these trends: reduced use of organophosphate-based insecticides; smaller amount of active ingredient used per field; increased prevalence of seed treatments; and wide acceptance of CRW-resistant hybrids.