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Agronomy Day 2007

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Is There a Future for Granular Soil Insecticides? Is a 20% Target Enough?

tour a
Michael E. Gray Michael E. Gray, Professor
and Extension Coordinator
Department of Crop Sciences
Tel: (217) 333-6652

Since the early 1960s, granular soil insecticides applied at-planting have served as the primary control tactic for corn rootworms and other secondary soil insects such as wireworms and white grubs. Historically, Illinois’ farmers have relied extensively on these products for corn rootworm control, treating nearly 90% of non-rotated corn acres each spring. Prior to the evolution of the variant western corn rootworm, farmers across the state typically treated fewer than 15% of their first-year corn acres. Following the aftermath of the widespread damage caused by the variant western corn rootworm in 1995, farmers have come to expect potential root injury in both rotated and continuous corn acres. Crop production inputs targeted at soil insect pests are now largely regarded by many farmers as insurance premiums they have elected to purchase. This management scenario has called into question the IPM paradigm, championed by land grant entomologists for many decades.

fig 1Of increasing interest to farmers are Bt corn hybrids, plants that have been genetically-engineered to prevent economic losses against a few key insect pests such as the European corn borer, some other lepidopteran pests (e.g., black cutworms, western bean cutworm), and corn rootworms. Not all Bt products are the same and farmers are encouraged to carefully review product literature before a purchase is made. We have learned that Bt hybrid performance can vary with respect to corn rootworm protection even with the same event. In most instances, transgenic hybrids have provided effective control against the two primary corn insect threats, hence the growing popularity of stacked Bt hybrids. However, Bt hybrids do not express insecticidal proteins that are effective against the wider array of secondary soil insects. Consequently, seed manufacturers continue to treat Bt seed with insecticidal seed treatments. The two most common products, clothianidin (Poncho®) and thiamethoxam (Cruiser®), are neonicotinoid insecticides. Each has systemic properties and the active ingredient is absorbed by the root system. These insecticides are now in great use across the U.S. Corn Belt. Resistance to neonicotinoid insecticides has developed previously in other insect pests such as the destructive Colorado potato beetle. After only three field seasons, Colorado potato beetle resistance to imidacloprid (a nicotinoid) was detected. No resistance management plan for the neonicotinoids has been advocated or developed, and none is anticipated. It seems almost certain that because of the ubiquitous use of these products on Bt seed, insecticide resistance is inevitable. Product performance of the neonicotinoids has been inconsistent against secondary insects. Against this backdrop, do the granular soil insecticides have a promising future? Will the private sector continue to invest in the production of these products and the human resources necessary to market and deliver these insecticides to farmers? Will a 20% market be attractive for some manufacturers?

fig 2Escalating use of transgenic Bt corn hybrids is anticipated as more acres are devoted to corn production to meet the increasing demand for ethanol. Favorable commodity prices, introgression of Bt events into elite corn germplasm, convenience and flexibility of planting stacked hybrids, larger farms, newer bulk planter designs, producer concerns over human health and safety during calibration and application, and perceived yield benefits of Bt hybrids compared with their non-Bt counterparts (even those non-Bt hybrids treated with soil insecticides), are among the many factors that will contribute to a continuing decline in the use of granular soil insecticides. However, granular soil insecticides can play an important role in the protection of corn planted into refuges. Thus far, insecticidal seed treatments have not offered consistent root protection in refuges when corn rootworm densities are high. If resistance to Bt emerges at some point in the future, the use of soil insecticides would again increase, assuming an infrastructure remains in place to implement this change. Ironically, resistance to granular soil insecticides has not developed despite their use for decades. Producers have unwittingly employed a refuge strategy each spring by banding these products; thus, exposing only a portion of the corn rootworm population each year to the chosen toxin. Granular soil insecticides now seemingly have a new and important role to play – as a refuge protector. In this role, they serve to prolong the potential usefulness of Bt events by delaying or preventing resistance to these new transgenic tools.