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Reduced Soil Insecticide Rates for Corn Rootworms: An Old Story Revisited

June 26, 2003

In recent years, renewed interest in the economics of various corn rootworm control tactics has emerged. Specifically, many producers are increasingly seeking more information regarding the potential benefits and drawbacks of using seed treatments, transgenic (Bt) hybrids for corn rootworms, liquid formulations, and reduced rates of traditional corn rootworm granular products. As the western corn rootworm variant continues to spread and damage first-year cornfields due to the oviposition of eggs in soybean fields the preceding year, many growers who have traditionally relied on crop rotation for management of corn rootworm larvae are beginning to seek answers for the first time regarding corn rootworm control.

In evaluating the potential benefits and risks of these various control tactics, don't forget to at least consider the well-documented research on the effi-cacy (regarding the prevention of corn rootworm larval injury) of reduced rates of granular soil insecticides. In the most recent issue of the Journal of Economic Entomology (June 2003), scientists at South Dakota State University (Billy Fuller and Paul Evenson) and North Dakota State University (Mark Boetel) reported the results of some research on reduced rates of terbufos (Counter 15G), tef-luthrin (Force 1.5G), and chloretho-xyfos (Fortress 5G). Their research was conducted in 1993, 1994, and 1996; therefore, these formulations reflect those that were used during this time period. The title of their article is "Emergence of Adult Northern and Western Corn Rootworm (Coleoptera: Chrysomelidae) following Reduced Soil Insecticide Applications." They offered the following concluding remarks: "Because application rates of these compounds have generally failed to impart marked impacts on corn rootworm feeding injury, survival, temporal emergence patterns, or reproductive capacity, pest managers choosing to apply one of these compounds at full-labeled rates should consider the option of reducing the application rate to a 0.75X level. Given the preponderance of evidence cited herein, this strategy should be both economically and environmentally beneficial if steps are taken to properly and accurately calibrate and apply these materials. Costs for applying these insecticides to protect corn from rootworm larvae could conceivably be cut by 25%. In addition, less toxic material would be released into the environment, thereby minimizing the potential for negative effects on nontarget organisms and reducing the risks of groundwater contamination. Notwithstanding, pest managers and producers should note that use of an application rate lower than that prescribed on the product label for control of the target pest voids any otherwise implicit war-ranty for an acceptable performance level."

The results of small-plot research con-ducted previously throughout the north-central United States at various universities were summarized and published by Billy Fuller and others in 1997 (Journal of Economic Entomology). These data also confirmed the utility of using reduced application rates (0.75X) of granular soil insecticides for corn rootworm larval control. On-farm research conducted across 10 northern Illinois counties (29 producers) in the early 1990s further con-firmed that 0.75X rates of certain granular soil insecticides (primarily evaluated terbufos [then Counter 15G] and chlorpyrifos [Lorsban 15G]) provided similar root protection and yields as compared with the labeled rates.

As producers evaluate (compare root injury in treated portions of a field with untreated areas) the return on their capital investment in rootworm protection this season, they may choose to consider the option of using 0.75X rates next spring. As stated previously, producers assume full lia-bility for using less than a labeled rate. In addition, time of planting should influence a producer's decision regarding rate selection of a given soil insecticide. If planting occurs in early April, the efficacy of granular soil insecticides may be compromised if they are applied at the 0.75X rate, particularly in years in which corn rootworm larvae hatch late (mid-June).--Mike Gray

Author: Mike Gray

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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