Eileen Cullen, Extension Entomologist

 

Recently, a County Agent colleague shared with me that he has heard of a few producers’ plans to apply a full corn rootworm rate of soil applied insecticide when planting a rootworm Bt corn hybrid. His impression was that growers and others have heard about Bt rootworm corn “failures” reported in neighboring states, may have seen lodged corn in an area of their own farm planted to Bt rootworm corn, and/or want to ensure total root protection from larval feeding.

 

This article stems from our discussion about this proposed practice to “double-up” on rootworm control tactics for the 2007 season. We (UW Madison Entomology and UW-Extension) do not recommend this practice, nor does it fit into a sound integrated pest management (IPM) approach to agronomic field crop production. This is not only a discussion taking place in Wisconsin, but a recent field crop newsletter article by my extension entomologist colleagues in Illinois are also cautioning against “loading up” on rootworm control tactics (Steffey and Gray 2007).

 

First, it is important to be knowledgeable about the instances of rootworm Bt corn failures. It is also important to examine the notion that any one insect control tactic, including rootworm Bt corn, will provide total (“bullet proof”) root protection in all cases under heavy rootworm pressure. That is not the case. Registered Bt insect traits for field corn in 2007 for corn rootworms include AgrisureÒ RW (mCry3A), HerculexÒRW (Cry34Ab1/35Ab1), YieldGard® Rootworm and YieldGardVTÒ Rootworm (Cry3Bb1). Bt corn product failures have been reported sporadically since 2003 when the first rootworm Bt corn trait became commercially available. One example comes from the University of Illinois 2004 corn rootworm efficacy trials in Urbana (Steffey and Gray 20004). Significant root injury occurred in research plots designed to produce severe rootworm pressure, the plots were early planted (04/19/04), and larval pressure was indeed severe. However, the rootworm Bt corn performed well at other IL trial locations that same year. In 2006, instances of unacceptable root feeding injury to Bt rootworm corn in eastern Iowa production fields were reported. The 2006 reports from production fields in eastern IA, were confirmed in one field by a visit from ISU entomology extension specialists (Tollefson and Rice 2006). They observed heavy rootworm feeding damage on corn roots, not only on the rootworm Bt corn, but also on the non-Bt corn in the refuge which had been treated with a soil insecticide. It is not known why these Bt rootworm corn failures occurred. Possible explanations, in the locations where they have occurred, include corn rootworm populations were exceptionally high, possibly the Bt gene was not adequately transferred into the commercial hybrid, or perhaps the Bt protein was not produced at high enough levels during the period when the larvae were feeding. These theories remain to be tested with careful experimentation. During 2003-2006, the average root injury across all Bt corn rootworm hybrids in the UW Madison Entomology test plots averaged a node injury rating of 0.04 (on a 0 to 3 scale, 3 being the worst damage).

 

Second, the best time to inspect corn roots for root injury due to corn rootworm larval feeding is in July. Growers and consultants can be most certain in July of accurately diagnosing corn root injury from rootworms (pruning, feeding scars), while damage is still relatively fresh. It is quite difficult to determine the original cause of lodging later in the season, or during harvest, when corn roots may be diseased or senescing in the field. If you have not evaluated roots before, or have questions about lodged corn in a field, please contact your UW-Extension County Office Agriculture Agent or Crops/Soils Educator for assistance in July, or no later than early August if possible.

 

Finally, remember that rootworm Bt corn hybrids are treated with an insecticide seed treatment (i.e., Poncho, Cruiser) at 0.25 mg / kernel before they are bagged and sold. Insecticide seed treatments on Bt rootworm corn are labeled for early season secondary pests (i.e., grubs, wireworms, seedcorn beetles, seedcorn maggot, etc.) not controlled by the Bt rootworm trait.

 

While options for corn rootworm control have expanded in recent years, there is no entomological data or expectation that applying a soil insecticide to rootworm Bt corn (already treated with a seed-applied insecticide for secondary pests) will ensure total rootworm protection. Corn rootworm populations have a history of developing resistance to control tactics, from chemical (Gray and Luckman 1994, Meinke et al. 1998) to crop rotation (Levine and Oloumi-Sadeghi 1996.). Insect resistance management (IRM) within an IPM program requires thoughtful use of multiple tactics based on field history and insect scouting records. It is recommended to rotate corn rootworm soil insecticide modes of action over time. It is also required by law (US EPA) to plant a non-Bt (i.e., non-YieldGard Rootworm, non-Herculex RW) corn refuge (20%) within or adjacent to each field of Bt rootworm corn on each farm. A previous article this spring addressed IRM when planting Bt corn hybrids and how to control rootworms within the non-Bt corn refuge areas (Wisconsin Crop Manager 14(3), http://ipcm.wisc.edu/WCMNews/tabid/53/EntryID/218/Default.aspx). Applying soil insecticide to a Bt rootworm corn hybrid is not a substitute for Insect Resistance Management (IRM), nor can it guarantee fail-safe root protection.

 

Cullen, E. 2007. Insect resistance management when planting Bt corn hybrids. Wisconsin Crop Manager 14(3): 23-25 http://ipcm.wisc.edu/WCMNews/tabid/53/EntryID/218/Default.aspx

 

Gray, M.E., Luckman, W.H. 1994. Integrating the cropping system for corn insect pest management. pp. 507-541, In R.L. Metcalf and W.H. Luckman [eds.], Introduction to insect pest management, 3^rd ed., John Wiley and Sons, Incorporated.

 

Levine, E., and H. Oloumi-Sadeghi. 1996. Western corn rootworm (Coleoptera:Chrysomelidae) larval injury to corn grown for seed production following soybeans grown for seed production. J. Econ. Entomol. 89:1010-1016.

 

Meinke, L. J., B. D. Siegfried, R. J. Wright, and L. D. Chandler. 1998. Adult susceptibility of Nebraska western corn rootworm (Coleoptera: Chrysomelidae) populations to selected insecticides. Journal of Economic Entomology 91:594-600.

 

Steffey, K., Gray, M. 2007. Loading up for rootworm control. The Bulletin Issue 1: March 23, 2007 http://www.ipm.uiuc.edu/bulletin/article.php?id=655

 

Steffey, K. Gray, M. 2004. Transgenic corn rootworm hybrid stumbles in Urbana experiment; Some producers also report severe lodging with YieldGard rootworm hybrids in commercial fields. Issue 22: September 2, 2004 http://www.ipm.uiuc.edu/bulletin/article.php?id=181

Tollefson, J., Rice, M.E. 2006. Bt rootworm corn failures: Understanding the issues. Integrated Crop Management IC-496(26), November 13, 2006 http://www.ipm.iastate.edu/ipm/icm/2006/11-13/btcorn.html