No. 16 Article 1/July 9, 2004

The "Buzz" About Corn Rootworm Damage Continues

As we rounded the corner into July, reports of significant damage caused by corn rootworm larvae continued. Recent storms have caused corn to lodge in many fields. Reports of inadequate to poor performance of rootworm-control products have been plentiful, and many growers and their agricultural consultants are scratching their heads, contemplating the possible reasons for these "failures." Explanations for the widespread damage and its severity in some areas range from the ridiculous to the sublime. Unfortunately, when all is said and done this season, it is likely that we still will not have one irrefutable explanation for all of the damage.

In some ways, the rather widespread occurrence of rootworm damage in insecticide-treated fields in 2004 reminds me of similar experiences in the mid-1980s. That dates me, I know. But it probably dates a lot of our readers, too, who will not soon forget all of the lodged cornfields they walked in July in 1983, 1984, and 1985. Poor performance of some of the insecticides way back then was explainable (i.e., enhanced microbial degradation of Amaze and Furadan). However, reports of inadequate performance of the other major soil insecticides being used then (e.g., Counter, Dyfonate, Lorsban) were also relatively common in northern Illinois. And you hear many of the same explanations today that you heard then--too much rain, too little soil moisture, high winds during and after planting, application problems. One of the most creative excuses I heard given to an affected producer was that when he cultivated his field, he lifted all of the larvae above the insecticide "layer," thereby moving the larvae out of danger of death by insecticide. Priceless.

I am as convinced today as I was in the mid-1980s that many factors contribute to poor performance of rootworm-control products. It is undeniable that degradation of soil insecticides is affected by, possibly accelerated by, environmental conditions. Accelerated insecticide degradation + early planting could very well lead to poor performance of an insecticide in the soil. It also is undeniable that soil insecticides may not be "activated" in dry soil conditions. However, one cause that may have contributed to poor performance of granular insecticides is inadequate application or incorporation. We don't talk about this frequently enough, and it may be time that we give it due attention.

From a study we conducted a couple of decades ago, we learned that incorporation of a soil insecticide in the soil is extremely important for the insecticide's performance against corn rootworm larvae. We applied the soil insecticide Counter 15G in four different ways with an old, four-row International Harvester planter:

The amount of rootworm larval damage in the plot increased as the degree of incorporation decreased. In other words, Counter applied behind the press wheel and not incorporated with tines resulted in a rootworm-control failure.

Planters have improved considerably since we conducted our study in the mid-1980s, but some of the same issues remain. Some people have suggested that furrow-closing wheels on some models of planters provide better incorporation of soil insecticides than press wheels. Some people also have suggested that a T-band application--some of the insecticide is applied in furrow, some is applied in a band over the row--provides better incorporation than a simple band application over the row. Both of these explanations seem logical; however, I am not aware of such comparisons in adequately designed and replicated research trials. What undoubtedly is true is that some incorporation of the insecticide in the soil is certainly better than none, especially if environmental conditions don't work in our favor. So maybe the experiences with rootworm damage in 2004 will persuade us to pay more attention to soil insecticide application.

In addition to our concerns about performance of rootworm-control products, there is the never-ending interest in the distribution of the variant western corn rootworm that lays eggs in crops other than corn. Rootworm larval damage has been observed in 2004 much farther west than has been observed in the past. We are aware of rootworm larval damage in fields of corn planted after soybeans as far west as Route 67, although more commonly as far west as Route 41. Apparently the variant has crossed the Illinois River. By summer's end, we should have a better handle on the expanded distribution of the variant western corn rootworm in Illinois.

One more note regarding severe rootworm larval damage in July. Some producers who encounter severe rootworm larval damage late in June or early in July want to do something about it. The fact of the matter is that unless a "rescue" insecticide can be placed near the bases of the corn plants, a "rescue" application should not be made. An insecticide broadcast over the tops of tall corn plants will not provide relief from rootworm larval damage. In the end, the extra money spent is not worth it. Rootworm larval damage is tough to accept, but throwing more money at the problem is not the answer.

Hopefully results from our numerous trials in 2004 will enable us to answer some questions about performance of rootworm-control products. If we or you are not satisfied with the answers, maybe we can work together to delve into the situation a little deeper. Whatever we learn, in whatever way we learn, we will share our observations. --Kevin Steffey

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