Western Corn Rootworms and First-Year Corn Injury: Questions and Answers

July 16, 1999
We continue to receive many questions regarding western corn rootworm injury in first-year corn (rotated corn). Along with these questions are continuing concerns regarding the performance of soil insecticides. The following questions and answers hopefully will shed some light on corn rootworms, soil insecticides, and first-year corn rootworm injury.

Is the rootworm injury worse this season than last year?

So far, reports from throughout the state and our own observations verify greater densities of corn rootworms this year. We have speculated about reasons for thisoccurrence in the previous issue of theBulletin (no. 16, July 9, 1999). Now is a good time to check forlarval injury in continuous corn as well as first-year corn in east-centralIllinois. The only way to evaluate soil insecticide performance is tocompare root injury (rate roots on the Iowa State 1­6 injury scale;see previous article) inuntreated strips (no soil insecticide used) with treated portions of agiven field. The bottom line: Grab a shovel, and be prepared to dig, wash,and rate roots in order to begin ade- quately assessing the return on yoursoil insecticide investment.

Are root ratings of 4.0 (one node of roots pruned) required before yield losses begin to accrue?

No. During a 4-year period, we evaluated the responses of 12 hybrids grown in DeKalb and Urbana to rootworm injury. In stressful growing seasons, characterized by soil moisture shortages during pollination, root rating averages as low as 2.0 (minor to moderate root scarring) reduced yields of certain hybrids. For some hybrids grown in a less stressful year, a root rating of 4.0 (one node of roots destroyed) may be required to elicit a negative yield response.

Are hybrids with large root systems and good root regeneration characteristics always the best "yielders"?

No. In wet growing seasons, particularly from mid-July to mid-August, certain hybrids with small root systems that lacked good root regeneration characteristics have been shown to be very competitive with respect to yield. In dry years, these smaller-rooted hybrids also have been shown to suffer significantly greater yield losses compared with larger-rooted hybrids. Having a large root system and possessing good root regeneration potential appears to be an asset in a dry year; this is not always the case in a wet season. The interaction among root injury, hybrid, and soil moisture with respect to yield can be quite complex.

Should I consider treating my soybean field in east-central Illinois to suppress the egg-laying activities of adult western corn rootworms?

No. We are continuing to learn more about the phenology of egg laying by western corn rootworms in cornfields and soybean fields of east-central Illinois. Unfortunately, we have much research to conduct before we can make responsible recommendations regarding the optimum time to trigger applications to prevent economic loads of eggs from being laid in soybean fields. If producers treat soybean fields for western corn rootworm adults, they will still be left with uncertainty about the effectiveness of the application. Because of this uncertainty, many producers also will be asked to consider the use of a soil insecticide at planting next spring. Two insecticide applications that are targeted at the prevention of root injury make neither environmental nor economic sense. This is especially true at the current extremely low commodity prices.

Should I consider the use of another type of sticky trap to monitor adult western corn rootworms in soybean fields? I've been told some other traps capture more adults.

Capturing more adults with another type of trap may not be an advantage. Counting more beetles requires more time and may not provide any additional information. Our economic threshold of two to seven beetles per trap per day was based on the use of Pherocon AM traps. If you use another type of sticky trap, be aware that comparing your trap counts with our threshold may be similar to comparing apples with oranges.

Can I use Pherocon AM traps in continuous corn systems?

Yes. In fact, most of the early research with Pherocon AM traps and corn rootworms was conducted during the 1980s in continuous cornfields in Iowa. In continuous corn, researchers suggested that 12 traps be evenly deployed throughout a field. If beetle densities reached six beetles per trap per day, producers were encouraged to rotate to another crop or consider the use of a soil insecticide the following spring. Researchers in this study also suggested that Pherocon AM traps should be used during the last 3 weeks of August.

Why hasn't the use of Pherocon AM traps been accepted in corn as a pest management approach?

During the mid-1990s, we conducted many on-farm trials (continuous corn) with producers in northern Illinois using Pherocon AM traps. The bottom line of the study: All of the 17 producers in our experiments indicated that traps were too difficult to locate in corn and consequently too inefficient to use. In addition, finding Pherocon AM traps within a pollinating cornfield during the heat of the summer was no picnic. The use of Pherocon AM traps within soybean fields has received a more positive reception. We believe that this is largely due to the much greater visibility of these traps across a soybean field. We continue to be impressed with the number of producers who express interest in Pherocon AM traps for monitoring western corn rootworm adults in soybean fields!

The next few weeks will undoubtedly bring more reports of rootworm problems across Illinois. Please let us know how your "rootworm season" progresses.--Mike Gray

Author: Mike Gray