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Issue No. 16, Article 2/July 8, 2005

When to Scout for Western Corn Rootworm Adults in Soybeans in 2005

The published protocol for trapping for western corn rootworm adults in soybeans is to establish Pherocon AM sticky traps (12 in each field) the last week of July and to change traps weekly and count beetles weekly through the third week in August. All of this is explained in detail on our Web site. (Please ignore the Illinois graphic displaying high-risk and moderate-risk areas. This map obviously is outdated.) The intent of this protocol, based on research conducted in the late 1990s, is to monitor for western corn rootworm adults flying from cornfields into soybean fields during the critical egg-laying period. However, the seemingly rapid onset of western corn rootworm adults emerging from the soil in 2005 has led to questions on the best timing for using yellow sticky traps to monitor for western corn rootworms in soybean fields this year. People are concerned that if they wait until the end of July to establish traps, they might miss a significant part of the egg-laying period.

This concern may be justified. After a relatively slow start in rootworm development this spring, the very warm temperatures in June sped things up considerably, compressing rootworm development. It is possible that peak emergence of adult western corn rootworms has occurred in some, maybe many, fields, in which case oviposition will begin sooner rather than later. Without any research to support a shift in timing of monitoring for rootworm adults in soybeans, we have to give some credence to logic. So you should consider placing yellow sticky traps in soybeans earlier in the summer this year rather than waiting until the last week of July. How much earlier? We can't provide a specific answer, so again, you'll have to use your best judgment. If you have the time and the dedication, you can begin trapping as early as mid-July and keep trapping through mid-August. This will virtually ensure that the traps will capture egg-laying female western corn rootworms if they are present.

We strongly encourage people in the so-called fringe area of the distribution of the variant western corn rootworm to trap for adults in soybeans in 2005. Without knowledge of the occurrence and density of western corn rootworm adults in soybeans, deciding about the need for a rootworm-control product for 2006 corn planted after 2005 soybeans will be guesswork. And despite the widespread use of rootworm-control products in the area known to be infested with variant western corn rootworms, trapping in soybeans is still encouraged, for all the same reasons. Growers can add rootworm control to their annual production costs, or they can determine from one year to the next whether an expenditure for control is necessary. Despite some beliefs to the contrary, not all cornfields are created equal when it comes to rootworm larval pressure.--Kevin Steffey and Mike Gray

Kevin Steffey
Mike Gray

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