No. 20 Article 3/August 6, 2004

Controlling Rootworm Adults in Soybean Fields

In what has become a frustrating refrain, we have learned that some people continue to recommend spraying insecticides to kill rootworm adults in soybean fields, with some hope that reduced numbers of adults in soybeans in 2004 will result in less larval damage in 2005. Unfortunately, more often than not, a rootworm-control product will be used when corn is planted in 2005. We have addressed the potential significant side effects of this strategy so many times that it has become boring, both for us and for you. However, we must state again that exposing adult corn rootworms to insecticides in late summer one year and then exposing corn rootworm larvae to insecticides again in the spring the following year is a recipe for trouble.

One of the most common questions we received during 2003-2004 winter meetings was "If I sprayed an insecticide to control soybean aphids in August [2003], did I kill enough rootworm adults that I won't need a rootworm-control product in 2004?" The most honest answer to this question is "I don't know." Undoubtedly, all of the insecticides applied to control soybean aphids in 2003 killed rootworm adults, too. However, given the limited residual effectiveness of most insecticides and the western corn rootworm's behavior of flying back and forth from corn to soybean to corn, timing an insecticide application to prevent significant egg laying in soybean fields would be difficult. We are aware that some cornfields in east-central Illinois that have been severely damaged by rootworm larvae this year were soybean fields treated for soybean aphids last year. I am certain that the converse is also true (fields of soybean treated for soybean aphids in 2003 with little or no rootworm larval damage in 2004). However, we do not have a database substantial enough to let us draw any reliable conclusions.

Neither a scouting procedure nor a threshold for controlling rootworm adults in soybean to prevent significant egg laying has been developed. Until these two crucial aspects of a rootworm adult control program have been worked out, we discourage the application of insecticides to control western corn rootworm adults in soybean. --Kevin Steffey and Mike Gray

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