No. 3 Article 3/April 8, 2005

Cruiser Registered for Use on Soybeans

By now, most of you know that the nicotinoid insecticide Cruiser (active ingredient thiamethoxam) was registered by the U.S. EPA for application to soybean seed to control bean leaf beetles, seedcorn maggots, soybean aphids, and wireworms. The seed-applied insecticide has been made available by Syngenta Crop Protection in CruiserMaxx Pak (a combination of Cruiser seed treatment insecticide and ApronMaxx seed treatment fungicide).

Here's a question. Is the application of a preventive insecticide necessary for production of soybeans in Illinois, or elsewhere in the Midwest for that matter? The reason I ask is that many producers seem to believe that this seed treatment is necessary for a crop that prior to 2000 rarely required an insecticide application. Over the past three decades, soybean producers occasionally had need of insecticides/miticides for control of bean leaf beetles, grasshoppers, twospotted spider mites, and sundry other insects. Occasionally, outbreaks of these insect or mite pests (in 1988, for example) justified widespread use of insecticides or miticides. However, annual application of insecticides to soybeans was not necessary and not common. So what has changed?

The most obvious answer is the arrival of the soybean aphid in 2000, with widespread and significant outbreaks occurring in Illinois in 2001 and 2003. Another answer is the relatively recent bean leaf beetle/bean pod mottle virus scare. However, neither bean leaf beetles nor soybean aphids have caused significant yield losses every year since 2000, and expectations for future occurrences of these insect pests still are uncertain. So why the mad rush to add another cost to soybean production? Will the addition of seed treatments result in yield benefits greater than the cost of the seed treatments?

Quite honestly, I can't answer these questions very well; limited data have been generated. We have data that reveal clearly that Cruiser controls early-season bean leaf beetles very well. However, densities of bean leaf beetles are very difficult to predict. For example, we established two rather robust trials in northwestern Illinois in 2004 in soybean fields surrounded by woods and alfalfa fieldsan ideal setting for bean leaf beetles. Results? Almost too few bean leaf beetles to count. Under such conditions, waiting to use a foliar insecticide only if necessary (and it wasn't necessary) was the more economic approach.

So what about soybean aphids, currently the most dreaded insect pest of soybeans? Some data have revealed early-season suppression of soybean aphid populations. But acceptable control of soybean aphids with Cruiser during a soybean aphid outbreak (i.e., population densities held below economic thresholds in late July, early August) is unlikely. Eillen Cullen, an extension entomologist at the University of Wisconsin, has written two excellent articles regarding Cruiser and soybean aphids in the newsletter Wisconsin Pest Manager:

I strongly encourage you to read both articles before assuming that planting insecticide-treated seed is the best approach for managing soybean aphids.

We and other entomologists throughout the Midwest will continue to establish field trials to determine whether seed-applied insecticides will be an added benefit for soybean production. As with the use of prophylactic insect-control products in corn (refer to "Protecting Corn from Soil Insect Injury" in this issue of the Bulletin), the use of prophylactic insect-control products in soybeans is not necessarily a tenet of IPM. We encourage producers to gather more facts before jumping on this bandwagon.--Kevin Steffey

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