Seed treatments and soybean aphids

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

This issue of the Field Crop CAT Alert includes an article by Terry Schulz and Kurt Thelen about seed treatments and soybean yield response. From a soybean aphid standpoint, I can confirm many of their findings.

Neonicotinoid seed treatments are recommended in soybean fields with a history of economic damage from seed corn maggot, white grubs and other soil insects. Bean leaf beetle is another early season pest that is controlled by seed treatments. In Michigan, early-season populations of bean leaf beetle are rarely high enough to merit treatment (Young beans can tolerate considerable injury.), but in western states where beetle populations are typically higher, seed treatments may be valuable. However, there is no good evidence that bean pod mottle virus infection (transmitted by bean leaf beetle) is less in seed-treated soybean.

Seed treatments for “insurance” or prophylactic control of soybean aphid are not recommended as a standard practice. Laboratory and field studies indicate that soybean aphid begins to survive on seed-treated plants 35 to 40 days after planting. In many parts of the United States and Canada, this is just when aphids begin to colonize fields. In outbreak years, populations in seed-treated fields often reach the 250 threshold, and require foliar insecticide sprays, at the same time as in untreated fields. Replicated university research trials across multiple states and years do not show a significant yield increases from using seed treatments under no or low aphid pressure. Many of these trials used small plots, but others were done in 1-acre blocks (Michigan, Iowa and Minnesota) or in replicated strips the length of a field (Nebraska). During aphid outbreaks, using an IPM approach based on crop scouting and thresholds to optimally-time a foliar spray results in significantly greater yield compared to using a seed treatment. An IPM approach to soybean aphid control also limits insecticide use to when and where it is needed, reducing pesticide exposure and selection for resistance. Formation of insecticide resistance is a growing concern, because neonicotinoids are available as both seed treatments and foliar sprays in many crops.

In Michigan, there is one small area where I believe past history may justify a seed treatment for soybean aphid in predicted outbreak years. Parts of Monroe County in southeast Michigan have some of the heaviest infestations of buckthorn (soybean aphid’s overwintering host) in the state. In the spring of 2005, colonization of soybean fields around Dundee, Monroe and other towns near the River Raisin was early and heavy, and aphids went over threshold by mid-June. Some fields were sprayed early, while other fields had been seed treated. In essence, the seed treatment replaced the first foliar spray. Later, fields were reinfested and many were sprayed a second time in late July. Remember, this is an exceptional area, based on the sheer amount of buckthorn. We know egg numbers were very high in this area this past fall and aphids may colonize fields early. Again, in this limited area of Michigan, the assumption is that early colonization sets up a two-spray system, and a seed treatment replaces the first foliar spray. From a time management standpoint, I realize that the seed treatment has advantages on the farm. From an IPM standpoint, though, two foliar sprays are equally as effective, and the sprays can be timed if and when they are needed.

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