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Time to Scout Soybean Aphids - They've Finally Arrived in the North

by Dr. Ian MacRae, U of MN Extension Entomologist

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Low populations of Soybean Aphid (SBA) have been reported throughout NW MN and NE ND. Populations are still low and generally not on more than 30% of the plants. The cooler weather will slow reproduction for a few days but it is predicted to warm up by the weekend, at which time we'll start to see some more population growth and dispersal across fields. Although most fields are well below treatment levels so far, it is time to start scouting the soybean fields, getting a handle on what populations you may have and tracking progress and population growth.

REMEMBER - the treatment threshold is 250 aphids/plant when 80% of plants have aphids. This threshold is backed by literally dozens of site years of data and has been adopted by ALL of the universities in the North Central Region. You will hear lower thresholds (some as low as 10/plant) - frankly, these just don't hold water. Some argue that as the cost of the commodity goes up and the cost of treatment goes down we should lower the 250 threshold. This would make sense if not for the simple fact that at 250/plant, soybean aphids are not yet causing yield loss, so treating at such a low level is a waste of money and a poor management practice. There is more than the mathematics at work here, the biology of the plant and the insects have to be taken into consideration.

Spraying early, unless at threshold levels, can set up the field for needing a second application later in the season. Spraying removes predators and parasitoids that help decrease establishment, it also does not protect any new plant material added after the application. End result - a lot of new plant material with little or nothing to prevent aphid establishment. Unless there are threshold levels of SBA in the field, do not spray early!

There are a number of guarantee programs available as well this year. Most seem to imply you should treat early and the chemistry for subsequent treatments, if needed, will be supplied free. A couple of problems with this strategy:

  1. Second applications aren't free, the chemistry may be, but the application costs are not,

  2. If you spray early, regardless of population, you'll likely need a subsequent application, and that subsequent application will be the same chemistry. Multiple applications of the same chemistry in the same year is one factor in the development of resistance, something we should be paying a little more attention to with this critter.

A lot to think about, but all-in-all, the best management option is to scout and treat when populations reach the 250 aphids/plant threshold.

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