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Frequent information updates for agricultural audiences

Soybean Aphids - from Mike Roegge

Posted by John Fulton -

We've been concerned with soybean aphids for the past few weeks, and have written to producers about the importance of scouting fields. Just within the past week some fields have seen an explosion of aphid numbers. Dave Simpson and Bill Cassady have both reported aphid numbers exceeding 500 per plant on some plants in fields they've scouted.

The economic threshold is 250 per plant. The economic injury level is 700 or more per plant. The difference is to give you time to line up treatment options as populations can rise rapidly in a short time. However, some useful facts to pass on to you as you go about determining if treatment is necessary.

Our 250 treatment recommendation is based upon soybean maturity from R1 (beginning bloom) to R5.5 (when the bean in the pod is about ½ size at one of the 4 uppermost nodes). The data really doesn't support treatment on soybeans that are R6 or later (when the bean in the pod is full in one of the 4 uppermost nodes). You must also remember that as the bean matures, the nutritional value of the bean declines which adversely affects the aphids, causing lower reproduction, and populations will decline.

When scouting for aphids also take into account the number of aphid dwarfs. These are whitish colored and about ½ size of the aphid. As the nutritional level of the bean declines, these dwarfs are produced. They are not nearly as detrimental to the soybean as the aphid is. This usually signals a decline in the aphid population.

Several specialists have mentioned the fact that you don't necessarily pull the trigger once the 250 threshold has been reached. Rather you need to go back out to the field in 2 days and determine if populations are increasing or not. So, if the field average today is 250 (or more) and the beans are at R4, contact the custom application and let them know that you may need them to schedule you. Then 2 days later, reexamine the field. If the aphid numbers have increased then our recommendation would be to control them. But, if the numbers are the same or declining, then schedule another visit in 2 more days to scout. Aphid populations can and do crash in a matter of days. Especially with the heavy dews and fog we've been experiencing of late. Also make a note of predators (lady beetles, etc.). If you find 2 or more per plant, you can expect them to provide some benefit of control.

Lastly, since the beans are growing in ideal conditions, they can withstand more stress than normal.

If you do apply a control, strongly consider a check strip to determine how effective the application was. Also, you might be interested in a "speed scouting" method of scouting for aphids. This might increase your effectiveness when scouting.

Note: Make sure you check populations of predators. The syphid fly numbers have exploded in the last two weeks, and in conjunction with other predators may provide at least partial control of the aphids.

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