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Scouting Soybean and Corn Insects

University of Minnesota Extension, Stearns County News
July 27, 2013         
Source:  Dan Martens, Extension Educator
University of Minnesota Extension
Stearns, Benton & Morrison Counties


Scouting Soybean and Corn Insects
By Dan Martens, University of Minnesota Extension

FOLEY, Minn. (07/21/2017) — SOYBEAN APHIDS. The threshold for Soybean aphids (SBA) continues to 250 aphids per plant on 80% of the plants check at random locations across a field with conditions favoring continued population growth. Ideal conditions for SBA are temperature near 80 degrees and somewhat on the dry side. Include immature nymphs and winged aphids in the count. Populations can grow and spread faster when winged aphids show up. Examine the whole plant. Take note of predator insects and dead aphids.

CORN ROOTWORM BEETLE ADULTS. Scouting for corn rootworm adult beetles (I’ll call them CRB here) might mean looking for them on silks through pollination and following. Sticky traps can be used. Consider numbers; and whether they are Northern CRB that are a uniform green color, or Western CRB that have black stripes on the wings that cover their back. 

If you’re planting varieties that do not have genetic traits for controlling corn rootworms, it is help to watch Corn Rootworm populations from year to year to see if trends are changing, or the proportion of northern and western beetles is shifting.

We used to think a 1 year corn / 1 year soybean rotation would keep them in check. But now some corn rootworm populations lay eggs that last two years in the soil. Some are also changing so they will feed and survive on soybeans also. Volunteer corn in soybeans contributes to survival.

It is important to monitor corn rootworm beetles if you are relying heavily on the genetic trait known as Cry 3Bb1 that has shown the most evidence of insect resistance so far. If you are using other genetic traits, you should watch to make sure they are performing well.

Corn Rootworm beetles do overwinter in Minnesota. So monitoring our fields is important for making management decisions for next year.

EUROPEAN CORN BORER. Extension Integrated Pest Management Specialist Bruce Potter also wrote recently that is important to scout for European Corn Borer (ECB) and other insect larvae that bore in the stalk and ear. Bruce reported last Friday that a Univoltine biotype of ECB had started a moth flight recently. That means eggs would be laid and new larva would be hatching. The Univoltine biotype, “uni” meaning one, lays eggs once per season. Isaac Popp, from Centra Sota in Little Falls, called on Thursday about finding small ECB larvae on some corn.

The Regular ECB first generation happens in early summer, mostly June. The Univoltine biotype is considered in the middle of the summer, mostly July. The Regular ECB second generation shows up later in the summer, mostly August. Weather conditions are a major factor in timing. 

This makes scouting and treatment decisions more difficult, especially where both types are present. Treatment would largely be considered for corn that does not have a genetic trait for controlling European Corn Borer. Populations of a new young larva could be high enough to merit treatment on corn without a genetic control trait. Farmers growing corn with genetic traits for ECB control, should check fields in the fall to be sure genetic traits are working well.

Thresholds are based on yield, corn price, control cost and effectiveness. A calculation determines larvae per plant that merits treatment. Obviously there are many answers; and charts that help to sort that out.

Call Extension Office, talk to your agronomist, or do an internet search for Minnesota Extension Crop News or Minnesota Extension Corn or Soybeans to learn more. Remember spraying insects routinely without evaluating thresholds is a waste of money and time, detrimental to predator and other beneficial insects, increases risk for resistance issues, and results in personal and environmental exposure to pesticides without good reason. Be sure to identify the pest correctly and follow label directions, including use of correct person protective clothing.

REMINDER: The Stearns County Dairy Tour will be held on Tuesday August 1st from 10:30 to 12:30 at Meyer Dairy, LLC, operated by Nick, Tara, Tyler & Maddie Meyer, located at 39957 360th Street, Sauk Centre, MN 56378. Questions can be directed to Emily Wilmes at 320-255-6169, extension 3.


Daniel Martens
Extension Educator, Ag Production Systems
(320) 968-5077
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