Monday, July 22, 2024

The use of biological insecticides for bollworm management in soybeans

• By Whitney Crow, Angus Catchot, Jeff Gore and Don Cook •

bollworm in soybeans
This bollworm larva is more than 0.5 inch and is too large to treat with Heligen. Larvae of this size  will require a more traditional insecticide — photo courtesy Mississippi State University

There are several biological insecticide options labelled in soybean; however, the most readily available and widely used product is Heligen. In the past week, many people have asked about the use of Heligen with the low bollworm numbers showing up in some soybean fields.

Heligen is not a traditional insecticide and needs to be treated as such. Expectations and limitations need to be understood when using biological products. Moving into next week, there will likely be an increase in the number of bollworms detectable with a sweep net, especially in later-planted beans that are flowering and with open canopies.

It is possible that many will run into situations with subthreshold levels leaving you questioning the next best management strategy. Questions then arise — do you spray and are populations high enough to justify the cost of a diamide or do you let them ride hoping populations will play out or crash?

This is a situation where both the dynamic threshold and Heligen come into play. By using the dynamic threshold with the current market price (approximately $9), the cost of Heligen plus the application (approximately $10), you are looking at a threshold of 4.9 larvae per 25 sweeps (Table 1).

Unlike if you are using 14oz of Prevathon (using the dynamic threshold), the treatment threshold would be ca. 9.8 larvae per 25 sweeps. The advantage to using this dynamic economic threshold over the standard (static) economic threshold is the ability to adjust based off market prices and input cost.

soybean dynamic thresholds

Things to remember when using Heligen:

• The goal is to target worms smaller than 0.5 inch. Do not use this product if larvae are greater than 0.5 inch.

• Generally, larvae will stop feeding within three days of the application and die within nine days.

• The rate of death will be dependent on temperature. The larvae that die from Heligen will generally release enough virus to cause secondary infections.

• Rate ranges are 0.7 -1.4 oz per acre; however, we recommend staying toward the middle to high end of the rate range depending on numbers and larval size.

• Heligen is a virus so it is slower acting. Generally, in five to seven days you will see larvae move toward the top of the canopy. Often sweep net numbers will actually increase in your samples five to seven days after application. This is normal and not a sign of a control failure. As larvae become infected and move to the top of the canopy, you simply catch more in the sweep net. Often, this is a sign that the product is actually working.

• Heligen is typically not a “zero out” product like many of the other products we use on bollworm in soybean. It typically keeps numbers well below threshold where economic damage is not occurring.

• In fields where larval numbers are high or where worms are greater than ½ inch use alternate products like Besiege, Prevathon, Elevest or Intrepid Edge.

Dr. Whitney Crow is an Extension entomologist; Dr. Angus Catchot, an Extension entomologist; Dr. Jeff Gore, a research and Extension entomologist and Dr. Don Cook, a research entomologist. All work for Mississippi State University.

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