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Just when you thought all the pests were done with your crop...

Posted Monday, September 13, 2010, at 3:34 PM

Just when you think all of your potential pests have had a chance at your crop, another shows its flexibility.

The soybean podworm (also known as the corn earworm) has been found in soybean fields in western and northwestern Missouri.

In northwestern Missouri, several fields have been completely destroyed. Podworms feed almost exclusively on pods. They eat away the pod wall and completely consume the seed. Pod feeding directly reduces yield and will also affect seed quality.

This pest is often found in late planted or double crop soybeans. Leaves, stems, and flowers can be damaged by the soybean podworm, but pods and seeds are especially vulnerable to attack by the third generation larvae. Infestations during peak flowering to early pod fill stages, can delay seed production and lower yields. In University research trials, an average of one large larva per row foot was found to reduce yields by approximately 1.9 bushels per acre.

It is important to check your fields to determine if you have significant numbers of podworms present. Control must be made before the significant damage to pods occurs

Soybean podworms may feed from two to four weeks and molt four to five times. Once the larva is full-grown, it crawls down the host plant and pupates n the soil. The next generation of moths emerges within 10 to 25 days.

If there is more than 1 podworm per 1 foot of row, or if more than 5% of pods are damaged, treatment is recommended.

When scouting late planted soybean fields, examine the size of the podworms. If the majority of the worms are 1.25-1.5 inches, they will pupate soon and their damage is already done. Damage varies from field to field, so it is very important to scout.

If podworms are small and pod damage is near threshold levels, then an insecticide application is recommended. As with any input cost, yield potential of your fields should also be considered. Podworm will feed on soybean up to the R6 (full seed) growth stage; they normally do not eat R6 soybean, so R1 (flowering) through R5 (beginning seed) are the most critical stages. Generally, soybean podworm will feed for at least two weeks, but if there is quite a range in the size of worms, the feeding time may be longer.

Numerous pyrethroid insecticides are labeled for soybean podworm in soybeans. It is important to use a high gallonage of water to cover the canopy and penetrate into it. For ground applications use 15 gpa and crop oil with the pyrethroid insecticide and 5gpa for aerial applications. Ttreat only if necessary as insecticide applications will reduce beneficial populations also.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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