Seed treatments and early season insect pressure

By Dusty Sonnenberg, CCA, Field Leader, a project of the Ohio Soybean Council and soybean check-off

Many farmers view seed treatments as a form of insurance. This is especially true in early planted crops. Seed treatments are chemicals applied to the outside of a seed coat. They are designed to be taken up by the plant as it germinates. The chemical is distributed in the plant tissue to provide protection against certain insects and diseases.  “Seed treatments are very water soluble. They need to be water soluble to be taken up through the plant and the vascular system of the plant,” said Dr. Kelley Tilmon, Professor and Entomologist specializing in Field Crop Insects with The Ohio State University. “That water solubility can prove to be a problem when the seed sits in the soil too long. There is an opportunity for the seed treatment to wash off the seed if there is enough exposure to rain percolating through the soil. Typically, if the conditions are excellent, you can expect the seed treatment to hold up on the seed before germination for 10-14 days, but experience shows that the seed treatment actually being available and ready for uptake by the plant is usually less than that. Under normal conditions, it may last a week, but it could be even less if there is a fair amount of rain.”

The availability of the seed treatment to be taken up by the plant is critical in adverse growing conditions. “What this means for pest problems, particular insects, is that certain soil born insect pests that you might normally have a level of protection against because of your seed treatment, could actually prove to be a problem that you might not expect in prolonged wet or adverse conditions,” said Tilmon. “Damage from pests such as seed corn magots and wire worms could show up as poor emergence or skips in the row or stunted plants, since the insects feed on the seed itself an on the developing root structure of the new seedling. Unfortunately, there are no rescue treatments, so the only option is to replant those areas or even the whole field if the situation is severe enough.”  

When evaluating fields that have poor stands or emergence issues, physically digging to look for the seed or analyzing a damaged seedling will tell the story. “When you go to the field, take a trowel or something to dig up the seed trench and look for the seed or the root system of plants that look poor. If that seed has been fed on it will be pretty obvious. You may even see the insect inside the seed, still feeding, or see the maggots feeding on the root system,” said Tilmon. “Another early season pest that can cause issues this time of year for seedling plants is slugs. It is important to note that slugs are not insects and will not be affected by strong insecticidal seed treatments.”

In replant situations, seed treatments are not as critical. “When replanting, the soil is typically warmer and dryer, and the seed will germinate quickly and has strong protection on the seed in the window when it is waiting to germinate,” said Tilmon. “Those pests are likely far enough along in their life cycle that they may not be needing to feed and have matured to the adult form that will not feed on the seed.”

According to Tilmon, there are no good ways to determine how long a seed treatment is still viable once the seed has been planted, especially visually by looking at the color of the treated seed. Typically, when seed is treated, the level of treatment used is higher than needed since it is known that some of the treatment will be lost over time due to the environment.  It is possible to lose some of the “color” of the seed treatment and yet still have a level of protection that can benefit the seed.

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