A Closer Look at Treated Seeds — Going Beyond Seed Catalog Descriptions

What Gardeners Need To Know About Treated Vegetable Seeds

If you’re growing sweet corn or another crop, you know that many seed catalogs offer treated vegetable seeds to protect growing plants against a variety of insect damage or disease. Depending on the pathogens involved, standard treatments might include mechanical processes, such as the use of hot water, or they might involve biological or chemical processes. Some such treatments may be toxic.

A Real Life Case Study

For many years my friend Gene has been purchasing his garden seeds from the same seed catalog. Last year he was tempted to try a new variety of sweet corn, for which top seed companies offer either treated or untreated.

Like many seed catalogs, this one offers scant information about how to grow corn and other vegetables, as well as when to plant vegetable seeds, apparently assuming customers are knowledgeable about such things. But the seed catalog also did not specify the nature of the seed treatment, which customers could not be expected to know.

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Although Gene has been growing sweet corn for 40 years, when this corn was ready to harvest he experienced no raccoon predation for the first time, even though he hadn’t gotten around to putting up his usual electric garden fencing to protect it. Gene became suspicious and began to wonder if the seed treatment had anything to do with deterring the raccoons.

Unlike the seed catalog, the corn seed packet specified that the seeds had been treated with Cruiser, a registered trademark of Syngenta. The active ingredient in Cruiser is thiamethoxam, one of a class of insecticides known as neonicotinoids.


Neonicotinoids (often called neonics) are systemic agricultural insecticides that resemble the natural (and highly toxic) insecticide nicotine. They work by affecting the central nervous system of insects, causing paralysis and rapid death. Other neonics in addition to thiamethoxam are: acetamiprid, clothianidin, dinotefuran, imidacloprid, nithiazine, and thiacloprid. Since these products were first registered, concerns have been raised about their effects on the environment, on honey bees and other beneficial insects, and on humans.

Thiamathoxam is used as a seed treatment for growing sweet corn and field corn, as well as for beans, coffee, cotton, melons, pumpkins, squash, stone fruit, and turf. It is absorbed and transported to all parts of a growing plant. It is harmful to insects, including bees, that eat pollen from the plant, as well as to organisms in the soil, and to aquatic life. Thiamethoxam breaks down into another potent neonic neurotoxin clothianidin, which was implicated in a massive bee die-off occurring in Germany in 2008, as well as contributing to honey bee population decline elsewhere.

The EPA evaluated the risk of thiamethoxam to consumers and has determined that it has toxicological effects in the liver, kidney, testes, and the body’s blood-forming system (EPA-HQ-OPP-2010-0324-0005 ). Additionally, the neurological effects of thiamethoxam are suspected in the impairment of brain development in infants and children.

So, were the raccoons turned off by the systemic toxin with which corn seed was treated? Gene, a biologist, was determined to find out.

Growing Sweet Corn From Treated and Untreated Seeds

This past summer Gene again grew the treated corn, but he also planted a nearby plot of the same variety of corn that was not treated. This time he experienced raccoon damage in both plots.

Interestingly, the raccoons ate 28% of the untreated ears, but only 14% of the treated ears. Being a scientist, Gene wanted to know if this disproportion could have been the result of random snacking on the part of the raccoons, or if it is a statistically significant indication of selective snacking.

Accordingly, he asked a colleague to run the numbers through a statistical (two-way chi square) analysis. The results of the analysis indicate that the raccoons did, indeed, deliberately avoid the treated corn in preference for the untreated corn.

So now Gene is left with this burning question: Should he eat the treated corn?

Would you?


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A Closer Look at Treated Seeds — Going Beyond Seed Catalog Descriptions