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CORVALLIS, Ore. - Encasing insecticides in microscopic plastic capsules--a common formulation for many pest sprays on the market--may make them more toxic than the active ingredient alone, according to a new study from Oregon State University.

Environmental toxicologist Stacey Harper and her team found that a common agricultural insecticide in its "capsule suspension" formulation--with molecules of the active ingredient encapsulated in tiny, inert plastic pellets--was more toxic than the same amount of active ingredient delivered straight up in water.

Their study appeared in this month's edition of the journal Environment International.

Harper, an associate professor in the College of Agricultural Sciences and the College of Engineering, and her doctoral student Alicea Meredith studied a commercial pyrethroid-type insecticide with an encapsulated active ingredient, lambda-cyhalothrin. The product is a broad-spectrum insecticide approved for use in many field and row crops. Its label warns that it is toxic to fish and other water-dwelling organisms.

The capsules encasing the product's active ingredient range from micron-sized (a red blood cell is about 8 microns in diameter; a human hair is 40-75 microns thick), to nanometer-sized, a thousand times smaller.

"We set out to see whether the size of the capsule made any difference in toxicity or environmental fate," Harper said. She hypothesized that the tinier capsules would be more toxic than the bigger ones, because they would be able to penetrate cells more readily.

The researchers spun the off-the-shelf product in a centrifuge and sorted its capsules into two size classes. There was a wide range of sizes; most capsules were in the neighborhood of micron-sized, but some were nanometer-sized.

They exposed the embryos of zebrafish to six successively stronger doses of the pesticide's active ingredient. One group got it in micron-sized capsules, and another group got the same dose in nanometer-sized capsules. As a control, a third group of embryos got the same dose of active ingredient, but it was not encapsulated.

In all cases, the lowest dose administered (20 micrograms of active ingredient per liter of water) was higher than any likely to be used in a commercial spray. "We started with a dose we knew to be toxic because we wanted to compare the toxicity of these two capsule sizes," Harper said.

Zebrafish, a fast-growing species common in home aquariums, are useful for toxicology testing, Harper said, because their bodies are transparent as they grow, enabling researchers to spot developmental anomalies from exposure to toxic chemicals.

Over five days the embryos showed the effects of pesticide poisoning, including physical malformations, tremoring, paralysis and death. But the pesticide in the smaller capsules was no more toxic than the pesticide in the larger ones, Harper said--the higher doses were more toxic across the board, regardless of capsule size.

"What was more surprising," she said, "was that the active ingredient alone was significantly less toxic than either of the encapsulated formulations. We didn't set out to test this, but it's what we found."

Chemical manufacturers have offered encapsulated formulations of pesticides for more than 50 years, Harper said, because encapsulation is thought to improve the product's dispersal and durability. "Our findings indicate that these formulations may be affecting where a chemical spreads through an environment and how it interacts with biological systems," she said.  

While the U.S. Environmental Protection Agency requires pesticide manufacturers to test a product's active ingredient for toxicity, it doesn't require testing of commercial formulations of the product, which are usually trade secrets. This means toxicity screening may underestimate--or perhaps overestimate--the actual environmental hazard of a chemical when it's used in real-life situations, said Harper.

"The testing assumes that the encapsulation makes no difference in the toxicity," she said, "but in this case, at least, it does. So it's important to figure out how the carrier of a chemical product affects its toxicity in order to determine whether our current risk assessments offer enough protection against products that incorporate this encapsulation technology."

Harper, also an environmental engineer, studies the environmental effects of human-made nanoparticles--microscopic bits of matter engineered to have commercially useful properties. Nanoparticles are widely used in pharmaceuticals, pesticides and personal care products, but little is known about their long-term environmental or health effects.

The study was funded by the U.S. Department of Agriculture National Institute of Food and Agriculture and by OSU's Agricultural Research Foundation.