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It’s been said that “the dose makes the poison.” However, when it comes to pesticides, it’s not just the dose that matters, but also the way that it’s packaged.
For about 50 years, pesticide manufacturers have prepared formulations in which the active ingredients are encapsulated in microscopic pellets of inert plastic suspended in a liquid carrier. This encapsulation, intended to make pesticides easier and safer for users to handle and apply, could lead to some unintended negative consequences for the environment, according to a study from Oregon State University.
Environmental toxicologist Stacey Harper and her team found that a common insecticide in its “capsule suspension” formulation can deliver fatal doses of poison to certain types of freshwater plankton. Specifically, the team found that the size of the capsules is significant, with nano-sized capsules making a “poison pill” that’s just the right size for the tiny creatures to ingest.
“Freshwater plankton are low on the food chain, and biomagnification of pesticides is a real concern,” Harper said. “While we don’t know what the broader impacts of this unintended exposure might be, our findings indicate that further study is needed.”
Harper, an associate professor in the College of Agricultural Sciences and the College of Engineering, and doctoral student Matthew Slattery studied a commercial pyrethroid-type insecticide with an encapsulated active ingredient, gamma-cyhalothrin. The insecticide is primarily used in the home and garden for ants, bed bugs, ticks and other insects.
That particular active ingredient was specifically designed to be hydrophobic so that it wouldn’t mix with water, Slattery said. But encapsulation defeats that feature. The capsules effectively become tiny submarines that carry the pesticide into streams. There, they can be encountered by species other than the ones they were intended to kill.
The capsules vary in size and can be measured on a scale of microns (a human hair is 40-75 microns thick) or nanometers (one-thousandth of a micron). The researchers spun the off-the-shelf product in a centrifuge and sorted its capsules into two size classes. Most were in the micron-scale range, but some were nano-sized.
Researchers exposed a species of water flea, Ceriodaphnia dubia, to standardized doses of the pesticide. (Despite its common name, the water flea is actually a tiny crustacean, less than a millimeter in length.) The species lives in freshwater lakes, ponds and marshes and, due to its sensitivity to pollutants, is used in toxicity testing of waterways.
One group received micron-sized capsules, and another group got the same dose in nano-sized capsules. As a control, a third group got the same dose of active ingredient, not encapsulated. The team found the impact on the water was fleas increased with the smaller, nano-sized capsules. The crustaceans were immobilized, leading to their death.
“These water fleas are filter feeders; they swim through the water and grab particles out of the water, normally bacteria and other food floating around,” Slattery said. “In our study, it was the size of the particles that mattered. The nanometer-sized particles were in the ‘Goldilocks zone’ – large enough for the water flea to collect it but not so large so that it couldn’t ingest it.”
Harper said pesticide manufacturers and regulators need to take the downstream effects of encapsulation into consideration.
“We need to think about considering encapsulation as an ingredient, because it alters how the active ingredient interacts with the environment,” Harper said. “Currently, the only testing that’s done after the final formulation are hazards like corrosivity and flammability. But not the potential for unintended toxic exposure. What we’ve found is that encapsulation makes a difference in exposure and that it is size-dependent.”
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, published in the journal Nanomaterials, was co-authored by Bryan Harper, a senior faculty research assistant in the Department of Environmental and Molecular Toxicology. The research was supported with funding from the U.S. Environmental Protection Agency, the U.S. Department of Agriculture’s National Institute of Food and Agriculture and the Agricultural Research Foundation of Oregon.