Why Are Neonicotinoids Safer Insecticides?

by Mika Pringle Tolson

John Casida leads a research lab at UC Berkeley to optimize pesticide use and minimize the hazards of exposure for humans and domestic animals.

John Casida, professor of Toxicology and Entomology at UC Berkeley has been studying pesticide mode of action and metabolism in his laboratory for decades. He spoke at the TSR&TP Symposium about mechanistic toxicology safety with one specific example - why neonicotinoids are better and safer insecticides than nicotine.

For more than 200 years, tobacco waste from producing cigarettes was used for insecticides. But, said Casida "Nicotine is more toxic to mammals than insects." It posed a danger to people and was never very effective at killing insects.

Neonicotinoids are the only major new class of insecticides of the last three decades. They were developed in the 1990s and now represent about 20 percent of the market share. "The pest insect is highly sensitive to these and people are not as sensitive," said Casida. His lab group worked on the snail model to study the pathways of metabolism in insects and mammals to determine the mechanisms of toxicity. The acetylcholinesterase binding protein in snails is a soluble surrogate for nicotine receptors. The researchers found that the neonicotinoid is a better fit at the insect than the vertebrate binding site. "Nicotine is ionized and it has to cross through iron barriers. Neonicotinoids can move directly in; there's no barrier," explained Casida.

A good example of how this has been applied are the neonicotinoid insecticides now available for cats and dogs with few side effects such as imidacloprid, nitenpryam, and dinotefuran. "The major compounds are so selective that controlling fleas on dogs or cats is moderately safe," said Casida. "Read the instructions."

Casida acknowledged his graduate students Tami Clark, Kevin Ford, Steve Lantz and Daniel Nomura, his collaborator at NIEHS Motohiro Tomizawa, the TSR&TP and the UC Berkeley campus in his efforts to optimize pesticide use, improve their selectivity and environmental characteristics, and minimize the hazards of exposure for humans, domestic animals and other nontarget species.

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