Adding new buzz to controversial insecticide

A team of US chemists has found a way to make the insecticide imidacloprid more effective, claiming that its faster-acting forms of the insecticide could be used in smaller amounts, reducing its exposure to the environment.

Imidacloprid is widely used around the world to control insect populations on crops, as well as reduce disease-carrying mosquitoes and disinfect indoor spaces.

Imidacloprid crystals
Imidacloprid crystals. Credit: Xiaolong Zhu

It’s a controversial substance: evidence of imidacloprid’s toxicity to bees led to it being banned by the European Union in 2018, along with some other similar pesticides. Imidacloprid is still approved for use in Australia.

A paper in the Journal of the American Chemical Society describes new crystal forms of imidacloprid, which are faster-acting on the bugs they’re targeting.

“By using modified forms of imidacloprid, we may have a sustainable strategy for improving the insecticide’s ability to control mosquito disease vectors while lessening the quantity needed,” says Bart Kahr, a professor of chemistry at New York University and co-author on the paper.

“This provides a pathway to minimise exposure and harm to other organisms, as well as delay the onset of the development of resistance by mosquitoes, an urgency where malaria is endemic.”

Read more: Neonicotinoids make bees antisocial and lazy

Imidacloprid crystals
Some of the imidacloprid polymorphs. Credit: Xiaolong Zhu

The research revolves around the “polymorphs” of a crystal – the different shapes a crystal can take based on its molecular structure. Ice, for instance, has at least 19 different polymorphs, although only one (ice Ih) commonly forms naturally on the Earth’s surface.

Different crystal polymorphs can be found by melting a substance down and then allowing it to cool and solidify. By modifying the surrounding temperature and pressure, new polymorphs can form.

The researchers had previously used this technique on the insecticide deltamethrin, finding several different polymorphs that worked more effectively than the commercial type.

Turning their attentions to imidacloprid, the researchers created three different crystal forms and tested them on three species of mosquito, as well as fruit flies. They found that all three polymorphs of imidacloprid were faster at killing the insects than the old-school form.

Imidacloprid crystals
Some of the imidacloprid polymorphs. Credit: Xiaolong Zhu

The fastest-acting polymorph – which was nine times faster at dispatching mosquitoes – is easy to make via heating and cooling, and is stable at room temperature.

“Given the widespread use of imidacloprid, we recognised the value in developing more active forms, which could be used in smaller amounts and hopefully spare pollinators,” says co-author Michael Ward, professor of chemistry at New York University.

Kahr says there’s more potential for crystal-growth chemistry to make better insecticides.

“Imidacloprid is a highly concerning composition of matter, yet at the same time it is extremely popular,” he says. “If regulators in North America will not prohibit it, simple interventions for minimising environmental exposure may have value.”

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