Coated seeds turn birds into mosquito-killing machines

Every year between spring and fall, many local governments and mosquito control districts in the US spray insecticides to kill adult and larval mosquitoes. Now researchers in California and Colorado are testing another approach to manage populations of these pesky and potentially disease-carrying insects.

The research teams are filling bird feeders with ivermectin-coated seeds. Ivermectin is an antiparasitic drug, but it can also paralyze and kill mosquitoes that ingest a blood meal laced with ivermectin. In many African countries where malaria is widespread, scientists have been mass administering ivermectin to humans to control malaria. “So it’s basically an extension of that same idea,” said Brian Foy, a vector-borne infectious diseases expert at Colorado State University.

He presented the work last week at the American Society for Tropical Medicine and Hygiene (ASTMH) conference in New Orleans.

In the continental US, West Nile virus is the most common mosquito-borne disease. Between 1999 and 2023, the US recorded cases of the disease in nearly 59,000 people—approximately 27,600 of whom were hospitalized, and about 3,000 people died.

Birds are the natural hosts of this virus, but female mosquitoes spread the disease when they bite an infected bird and then bite a person. In laboratory trials, Foy and other scientists found that ivermectin-dosed blood meals could kill Culex mosquitoes—the primary carriers of West Nile virus. His idea was to somehow give birds ivermectin to control mosquito numbers and thus the spread of West Nile virus. Foy’s team identified doses that could be safe for birds to ingest repeatedly but potent enough to kill Culex mosquitoes.

But how to give birds ivermectin? Foy and his colleagues spray-coated seeds with a solution containing veterinary-grade ivermectin powder. Next, they sprayed the seeds with an inert polymer that protects the ivermectin from sunlight and environmental degradation. “It [the coating] is clear; the seed looks like a seed,” Foy told C&EN. And the birds “can’t taste the ivermectin on it; they just eat it.”

This summer, the team recruited 40 volunteers in Fort Collins, Colorado, and Davis, California, who filled their bird feeders with the ivermectin-coated seeds. They used caged tube feeders that limited access to small birds such as sparrows or finches and deterred squirrels. To assess if the experiment worked, Foy’s team drew blood from birds after they’d visited the feeders to test for ivermectin presence. They also set up mosquito traps nearby to determine if their population and proportion of females changed. “The hope is that you could lower people’s risk of West Nile virus during the summers,” Foy said.

The researchers plan to repeat the experiment at least twice to determine whether it works. Sarah Hamer, a veterinary ecologist at Texas A&M University, who chaired the ASTMH session and wasn’t involved in the work, called this mosquito-control approach “completely innovative.” Turning birds into mosquito-killing machines could be one more tool that communities adopt during peak mosquito season, she told C&EN.