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It was an avoidable massacre. Beekeepers in Dorchester County, South Carolina, saw 48 of their hives killed off on 28 August. The culprit was a pesticide, sprayed from a plane with the aim of killing mosquitoes that can carry the Zika virus.

But South Carolina’s mosquito population isn’t yet known to carry Zika – and even if the virus is present, there are ways to kill the mosquitoes without killing bees.

In response to four local cases of Zika, Dorchester County sprayed a pesticide called Naled, a neurotoxin which kills adult mosquitoes and other insects. The four people infected all caught the virus before arriving in South Carolina: no one in the state has yet acquired Zika locally.


The aerial spraying killed millions of bees. Commercial beekeeper Juanita Stanley lost 46 hives, and a hobbyist beekeeper lost two. “Of course this is a tragedy,” says Michael Weyman at Clemson University, South Carolina, which is investigating claims by the beekeepers that the pesticide was misused.

Any actions that cause mass bee deaths are particularly concerning, given the fragility of bee populations in general – a consequence of the mysterious colony collapse disorder.

Bees can be spared by spraying at night instead of in the early morning. Bees don’t fly at night, and Naled only kills insects while they are airborne, says Mark Latham, director of Manatee County Mosquito Control in Palmetto, Florida, an area with commercially important beekeeping. Latham has carried out aerial spraying for 35 years, including of Naled, and almost always sprays at night.

The approach should work even though Aedes aegypti – the mosquito that can carry Zika – is a species active by day.

Spray at sunset

“We can target them effectively with aerial spraying starting just before sunset and for the following 30-60 minutes,” says Latham. “Bees are usually ‘back home’ in their hives well before sunset.”

When sprayed, Naled hangs in the air in the form of tiny droplets, and this is why insects in flight are vulnerable. Once the pesticide settles on the ground or plants, it quickly breaks down and becomes inactive. So spraying at a time when mosquitoes are flying but bees are not is a good way to make the spray safer for bees.

Since the Dorchester County spraying killed honeybees, it probably also killed wild bees and other pollinators, says Aimee Code, the pesticide programme director of the Xerces Society for Invertebrate Conservation. Honeybee deaths are just easier to notice, because wild bees are mostly solitary.

Other pesticides that kill adult mosquitoes – known as adulticides – are no safer for bees, says Latham. Even pyrethrin, which is produced from chrysanthemums, is toxic to bees. Latham says he uses a lot of larvicides, which kill mosquito larvae and are safer for bees.

“Adulticiding is in a sense a method of last resort, if you haven’t killed mosquitoes when they’re larvae,” he says. Another effective and safe control method for Aedes aegypti is to eliminate the standing water in gutters, birdbaths and backyard pools where the mosquitoes typically breed.

Code does not rule out using adulticides like Naled, but agrees that they should be “really a last resort, when the disease is established”.

For example, they could be used in Florida, where mosquitoes are clearly spreading Zika. What happened in South Carolina “feels like it was more fear-based”, says Code. “We need to plan ahead, not be reactive.”

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