Swarms of giant hornets have killed 42 people and injured 1,640 in China’s northwestern province of Shaanxi in recent weeks, the official Xinhua news agency said on Thursday.

Authorities are destroying nests, putting up warning signs and raising public awareness via television, radio and the Internet, it said. Residents have been told to wear long sleeves when they go out, and not to try to drive away the swarms.

“The National Health and Family Planning Commission has deployed emergency medical treatment and stepped up prevention and propaganda work to help minimize casualties,” Xinhua said.

Hornets mate and migrate in September and October when they are at their most aggressive, according to local media. Their stings can cause allergic reactions and multiple organ failure.