A rash of deadly hornet attacks in northwestern China has killed 42 people and injured more than 1,600, local officials said Thursday.

The attacks began in July and have centered on three cities in China’s Shaanxi province: Ankang, Hanzhong and Shangluo. Local authorities believe a particularly venomous species, known as the Asian giant hornet, is behind the attacks.

They say the hornets, which can grow up to 2 inches long, are most active in September and October, when they breed and migrate. But experts quoted by the state-run New China News Agency offered different reasons for the unusual number of attacks this year. 

Huang Rongyao, an insect control expert in the worst-hit city, Ankang, said increased vegetation growth was attracting hornets to the area while warmer than usual temperatures were making the insects more active.

Hua Baozhen, a professor of entomology at Northwest Agriculture and Forestry University, cited a decrease in the hornets' natural enemies, including spiders and birds.

Police and firefighters have been destroying hornet nests and medical experts have been dispatched to the region to help treat critical patients, local news reports said.

One patient, Chen Changlin, told the official China Youth Daily that he was attacked while harvesting rice last month.

“I ran to the side of the road for help, but the hornets chased me about 200 meters [yards] and stung me for nearly three minutes,” he told the paper.

There have also been reports of hornet attacks in China’s southern Guangxi Zhuang Autonomous Region. Twenty-three primary school children, their teacher and six villagers who came to their aid were injured in one such attack last month, the official news agency reported.

The teacher, Li Zhiqiang, told the children to hide under the tables while he fought off the insects until he passed out, according to the report.

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