The time-tested method of repelling locusts from crops in East Africa is to bang on a metal bucket and whistle loudly. When swarms afflict five countries and swell to the size of Moscow, more drastic measures are needed.

In Kenya, police facing the country’s largest outbreak in 70 years have fired machine guns and tear gas into swarms in an effort to prevent them from consuming fields. Ethiopia is spraying pesticide from small planes to displace hovering throngs, though swarms have forced passenger jets in the region to make emergency landings.

In Eritrea and Djibouti, teams in the hundreds are chasing swarms with hand-held pesticide pumps and truck-mounted sprayers.

The rising number of desert locusts presents an unprecedented threat to food security and livelihoods in the Horn of Africa, the United Nations Food and Agriculture Organization said on Wednesday.

“This has become a situation of international dimensions that threatens the food security of the entire subregion,” Qu Dongyu, director general of the FAO said last week.