Uganda scrambled to respond to the arrival of the biggest locust outbreak that parts of East Africa have seen in decades, while the United Nations warned Monday that “we simply cannot afford another major shock” to an already vulnerable region.

An emergency government meeting hours after the locusts were spotted inside Uganda on Sunday decided to deploy military forces to help with ground-based pesticide spraying, while two planes for aerial spraying will arrive as soon as possible, a statement said. Aerial spraying is considered the only effective control.

The swarms of billions of locusts have been destroying crops in Kenya, which hasn’t seen such an outbreak in 70 years, as well as Somalia and Ethiopia, which haven’t seen this in a quarter-century. The insects have exploited favorable wet conditions after unusually heavy rains, and experts say climate change is expected to bring more of the same.

U.N. officials warn that immediate action is needed before more rainfall in the weeks ahead brings fresh vegetation to feed new generations of locusts. If left unchecked, their numbers could grow up to 500 times before drier weather arrives, they say.

“There is the risk of a catastrophe,” U.N. humanitarian chief Mark Lowcock told a briefing in New York on Monday, warning that a region where 12 million people already face severe food insecurity can’t afford another jolt.

Without enough aerial spraying to stop the swarms, the locust outbreak could turn into a plague, “and when you have a plague, it takes years to control,” Dominique Burgeon, emergency and resilience director with the U.N. Food and Agriculture Organization, told The Associated Press last week.

The outbreak also is moving toward South Sudan, where another several million people face hunger as the country struggles to emerge from civil war.

The U.N. has asked for $76 million in immediate aid. So far just under $20 million is in hand, officials said. The United States said Monday it has released $800,000 and the European Union has released 1 million euros.

“The response today is not gonna work, unless there’s a big scale-up,” Lowcock said.

The locusts are eating the vegetation that supports vibrant herder communities in the region, and Kenyan Ambassador Lazarus Amayo warned of the “inherent risk of communal conflict over pastures.”

The outbreak is so severe it might even disrupt the planting of crops in the coming weeks, he said, adding that the locusts “do wanton damage.”