Swarms of desert locusts have been recorded in the region since biblical times, but unusual weather conditions exacerbated by climate change have created ideal conditions for insect numbers to surge, scientists say.
Warmer seas are creating more rain, wakening dormant eggs, and cyclones that disperse the swarms are getting stronger and more frequent.
Aided by those weather patterns, and sometimes inadequate measures to stop it in countries that are at war or lack resources, the outbreak has devastated crops in a region already struggling with food security.
Breeding areas along the Red Sea and Gulf of Aden coasts
Breeding continues along the coast
Swarms from Yemen invade Ethiopia and Somalia at the end of June 2019
It started in Yemen. Ignored in the chaos of the civil war, locusts hatched in winter breeding areas. They migrated across the Red Sea to the Horn of Africa and spread across East Africa.
In addition to the favourable weather conditions, wet and ample vegetation have helped locusts to boom.
Adult desert locust swarms can fly up to 150 km (93 miles) a day with the wind and adult insects can consume roughly their own weight in fresh food per day. A single square kilometre swarm can eat as much food in a day as 35,000 people.
They feed on nearly all green vegetation — leaves, flowers, bark, stems, fruit, and seeds — and crops including millet, rice, maize, sorghum, sugarcane, barley, cotton, fruit trees, date palm, vegetables, rangeland grasses, acacia, pines and banana.
Desert locusts change their behaviour from acting as individuals to becoming part of a group, forming dense and mobile hordes. Swarms can be several hundred square kilometres and extremely dense, with up to 80 million adults in each square kilometre.
Meanwhile, the locusts, which have a life cycle of three months, are already breeding the next generation, which the U.N. Food and Agriculture Organization (FAO) says could be on average 20 times more numerous.
When eggs hatch the hungry young locusts are earthbound for two weeks and more vulnerable to spraying than when they grow wings.
After that, they take to the air in swarms so dense that they have forced aircraft to divert.
An unusually rainy season, lush vegetation and warm weather have an encouraged locust reproduction in East Africa.
As a desert locust populations grow and become more dense, they change behaviour to act as a group. Immature gregarious locusts change colour to pink.
If allowed to breed unchecked in favourable conditions, locusts can form huge swarms that can strip trees and crops over vast areas, posing a particular danger in conflict-ridden areas with populations already facing the threat of food shortages.
Countries on the frontline of the battle against the infestation are short of vital supplies. Kenya ran out of pesticides. Ethiopia needs more planes. Somalia and Yemen, wracked by civil war, can’t offer security to exterminators.
As the threat increases, Uganda has deployed the military and Kenya is training hundreds of youth cadets to tackle the swarms. In Somalia, a regional environment minister said security forces have resorted to shooting anti-aircraft guns at the insects. Everyone is racing against time before the rains begin in a month, and the next generation of larvae hatches.
The last major infestation was in 2003-2005 when more than 12 million hectares were treated in west and northwest Africa, costing hundreds of millions of dollars,w including food aid.
If not contained, the number of locusts in East Africa could explode by up to 400 times by June, FAO has said, potentially devastating agricultural production in a region where more than 19 million people already go hungry.