The locust crisis that has now reached 10 countries could carry on to endanger millions more people, forecasters have said.

Climate change created unprecedented conditions for the locusts to breed in the usually barren desert of the Arabian gulf, according to experts, and the insects were then able to spread through Yemen, where civil war has devastated the ability to control locust populations.

It was Cyclone Mekunu, which struck in 2018, that allowed several generations of desert locusts the moist sand and vegetation to thrive in the desert between Saudi Arabia, Yemen and Oman known as the Empty Quarter, breeding and forming into crop-devouring swarms, said Keith Cressman, locust forecasting expert for the UN’s Food and Agriculture Organization (FAO).

“That’s fine, that’s quite good in itself, but just about when those conditions are drying out and the breeding is coming to an end, a second cyclone came to the area,” he said.

“That allowed the conditions to continue to be favourable and another generation of breeding, so instead of increasing 400-fold, they increased 8,000-fold.

“Usually a cyclone brings favourable conditions for about six months and then the habitat dries out, and so it’s not favourable for reproduction and they die and migrate.”

The amount of cyclones in the area seem to be increasing, said Cressman, making it likely that locust swarms will also become more common.

The FAO has warned that the food security of 25 million people could be endangered by the locusts, which according to the agency’s locust monitoring service have been spotted in at least 10 countries over recent months. One swarm recently reported in Kenya covered an area the size of Luxembourg.

The organisation has requested $140m (£120m) to help fight the ongoing breeding of the insects, predicting that a continuation through late March and April could see the existing number of locusts grow by 400 times by June.

The current crisis is considered the worst in decades, and there are fears it could last longer than previous locust outbreaks.

Alongside the climate emergency impact, the war in Yemen is a key factor.

Cressman said Yemen is a “frontline” country for locusts, with the insects typically present throughout the year. But its once effective locust programme no longer has the same impact in cities where control is now divided between the government and Houthi rebels.

The head of the locust programme, Adel al-Shaibani, is based in the Houthi-controlled capital, Sana’a.

“Before the war we had a good ability to reach anywhere in Yemen,” he said. “In current times we’re just able to cover the Red Sea coastal areas – but not all – and some areas in the interior.”

He explained that there were two separate locust control centres in Yemen but neither was able to combat the outbreak effectively alone.

The Sana’a-based centre carried out control operations wherever they could in 2018, but they have been underfunded and have lost some of their vehicles.

“In spite of all our hard efforts, some areas remained out of control due to security reasons near the border with Saudi Arabia. The desert locust outbreak occurred and some swarms formed and moved to other areas,” said Shaibani.

A young desert locust without wings is seen on a stalk near Geerisa town, in Somaliland’s Lughaya district. Photograph: Daniel Irungu/EPA

By late 2019, the locusts had moved into the Horn of Africa, finding favourable conditions when an unseasonaal cyclone hit Somalia in December. This extended breeding time and allowed them to spread to areas authorities could not control because of the country’s security problems.

“This crisis could be quite long because of the Yemeni and Somali areas that cannot control the populations,” said Cyril Piou, an expert with the French Agricultural Research Centre for International Development.

He said that in previous decades locust outbreaks had only lasted roughly two years but, without preventive systems, they will last longer, happen more frequently and spread further.

“We are all linked in some way, what is happening somewhere else affects us all,” he said.

The last comparable locust outbreak was in the late 1940s and 50s, but Cressman said that was in a time when monitoring and reporting was a slow, cumbersome process and chemical pesticides were readily available for control operations.

Historically, the Arabian Gulf has very few cyclones. But the past decade has brought a significant increase thanks to the Indian Ocean dipole, a phenomenon linked to flooding in the western Indian Ocean, dry weather in the east and wildfires in Australia.

Cressman, part of whose job involves looking at historical conditions to understand current developments, said the climate’s behavioural changes made that difficult.

“This analogous forecasting methodology used to work pretty good up until five years ago, and it’s just not working very well any more at all because of the rainfall, the timing, the distribution. It’s very different,” he said.