A throng of crop-eating caterpillars is threatening food supplies across west Africa, and could prove hard to control with pesticides. The crawling menace has appeared in northern Liberia, where hundreds of millions of the black larvae are devouring plants, fouling wells with their faeces and even driving farmers from fields.

They are now crossing into neighbouring Guinea, and the UN Food and Agriculture Organization (FAO) warns that in two to three weeks they will turn into moths that can fly hundreds of kilometres and could spread across west Africa, worsening food shortages in the region.

"The species is so far unknown," FAO entomologist Winfred Hammond told New Scientist from Accra in Ghana, where Liberian specimens were being flown for analysis.

Several species of African caterpillars swarm over crops but unlike the best-known species, army worms, the newly discovered Liberian caterpillars reportedly climb trees to get away from hand-sprayed insecticides on the ground. Aerial spraying might kill them but the FAO fears this will contaminate water supplies.

Although too late to help farmers this year, a bio-pesticide already developed for army worms could be adapted for the new species. The idea would be to infect the caterpillars with a virus specific to that species, and then use mashed-up infected caterpillars against the rest. Such a pesticide could be made locally for a third of the cost of chemical sprays.

But there is a hitch: though UK agencies funded the pesticide's development, commercialisation has stalled for lack of funds. "It has been "extremely difficult to get £1.25 million for producing it locally," says Ken Wilson of Lancaster University in the UK.

Update: The UN Food and Agriculture Organization has confirmed that the caterpillars in the Liberia outbreak are of the same species of armyworm that is a well known pest in east Africa. The viral pesticide developed against the armyworm should therefore work against the new outbreak.