The caterpillars have eaten their way through entire farms in days
A mystery pest which has devoured crops and contaminated water in Liberia and Guinea has finally been identified.
The insects, thought to be armyworms, are in fact the caterpillars of the moth Achaea catocaloides, says the UN's Food and Agriculture Organization.
Cornering the culprit will allow the government to select the best pesticide to tackle the outbreak - the worst seen in Liberia since 1970.
More than 20,000 people have so far had
to evacuate their homes.
As well as devouring coffee, cocoa and plantain crops, the invaders have polluted drinking water sources with their faeces.
Liberian President Ellen Johnson-Sirleaf declared a national state of emergency after caterpillars were reported to have infested more than 100 villages, including several over the border in Guinea.
During a field investigation last week, entomologists from the FAO and the Liberian Agriculture Ministry took samples of the caterpillars, their larvae and their pupae.
The insect scientists established that the insects were not armyworms, as had been reported, but could not identify the species.
The team sent pictures of both moths and caterpillars to experts at the International Institute of Tropical Agriculture in Nigeria.
They confirmed that the species is A.catocaloides, a pest endemic to West Africa.
"The farmers in Liberia know these caterpillars but they have never seen them on this scale. It is extraordinary what has been seen here," said Dr Winfred Hammond, an FAO entomologist, based in Accra, Ghana.
The moths' distinctive wing patterns helped experts identify the caterpillars
Farmers were described as "shocked" by the scale of the swarms, in a report by the investigations team.
"The citizenry were horribly alarmed and frightened as they saw their environment (crops, water and buildings) covered by swarms of caterpillars and therefore had to cry for help," said the report.
"The government responded positively... but the country was ill-prepared for containment since it lacked the capacity for actual diagnosis of the situation and institutional structures and resources for efficient and effective containment."
One piece of good news for farmers is that the caterpillars are likely to be easier to control than armyworms would have been.
They spin their cocoons on the ground under fallen leaves, which leaves them relatively exposed. Armyworms bore 4-5cm into the ground to pupate and are thus much tougher to eradicate.
The next stage is to step up a program of pesticide spraying which until now has failed to reach any more than a dozen of more than 100 affected villages, according to Dr Hammond.
"We really have some homework to do now - because the caterpillars are still spreading. They are in Guinea already," he told BBC News.
"We cannot avoid applying pesticides. Now that we have an accurate identification, we can choose a pesticide which is specific to this species of caterpillar.
"But in the long term, we have to take this as an opportunity to develop early warning strategies for countries in West Africa. Not only for these caterpillars, but for other migratory pests."
Dr Hammond spoke on his way to an emergency meeting in Liberia, where representatives from the governments of Liberia and Guinea, the regional grouping Ecowas, and the FAO will discuss strategies for halting the spread of the caterpillars.
One challenge will be reaching the sites of the caterpillar eggs - which are laid on the leaves of very tall Dahoma trees.
The spraying programme may need to intensify to contain the outbreak
These eggs hatch into caterpillars which feed on the leaves of the trees until they mature and fall to the ground, where they pupate.
Caterpillars which are not yet mature begin migrating in search of food - leading them to crop fields, into water bodies and residential areas.
The cause of this year's unexpectedly large outbreak is likely to be unusual weather patterns, according to Dr Kenneth Wilson, an ecologist from Lancaster University.
"With such widespread outbreaks, something has to be done," said Dr Wilson, an expert on armyworms.
"Because the larvae pupate above ground, destroying these by trampling or fire is an option, but if the outbreaks are really extensive then this might have little impact at national level.
"Vigilance is the key, as control will be much easier if that they stamp down on the next generation of outbreaks early on, when the caterpillars are small and vulnerable.
"However, if they are feeding in tall trees, this can be extremely difficult without aircraft sprayers."