In what is being described by the UN food agency as Liberia's worst plague in 30 years, caterpillars are destroying crops in the country and posing a major threat to the precarious food security situation there.

The caterpillars, two to three centimetres in length and described by villagers as "black, creeping and hairy," are advancing in the tens of millions, devouring all plants and food crops in their path and in some cases overrunning homes and buildings.

The situation in Liberia is a national emergency and is likely to escalate into a regional crisis involving neighbouring Guinea, Sierra Leone and Coted d'voire, Representative of the UN Food and Agriculture Organisation (FAO) in Liberia, Winfred Hammond, has reported.

In some of the affected villages, people are unable to reach their farms because they are completely surrounded by the pests, suspected to be African armyworms (Spodoptera spp). In addition, many wells and waterways in the affected areas are unfit for human consumption because of the huge volume of faeces dropped by the caterpillars.

Aerial spraying of pesticide is not an option as it is likely to further contaminate the already precarious water supply in the area. Whatever solution is found, the agency recommends the use of less risky pesticides, including bio-pesticides when they are effective.