First, catch your larvae...

Mopane 'worms' are high in protein and delicious roasted (no, really). They could also provide a lifeline for African farmers, says Simon Hadlington

In the beer halls of Harare, a new nibble is becoming trendy. Forget pork scratchings or crisps, the flavour of the month is roasted, salted caterpillar. It is called the mopane (pronounced mo-pahnee) worm, and its growing popularity could be good news for the impoverished rural communities in parts of southern Africa that harvest and process the worms to sell and to store for themselves. For these villagers, the mopane "worm" - not strictly a worm but an insect larva - is becoming a mainstay of the local economy.

In the beer halls of Harare, a new nibble is becoming trendy. Forget pork scratchings or crisps, the flavour of the month is roasted, salted caterpillar. It is called the mopane (pronounced mo-pahnee) worm, and its growing popularity could be good news for the impoverished rural communities in parts of southern Africa that harvest and process the worms to sell and to store for themselves. For these villagers, the mopane "worm" - not strictly a worm but an insect larva - is becoming a mainstay of the local economy.

A poor worm harvest could leave families without cash to send their children to school. If the crops also fail, entire villages could find themselves having to sell their household possessions to pay for food. To help these communities to make the most of this crucial natural resource, an international team of ecologists, economists and social scientists has carried out the first large-scale, systematic study of the biology and economy of the mopane worm.

The research project's leader is Dr Jaboury Ghazoul, an ecologist based at Imperial College London's Silwood Park campus in Berkshire. "This project was started because there was a recognition that mopane worms are an extremely important resource for both subsistence and income generation for the poorest members of southern African society," he says.

"Rural communities are isolated and have little access to sources of income. Their crops and livestock are also often of poor quality. The mopane worm has become more commercialised in the past 15 years, making it a vital source of cash. We have tried to identify ways to improve income generation through better commercialisation, production and adding value to the product, including ideas such as covering mopane worms with chocolate, to help these communities to get out of dire poverty."

The worm is the larva of the emperor moth, a member of the silkworm family. The insect occurs almost exclusively in mopane woodlands, which are themselves unique because they consist almost exclusively of the mopane tree. Mopane woodlands typically occupy soil of low fertility in areas that are prone to drought, covering large swathes of southern Africa, including Namibia, Botswana, Zimbabwe, Mozambique and northern parts of South Africa.

Outbreaks of mopane worms usually occur twice a year, with larger numbers hatching around November and fewer around March. The adult moth lays eggs on leaves of the mopane tree. Once the eggs hatch they rapidly grow into spectacularly coloured, slightly ferocious-looking caterpillars, covered in spots of yellow, red, black and white, and with thick black spines along the back. When the worm is 10 to 12cm long, just before it starts to pupate, it is collected. "The timing of the main mopane worm outbreak is extremely important because it occurs just before what is often termed the hunger season," says Dr Ghazoul.

"There are not many crops available for harvesting and there is a big need for household expenses such as school fees. In this part of the world, many poor people use income from the mopane worms to buy basic foodstuffs and pay for their children's schooling. If there was no mopane worm for an extended period, some households might be forced to take their children out of school. A poor worm harvest coupled with a poor crop harvest could put some communities in real difficulty."

The villagers are alerted to an outbreak when they see the white eggs on the leaves of the mopane trees. When the caterpillars are ready for harvesting, members of entire villages drop whatever they are doing, children are taken out of school and everyone heads into the woods to collect the larvae. The worms are plucked off the leaves and placed in large baskets. Harvesting can last for two or three weeks.

Back home the messy business of processing the worms takes place. First the worm has its guts removed by squeezing the body between forefinger and thumb, expelling the viscera. The gutted body is then washed and either boiled or roasted over charcoal before being left to dry in the sun. Once dried, the worms are packaged.

A single household might typically collect between 10 and 30 tins of worms, each containing about 5kg of processed product and with each tin worth between $8 and $10. "In the remote villages the householders then wait for the traders to arrive, or they might take the worms to the local market," says Dr Ghazoul.

"There is a highly complex economy surrounding this resource. Collectors mostly rely on middlemen who come to the villages and take the harvest to district markets, where it is often sold on to other traders and wholesalers who have access to the much bigger markets in the large commercial centres such as Harare. The value of mopane worms increases with each transaction and processing step along these chains, benefiting poor collectors and traders in rural and district markets alike. You also get poor small-scale vendors in the large urban centres who might buy a few kilograms at a time, fry them in oil and season them and sell them in beer halls or bars."

Many of the worms find their way into urban supermarkets, where they are bought as a cheap source of "meat". They are not only tasty but also highly nutritious with 45 per cent digestible protein content.

One of the aims of the project, which has been funded by the British Government's Department for International Development, has been to overcome the intrinsic unpredictability of the outbreaks. "No one knows when or where an outbreak is likely to occur," says Dr Ghazoul. "As part of this project we have been looking at the feasibility of domesticating the mopane worm."

In mopane woodland in Botswana, Dr Alan Gardiner designed and built the world's first mopane worm farm. The idea was to see if it is possible to raise the larvae in more controlled conditions to increase the density of production and improve the quality of the product and provide a more predictable supply.

The team collected eggs from the wild, hatched them in containers and reared the larvae until they were around a centimetre long, then placed them on trees whose branches had been covered with sleeves of muslin to protect the larvae from predators and parasites. "When the caterpillars had grown to around four centimetres we put them in large 'shade-houses'," says Dr Ghazoul. "These are areas of mopane woodland, 20 or 30 metres square, enclosed by cloth to keep the environment cool and moist."

Once the larvae were large enough to harvest they were removed, with a proportion retained to breed from. There was a serious problem with disease, however, with whole populations of larvae wiped out. "One possible solution might be to have many small farms dotted throughout communities, rather than a single large farm," Dr Ghazoul says. "This would spread the risk to disease and should encourage farmers' own innovations."

The research team also carried out new work on the life cycle of the mopane moth. Dr Gardiner discovered that the belief that the adult moth takes six months to emerge from the pupa was not necessarily correct. "We found that in some cases it took up to two-and-a-half years for the adult to emerge," says Dr Ghazoul. "We would like to be able to understand why this is the case: are there environmental cues that stimulate emergence? In general terms we do not know what are the important factors that control the size of the population of the mopane moth and the timing of outbreaks."

One factor seems to be the prevalence of parasites - small wasps that lays their eggs on the mopane worm. When the eggs hatch, the larvae live within the mopane worms and eventually kill them. "We would very much like to get enough biological and environmental data on life cycle factors," says Dr Ghazoul. "The idea would be to develop mathematical models that include variables such as the quality of forage material, the density of parasite populations and weather patterns, which might allow us to predict when and where outbreaks might occur allowing farmers to plan ahead and prepare for big mopane worm events."