Box 1.3. Sustainable use of mopane worms
An important economic insect in Africa is the larva (caterpillar) of emperor moths, especially Imbrasia belina (see Plates 1.4 & 1.5, facing here), which is harvested for food across much of southern Africa, including Angola, Namibia, Zimbabwe, Botswana, and Northern Province of South Africa. The distribution coincides with that of mopane (Colophospermum mopane), a leguminous tree which is the preferred host plant of the caterpillar and dominates the “mopane woodland” landscape.
Early-instar larvae are gregarious and forage in aggregations of up to 200 individuals: individual trees may be defoliated by large numbers of caterpillars, but regain their foliage if seasonal rains are timely. Throughout their range, and especially during the first larval flush in December, mopane worms are a valued protein source to frequently protein-deprived rural populations. A second cohort may appear some 3–4 months later if conditions for mopane trees are suitable. It is the final-instar larva that is harvested, usually by shaking the tree or by direct collecting from foliage. Preparation is by degutting and drying, and the product may be canned and stored, or transported for sale to a developing gastronomic market in South African towns. Harvesting mopane produces a cash input into rural economies — a calculation in the mid-1990s suggested that a month of harvesting mopane generated the equivalent to the remainder of the year’s income to a South African laborer. Not surprisingly, large-scale organized harvesting has entered the scene accompanied by claims of reduction in harvest through unsustainable over-collection. Closure of at least one canning plant was blamed on shortfall of mopane worms.
Decline in the abundance of caterpillars is said to result from both increasing exploitation and reduction in mopane woodlands. In parts of Botswana, heavy commercial harvesting is claimed to have reduced moth numbers. Threats to mopane worm abundance include deforestation of mopane woodland and felling or branch-lopping to enable caterpillars in the canopy to be brought within reach. Inaccessible parts of the tallest trees, where mopane worm density may be highest, undoubtedly act as refuges from harvest and provide the breeding stock for the next season, but mopane trees are felled for their mopane crop. However, since mopane trees dominate huge areas, for example over 80% of the trees in Etosha National Park are mopane, the trees themselves are not endangered.
The problem with blaming the more intensive harvesting for reduction in yield for local people is that the species is patchy in distribution and highly eruptive. The years of reduced mopane harvest seem to be associated with climate-induced drought (the El Niño effect) throughout much of the mopane woodlands. Even in Northern Province of South Africa, long considered to be over-harvested, the resumption of seasonal, drought-breaking rains can induce large mopane worm outbreaks. This is not to deny the importance of research into potential over-harvesting of mopane, but evidently further study and careful data interpretation are needed.
Research already undertaken has provided some fascinating insights. Mopane woodlands are prime elephant habitat, and by all understanding these megaherbivores that uproot and feed on complete mopane trees are keystone species in this system. However, calculations of the impact of mopane worms as herbivores showed that in their six week larval cycle the caterpillars could consume 10 times more mopane leaf material per unit area than could elephants over 12 months. Furthermore, in the same period 3.8 times more fecal matter was produced by mopane worms than by elephants.
Elephants notoriously damage trees, but this benefits certain insects: the heartwood of a damaged tree is exposed as food for termites providing eventually a living but hollow tree. Native bees use the resin that flows from elephant-damaged bark for their nests. Ants nest in these hollow trees and may protect the tree from herbivores, both animal and mopane worm. Elephant populations and mopane worm outbreaks vary in space and time, depending on many interacting biotic and abiotic factors, of which harvest by humans is but one.