Mopane woodlands and the mopane worm

Biology of the mopane worm

Adults of the Mopane Emperor Moth (or the Anomalous Emperor moth)
on the trunk of a mopane tree. Click on this image or any others on this
page to see a larger picture.

Imbrasia belina (Westwood), the mopane worm or Anomalous Emperor Moth, is a saturniid lepidopteran which is widely distributed throughout southern, central and east Africa. Its distribution in southern Africa follows that of its primary host plant, the mopane tree (Colophospernum mopane), which occurs in a broad band extending from the Northern parts of South Africa into Zimbabwe and Botswana, and west into Namibia. I. belina feeds on a number of tree species but C. mopane is the most suitable host in terms of developmental periods, number of emerged adults and nutritional quality.

Final instar mopane worms feeding on a mopane tree.

Life cycle

Across most of its distribution, the species is bivoltine with the first generation emerging from pupation in November to December and the second in February to March, only in more arid areas is it univoltine. Adult moths lay a single cluster of 50 to 200 eggs around twigs or on the leaves of host plants over a two month period. After approximately ten days the larvae emerge and then pass through five instars before pupation. Instars I to III of the caterpillars are strictly gregarious and will forage together in aggregations of 20 to 200 individuals. After moulting into instar IV caterpillars disperse immediately to become solitary. The larval stage lasts approximately 6 weeks during which time the caterpillars undergo a 4000 fold increase in body mass. At the end of the larval stage the fifth instar caterpillars burrow into the soil, where they undergo a period of diapause. Eclosion occurs either one or six to seven months after pupation, depending on the generation. The non-feeding adult stage lasts only two to three days, during which time the only function of the imago is to find receptive mates and to oviposit.

A stand of mopane trees defoliated by an outbreak of mopane worms

Effect on host plants

There is has been little work conducted on the impact that I. belina have on their host plants. A field study in Botswana by Ditlhogo et al. (1996) found that only 14 percent of I. belina defoliated C. mopane trees produced seeds compared with 84% of undefoliated trees (Ditlhogo et al., 1996). They also found a relationship between tree size and seed production following defoliation with larger trees able to withstand the impacts of caterpillar grazing compared with smaller trees. However, this was a limited study and further work is needed to fully quantify the influence that I. belina has on its hosts.

Natural enemies

There are a number of egg parasitoids known to attack I. belina, and 40% or more of the eggs of any generation are often parasitised. The caterpillars of I. belina contribute to the diets of at least 34 species of birds and several mammals. The caterpillars are also sometimes infected with viruses which may cause considerable mortality.

A final instar mopane worm killed by a virus

Population ecology

There are no reliable data available on the population dynamics of I. belina, but there is considerable anecdotal evidence to suggest that it displays outbreak dynamics similar to those known from many species of Northern Hemisphere woodland Lepidoptera. I. belina conforms to the picture of a 'typical' outbreak lepidopteran closely: eggs in masses, gregarious, large, brightly coloured larvae which begin feeding early in the season and which are reasonably polyphagous.
There is a pressing need for quantitative data on where when and why outbreaks occur and particularly high quality data on the spatial and temporal population biology of the moth. This will be an important focus of the project.

Woodland management and the mopane worm

The abundance of the caterpillars is, apparently, declining (Roberts 1998) as a result of increasing exploitation and a decline of selective harvesting (Hobane 1995) and a general increase in pressure on mopane woodlands. There have been reports of the disappearance of the mopane moths from parts of Botswana after heavy harvesting (Bartlett 1996). Suggested threats to mopane worm abundance, in addition to over-harvesting, include deforestation of mopane woodland, and increasing drought. Increased harvesting pressure may have wider effects on the mopane ecosystem itself. Mopane trees may be felled or branches lopped to facilitate harvest of the worm, and the tallest mopane trees may be most at risk as they are the most heavily laden with caterpillars which are beyond the reach of harvesters. Although mopane trees are in no danger of becoming scarce, selective destruction of the tallest individuals may have profound consequences for the abundance of mopane worm and other mopane herbivores such as elephant and various host-specific insects. Research into the extent of tree felling for gathering mopane worms and the effects of this practice on mopane worm populations and other resources provided in mopane woodland is needed to ensure continued supply of these resources.

Perhaps the greatest impediment to the further commercialisation of mopane worms is their temporally restricted abundance. If it is possible to raise mopane worms artificially through simple domestication programs then the seasonal dependence on the market and the degree of exploitation on the natural occurrence of the worms would be reduced.

References

Bartlett, E. (1996) Hold the turkey. New Scientist, 152 (2061/2): 58-59.

Ditlhogo, M., Allotey, J., Mpuchane, S., Teferra, G., Gashe, B.A. and Siame, B.A. (1996) Interactions between the mopane caterpillar, Imbrasia belina, and its host, Colophospermum mopane in Botswana. In Flower, C., Wardell-Johnson, G. and Jamieson, A. (eds.) Management of mopane in southern Africa. Ch.9, 46-49.

Gaston, K.J., Chown, S.L. and Styles, C.V. (1997) Changing size and changing enemies: the case of the mopane worm. Acta Oecologia 18: 21-26.

Hobane, P.A. (1995) Amacimbi: the gathering, processing, consumption and trade of edible caterpillars in Bulilimamangwe District. Centre for Applied Social Sciences, University of Zimbabwe. 33 pages

Klok, C.J. and Chown, S.L. (1999) Assessing the benefits of aggregation: thermal biology and water balance of anomalous emperor moth caterpillars. Functional Ecology 13:417-427.

Oberprieler, R.G. (1995) The Emperor Moths of Namibia. Sigma Press. Pretoria. South Africa.

Roberts, C. (1998) Long-term costs of the mopane worm harvest. Oryx, 32(1): 6-8.

van den Berg, M.A. (1971) Studies on the egg parasites of the mopani Emperor moth Nudaurelia belina (Westw.) (Lepidoptera: Saturniidae). Phytophylactica 3: 33-36.