Economic importance

A. acerata is a production constraint in some localities of Eastern Africa. Outbreaks are sporadic and seasonal and usually occur at the beginning of the dry season. Complete defoliation and/or repeated defoliation results in reduced production.

Geographical distribution

Africa (Kenya, Tanzania, Uganda, Rwanda, Burundi, Congo, Zambia, Ethiopia and Nigeria) and South America

Damage

The caterpillars feed on the leaves of sweetpotato, generally working from the edge inward, and leaving only the midrib and main veins.  They can completely defoliate plants.

Young caterpillars hatch in large groups inside a nest formed of several leaves joined with web.  They feed on the upper leaf surface, and the leaves of the nest become brown and shrivelled and covered with frass.  Older larvae become solitary and feed at night, usually remaining on the soil during daylight.

Morphology

Egg. The pale yellow eggs measure about 0.5 mm wide and 0.7 mm long, and are laid on the leaf surface in groups of several hundred.

Larva. The mature larvae are greenish-black and covered with fleshy, branching spikes. Caterpillars reach a size of 20-24 mm before pupating.

Pupa. The pupae are cream to blackish with brownish banding on the back, and measure 12-15 mm.

Adult. The adult sweetpotato butterfly has orange wings with brown margins.  Wingspan is 30-40 mm.

Biology and ecology

 A. acerata is found in all zones of sweetpotato production in Eastern Africa, but is considered a constraint in relatively dry agroecological zones. Outbreaks are common in Rwanda.

Females lay eggs in batches of 70 to 500 eggs on both surfaces of the leaves. Development is temperature-dependent. The egg stage takes 5 to 10 days.  The larva passes through five larval stages in 16 to 26 days. The larvae are concentrated in a protective webbing for the first 2 weeks after hatching. They then become solitary and hide from the sunlight on the ground during the day. For pupation the caterpillars crawl up the plant or any convenient support, such as tall grass or a wall bordering the sweetpotato field. Here the pupa is suspended in a vertical position. The pupal stage takes 4 to 10 days.

Host range

The insects prefer sweetpotato. Observations were made that A. acerata completed all its life stages on several Ipomoea spp., which are indigenous to Africa.

Detection and inspection

The "nests" of young caterpillars can consist of several leaves and are easily spotted in the field. Pretty, orange butterflies, swarming over a sweetpotato field, are easily recognisable.

Management

Mechanical control

The traditional method of controlling outbreaks has been to handpick and destroy nests of young caterpillars. A limiting factor might be lack of labour.

Chemical control

Severe outbreaks might warrant the use of contact insecticides.

Biological control

No parasitoids were recovered from eggs or adults. Larvae have been observed to be parasitised by the hymenopterans Apanteles acraea Wlkn, Zenillia vara Cuarran, Charops sp. and Meteorus sp. and one dipteran, Carcelia normula. Total parasitism could reach 25% in the dry season. Pupae have been observed to be parasitised by two species of Brachymeria (Chalicididae) while the ants Camponotus rufoglaucus Forel and Pheidole megacephala (F.) preyed on larvae. The pathogenic fungus Beauveria sp. was observed on larvae in the field during the rainy season.

Cultural control

Intercropping sweetpotato with onion and/or the silverleaf desmodium, Desmodium uncinatum, might reduce the number of eggs laid by the females on sweetpotato. 

References

Ames, T., Smit, N.E.J.M., Braun, A.R., O’Sullivan, J.N., and Skoglund, L.G. 1996. Sweetpotato: Major pests diseases, and nutritional disorders. International Potato Center (CIP). Lima, Perú. 152 p.

Hitimani, N., 2001. Host plant finding by Acraea acerata Hew. (Lepidotera: Nymphalidae), the sweet potato butterfly: Implications for pest management. PhD thesis, The University of Edinburgh, U.K., 286 p.

Lefèvre, P.C., 1948. Acraea acerata Hew. parasite de la patate douce. Bulletin Agronomique du Congo Belge. 39: 49-76.

Smit, N.E.J.M., Lugojja, F., Ogenga-Latigo, M.W. 1997. The sweet potato butterfly (Acraea acerata Hew., Nymphalidae): a review. International Journal of Pest Management, 43: 275-278.

Contributed by: Nicole E.J.M. Smit