In recent years several insect species have migrated to the Western Cape and have become conspicuous in our gardens. Observant gardeners and naturalists would wonder what they are.
One of them is the butterfly Charaxes brutus natalensis (white-barred emperor), a real beauty that usually skips gracefully from plant to plant in gardens.
It comes from the Eastern Cape and KwaZulu-Natal where its larvae feed on Natal mahogany and Cape ash, trees that are also common in Cape gardens. Even the exotic syringa can serve as a host.
Another is a big beetle, longer than 2cm, with yellow markings that has recently been noticed in a Somerset West garden where it fed on the red berries of a shrub. This is the zigzag beetle, named for its markings, a fruit chafer or “vrugtetor” (Anisorrhina flavomaculata), also from the north-eastern parts of the country.
Previously it did not occur west of Port Elizabeth, but then it
was spotted in Swellendam and now in Somerset West. The butterfly and the fruit chafer follow the route of another butterfly and some other beetles that have also expanded their range south-eastwards. Perhaps many more less conspicuous insects have done the same thing without being noticed.
How does one explain this? Of course range expansion can happen when insects are transported deliberately or accidentally, e.g. with plant material, from one country or region to another.
This is the case with many of our exotic pest species, such as the woolly whitefly from the Caribbean region that two or three years ago became so abundant on our lemon trees, but has now all but disappeared after a parasitic wasp was introduced to control it.
Also, the so-called cabbage white (Pieris brassicae), a butterfly from Europe, arrived here in 1994 and is now very common in our gardens, where its larvae can devastate nasturtiums and cruciferous plants.
This may also be the case with the tiny (4mm) garden-bane weevil (Elliministes laesicollis) from the northern parts of the country that suddenly appeared in Cape Peninsula gardens a few years ago, and has now spread to gardens in Stellenbosch, where it will give gardeners many headaches in future.
In the case of gradual range expansion southwards, never northwards in South Africa as far as we know, one is tempted to speculate that it is a response to climate change, especially since it has been established, that in the northern hemisphere, the range of certain well known insects is expanding northwards. Insects are known to be very sensitive to atmospheric conditions, e.g. termites “knowing” that it is going to rain, and can respond to subtle changes.
The climatic barriers that restricted them to certain areas in the past might have disappeared, but there is no evidence to support this hypothesis or the one that ascribes it to changed agricultural practises.
Professor Jan Giliomee is a researcher in ecology and entomology in the Department of Botany and Zoology at the University of Stellenbosch.