on the move

With temperatures rising steadily, tropical insects are moving south, writes MARTYN ROBINSON

One of the impacts of climate change is that tropical animal and plant species are now able to live further south than ever before. A number of winged insects have led the exodus, including the plumbago blue butterfly (Leptotes plinius), which needs plumbago as a food source. Australian white-flowered Plumbago zeylanica only grows in rocky tropical and subtropical areas, as far south as northern New South Wales, which may have previously limited the spread of the butterfly.

The butterflies aren’t fussy about the plumbago species or flower colour they feed on, however, so when gardeners further south started growing blue-flowered Plumbago auriculata (syn. P. capensis) from South Africa, they readily switched to that. In warmer regions, they fed freely on the new blue variety most years, but temperatures weren’t warm enough in many areas. Around Sydney, they only appeared during long, hot summers with northerly winds, but as hot weather became more consistent, they were able to linger longer. Now they are resident right down near the Victorian border.

So how do you know if you’ve got plumbago blue butterflies? Well, if you have a plumbago plant in your garden, you will have them. They are small, with a wingspan of only 2–3cm. Many people mistake them for moths, but on closer examination you can see that they have the typical drumstick-shaped antennae of butterflies. Their wings have a purple-blue sheen on the upper surface and a marbled effect on the underside. The caterpillars are difficult to see when resting in the sticky buds and seed pods, but they do little harm.

Other tropical insect species have also moved to Sydney. At least two butterflies, one moth, a katydid, a mantis, a native cockroach and a spider are now breeding residents, rather than rare summer visitors.