Millions of monarch butterflies (Danaus plexippus) are about to begin their mass migration from southern Canada and the northern United States to their wintering grounds in California and Mexico.
Although native to continental America, monarchs have spread far from home. They are widely distributed in Hawaii (where a black and white form occurs on the Big Island) and Guam. Stragglers were apparently reported from eastern Australia as early as 1856. They were well established by 1878, when this drawing was made of a caterpillar in Tasmania.
The caterpillars feed on poisonous milkweed (Asclepias), cotton-bush and swan plant (Gomphocarpus), all of which have been introduced to Australia. They are undeterred by the plants' toxins. So undeterred that they store them to use for their own defence. To warn potential predators of the danger, the caterpillars sport vivid aposematic coloration.
Making a meal of poisonous weeds stands them in good stead. They keep the toxins through metamorphosis into the adult stage.
With their prodigious powers of flight, wanderers have dispersed across Australia. Their dependence on introduced plants has restricted their establishment in many areas but as the weeds spread, so do they. Although spectacular, the butterflies are signalling the decline in our natural environment.