Monday 14 September 2015 9:02AM
Image: Monarch butterflies aggregated this year in the paperbark grove at the Australian Botanic Garden at Mount Annan on the outskirts of Sydney. (Ann Jones)
The monarch butterfly is seemingly embraced by Australians as a part of the ecosystem, but it’s actually native to North America and depends on plant hosts from other parts of the world. Ann Jones investigates how this pretty insect came to make its home on Australian shores.
The arrival of the monarch butterfly in Australia (Danaus plexippus) probably occurred with little fanfare. In fact, it’s almost impossible to pin down exactly when this pretty brown and black striped butterfly with white spots arrived on our shores.
They are first recorded here in the summer of 1870/71. In a 2004 article in Biological Invasions, Anthony Clarke and Myron Zalucki put forward that there were many almost simultaneous ‘first records’ around that time, which suggests that the monarch butterfly was already established.
They’re so cute and furry and you can’t help but like them when you see them. But they are potentially a tremendous pest.
Dr Dave Britton, Australian Museum
How had such a thing happened? Through an extraordinary stroke of luck, which can only be understood by understanding the butterfly’s interwoven relationship with its environment.
The monarch butterflies’ host plant, which it relies upon for food and protection in the caterpillar stages, is a milkweed, a group of plants which exudes a milky, latex-like poison when its external skins are penetrated.
‘She’ll alight, her feet will be touching the surface, her abdomen will arch, will bend under, and she’ll put her egg underneath,’ says Professor Myron Zalucki of the University of Queensland.
‘And she tastes that leaf with her abdomen—her ovipositor has a whole bunch of chemo-sensory hairs that actually sense what’s in the leaf surface-waxes of the plant—and make that final decision to lay or not to lay.’
This is a calculated risk by the butterfly mother. The young caterpillar that will emerge from the egg has a certain tolerance of the plant’s own defence mechanism, a milky poison that would kill other insects. In fact, the female aims that her offspring will hitch a ride on the back of the plant’s defence mechanism to protect themselves in turn.
‘So [the plant] has these cardiac glycosides—a whole suite of them,’ Zalucki says.
‘The caterpillar has dealt with them by excreting them to its exoskeleton, so it essentially stores a lot of these in its own skin, so presumably it tastes bitter.’
Image: Female monarch butterfly (Kenneth Dwain Harrelson. Licensed under CC BY-SA 3.0 via Commons )
Exuding a bitter taste to birds or other potential predators is a strong defence. The predators soon learn not to eat the brightly coloured caterpillars.
But how did such a butterfly get to Australia all the way from North America?
‘Monarchs were talk of the town, so to speak, back in the mid-1800s, because the species was about to engulf the entire globe,’ Zalucki says.
‘They had colonised the Pacific and I think the traffic between California and Australia during the gold rush era of around about the 1840s would have been quite high and probably would have facilitated the colonisation of the islands, and what happened on these islands was quite interesting.
‘Arriving in virgin territory with lots of milkweed, their numbers just exploded. So we have reports of millions of monarchs, milling around. Now they’re going to disperse as well, so their chances of getting somewhere nearby are that much higher simply because the numbers are higher. [They] made it to Australia by 1869, 1870, or thereabouts—probably blew in from New Caledonia.’
Movement of small, light, things can happen naturally. Storms and cyclones may be responsible for the movement of the monarch through the Pacific Islands and onto Australia.
But in order to establish a successful breeding population an appropriate host plant, a milkweed, sometimes called cotton bush, would have to be present. There are milkweed species, like the bush banana, in Australia. But the butterfly does not seem to use them. Instead it predominantly uses two species, also imports from other parts of the world—neither of which are from North America.
‘In the case of some of the milkweeds that the monarch butterflies use [in Australia]—the Gomphocarpus species have come from southern and tropical Africa, and the Asclepias speces have come from the new world—from the Bahamas and the Caribbean,’ says Dr Paul Forster from the Queensland Herbarium.
‘We have early records of both of them starting from around 1860 onwards in eastern Australia.
‘Of course, in those days there was no quarantine, so a lot of goods came into the country and they probably just came in as contaminants in goods, or perhaps, in the case of Asclepias, they had been brought in originally as a garden plant, because they have attractive red and orange flowers.
‘They like disturbed soil, and once they arrived, they more or less explosively dispersed right up the eastern seaboard of Australia in the agricultural areas.’
Image: Male Monarch butterfly (Derek Ramsey. Licensed under GFDL 1.2 via Commons)
If these plants were not present when the first monarchs flew to ground after the wind storm, or off the ship, then it’s entirely possible that they would not have successfully established a breeding population. But they found these bitter, poisonous plants on which to lay eggs, and the rest is history—and the present, and probably the future as well. The plants are going quite strongly and populations of the butterflies can be seen in various spots along the eastern seaboard.
Why is it that this exotic, invasive, non-native, butterfly is seemingly embraced by Australians as a part of the ecosystem? There are hobbyists out there who breed up more monarchs to release. There are companies which will breed monarchs and ship them to you in a pretty container for you to release on your wedding day.
If this was happening with cane toads, this would surely be railed against, so why not with these butterflies? The answer is probably multifaceted, and includes the fact that the monarchs eat a weed rather then a native plant. Zalucki thinks the nectar competition that they provide the native butterflies would be negligible.
‘It’s very much a perception thing, and we get the same thing with the introduced bumble bee in Tasmania, Bombus terrestris. They’re so cute and furry and you can’t help but like them when you see them. But they are potentially a tremendous pest,’ says Dr Dave Britton, the collections manager at the Australian Museum.
‘We call the plants weeds in these particular cases, but the reality is that the butterfly is a flying weed,’ says Paul Forster.
Editor’s note: Myron Zalucki was a plenary speaker at the recent XVIII International Plant Protection Congress in Berlin.