Catriona MacLennan: Monarch's reign is in danger of ending

Aucklanders can do their bit to help attract monarchs. Photo / Chris Gorman

Opinion

Loss of habitat, extreme weather, chemicals and garden trends are decimating numbers of beautiful butterfly.

One of the uplifting sights of a Kiwi summer is the soaring, swooping monarch butterfly. And it is not only in New Zealand that the majestic insects are appreciated.

Since time immemorial, millions of Northern Hemisphere monarch butterflies have flown from Canada and the United States to spend the winter in Mexico. So regular has been the migration that Mexicans link the arrival of the king of the butterflies to two important cultural events: the corn harvest and Dia de los Muertos on November 1.

In the language of the native Purepecha Indians, the monarch butterfly is called the "harvester butterfly" because it appears when it is time to harvest the corn.

But last year and again this year, the butterflies have not appeared in their usual numbers. Last year, the tally of monarchs making it to Mexico plunged by 70 per cent to 60 million. This year, the position is even worse: the butterflies arrived 10 days after November 1 and only around three million have been seen.

Graphs predicting this northern winter's fall roost numbers and Mexican forest area occupation by the butterflies, show figures in freefall. Basically, monarchs are disappearing.

In Montreal, researchers estimate that eastern Canada's monarch numbers are down by 90 per cent. The population collapses are attributable to four factors - habitat loss, severe weather, the use of pesticides, and garden design trends.

In the United States, as much as 6000 acres (2428ha) of uncultivated land a day is ploughed under for development. Parks and backyards are being squeezed out of cities as population pressures intensify. In addition, rising corn prices have resulted in the conversion of additional land to cornfields, replacing what used to be conservation areas with intensive cropping. This destroys the monarchs' habitat and deprives them of food.

Two studies found that between 60 and 90 per cent of swan plant in the state of Iowa had disappeared.

More - and increasingly severe - weather events threaten the butterflies, with climate fluctuations in the US and Canada drying out eggs and killing larvae. Pesticides are also a major threat. Roundup wipes out almost all vegetation except that which is genetically modified to survive the herbicide. In the US, millions of acres of native plants - and particularly swan plant - have been destroyed by chemicals.

Globally, backyards which used to feature varied displays of colourful, nectar-rich plants have been replaced by courtyards with paving or lawn and uniform, monochrome displays.

American native oak trees host as many as 537 species of caterpillars, while willows provide a haven for up to 456 different species. By contrast, the non-native gingko supports only three.

So why should humans care about the collapse of the monarch butterfly population? The reason is that insects are essential to human survival. Butterflies, like bees, are pollinators and pollination is integral to crop production. Bee populations around the world are also collapsing. Without bees, 80 per cent of crops would disappear.

Aucklanders can help monarch butterflies by cultivating them in their gardens, on their balconies or in any small courtyard. Swan plant is their main food Swan plants contain toxins called cardiac glycosides that the caterpillars must ingest. The glycosides make the monarchs poisonous to most predators.

Swan plants planted in summer and left over winter will be 2m tall the next year and will provide ample food for monarchs.

If no eggs or caterpillars appear on the swan plant, it is because they are being eaten by predators such as wasps and praying mantises. To protect the eggs and caterpillars, keep the swan plant inside until the butterflies hatch or else wrap the plant in an old mesh curtain.

It is also important not to use poisons and sprays in the garden - they will kill the plants and damage the eggs and caterpillars. Plant a range of colourful, nectar-rich plants to attract butterflies.

For more information visit www.monarch.org.nz