Actions taken by Mexico the US and Canada, it is claimed, have improved the butterfly’s prospects; at any rate, more of them are being recorded along the country’s northern border. Most will spend the winter at the Monarch Butterfly Biosphere Reserve in a mountainous forested area 100km north-west of Mexico City.
An increase in monarch numbers would be especially welcome; the population has been declining for 20 years. There were about one billion monarchs in Mexico during the 1990s but only 35 million were recorded in the annual census of 2009. Numbers had hit an all-time low. The 2015/16 winter census begins in December.
The orange and black monarch, with a wingspan of up to 10cm, is the world’s most famous butterfly. Breeding mainly in the US and Canada, those from east of the Rocky Mountains migrate south to Mexico, up to 5,500km away, in autumn. Canadian ones have the greatest distance to travel but each insect completes only part of the journey.
Seeking out their favourite plant, milkweed, monarchs breed at locations along the way. Four generations will be born en route. The first three will die without ever seeing the Promised Land; the fourth one will hibernate in Mexico. The multi-generational process is repeated in spring; the butterflies breed on the eight-month journey northwards. Nobody knows how details of the route are passed from generation to generation or how the butterflies manage to find their way.
The eggs are laid on common milkweed, a plant with large broad leaves which grows to a height of 1.5 metres.
Carl Linnaeus named it after Asclepius, the Greek god of healing. Its vernacular name comes from the milky toxic sap. The butterflies absorb the toxin and retain it in their tissues. This renders the adult insect and the caterpillar poisonous to potential enemies. The bright orange colouring of the flying insect and the wasp-like black-and-yellow markings of the 5cm long larva help deter predators.
So many butterflies congregate at the famous reserve that the trees where they perch take on a magical bright orange appearance. The branches bend under the weight. There may be a thousand million in clusters in the forest. It’s futile trying to count them; there are far too many. The scientists measure the area covered and estimate the numbers from that.
In the December 2009 census, only 1.92ha were occupied, the lowest ever recorded. In 1996, the area covered was 21ha. Numbers depend mainly on how many insects survived the previous winter and how well they bred during the summer. Logging, critics claim, is altering the micro-climate on which the insects depend. Reduced availability of milkweed plants in the US may also be a factor.
Monarch butterflies, lost during migration, occasionally arrive in Ireland. David Nash, Trevor Boyd and Deirdre Hardiman, in their definitive Ireland’s Butterflies a Review, say that the species was recorded here for the first time at Castletownshend in 1916. One visited Corbally, near Limerick City, 16 years later; 1999 was an exceptional year for monarchs; there were four records. Most sightings are made on the south and west coasts in September and October.
It’s thought that the visitors come from north America as birds from there arrive at the same time, carried on Atlantic hurricanes. Milkweed doesn’t grow here so vagrant butterflies can’t survive our winter. Tourists visiting Madeira and the Canary Islands should keep an eye out, as monarchs have been resident in coastal areas there since 1880. They settled in the south of Spain 100 later.
- David Nash, Trevor Boyd and Deirdre Hardiman. Ireland’s Butterflies, a Review. Dublin Naturalists Field Club. 2012.