The poisonous caterpillars and adult butterflies have distinctive warning colouration to deter predators.
The Monarch Butterfly (Danaus plexippus) breeds in North America and Canada during the summer, but it migrates south to overwinter. The caterpillars feed exclusively on milkweed and this makes them, and the adult butterflies, highly poisonous. The adult butterflies feed on nectar and seem to be very fond of Buddleia and Joe-Pye Weed.
Poisonous
Milkweed contains poisons (cardiac glycosides) which pass into the caterpillar as it feeds. These toxins are still present in the adult butterfly and both stages in the life-cycle are avoided by most predators. The Black Headed Grosbeak is one of the few exceptions in America, it is not harmed by cardiac glycosides.
Warning Colours
Both the adult butterfly and the caterpillar carry distinctive ‘warning colouration’- black, white and yellow bands in the caterpillar, and a striking black, white and orange pattern on the wings of the butterflies. Any vertebrate animal which eats a monarch caterpillar will have a nasty experience. That individual will learn from this and avoid any potential ‘food’ marked in a similar way. (Nature seems to have settled on black and yellow stripes as a warning sign in many insect species.) The adult butterflies have large wings which sometimes seem to have triangular pieces missing. Presumably birds have taken a bite and found it unpleasant!
Migration
Monarch Butterflies breed in the northern United States and southern Canada through the summer, but as fall approaches they migrate south to their traditional overwintering sites in California and Mexico. The adults who migrate south are not the ones who migrated north – they are their descendents, often several generations removed. They seem to be able to navigate using the sun and magnetic fields, and the migrating butterflies often use the same individual trees their ancestors used in the previous winter.
Monarch Butterflies need to make this long migration each year (as much as 2,000 miles) because the northern winter is too cold for the adults and the southern overwintering places are too far south for the milkweed that the caterpillars require.
Butterfly Gardens
There are many reasons why a small butterfly garden might be a good idea. Flowers attract butterflies (and bees) with bright colours and fragrant perfumes. Fortunately humans seem to share the same preferences, so a part of the garden that attracts butterflies will also look good to us! In addition the butterflies themselves are beautiful and their visits add a new dimension to the garden. But, if you decide to go this way, then you must avoid pesticides and maybe learn to live with a few caterpillars.
(Read an article about habitat loss and the Monarch Butterfly)
Other articles by John Blatchford
