October, 2006

Monarch Butterfly

The striking orange and black monarch (Danaus plexippus) is perhaps the best known butterfly in the world. This familiarity makes it a favorite; it is big and colorful and abundant over our whole continent so that just about everyone has seen it. The spectacular migration of the monarchs is so noteworthy that no book on butterflies ever neglects it. The monarch is a member of the family Danaidae or the subfamily Danaiinae, depending on your taxonomic preference. Of some 300 species of related butterflies, the monarch is the only one to regularly move into temperate areas. Most of the danaids are tropical and cannot stand freezing temperatures. Males and females look similar but the male (shown above) has a pair of black spots on its hindwings, near the abdomen, which produce pheromones used in mating.

Here in central Texas we see the most monarchs in the early spring and late autumn. They move up from Mexico when weather permits and start to lay eggs as soon as they find milkweeds, their larval food. Subsequent generations fly farther north, until they are found all the way into Canada. Towards the end of summer, the last generation to be born starts to head southward. Those that make it all the way to their wintering grounds in Mexico will return in the spring to start the next cycle. There are non-migratory populations of monarchs, but only in tropical areas of southern Texas and Florida.

Because monarch caterpillars feed on plants in the milkweed family (Asclepiadaceae), they ingest toxic chemicals from the sap. This makes both the caterpillars and adults distasteful to birds and some other predators, affording the insects protection, which they advertise with their warning colors. The poison does not work against spiders and insect predators such as mantids, so monarchs do still face a great number of larval and adult hazards.

As we head into the end of October, the fall monarch migration should be at its peak, and the butterflies can be found just about anywhere on a sunny day, but especially at flowers, as they need a lot of energy to make it all the way to Mexico.