This monarch caterpillar was munching on common milkweed at my garden this weekend. As noted below, habitat destruction has reduced the amount of milkweed growing in the Midwest, which is the only food source for the graceful Monarch butterfly. That’s one of the reasons I don’t kill off all the milkweed in my gardens. Another reason: the scent of milkweed flowers, as another gardener once described it to me, is intoxicating. Unfortunately, my nemesis, the Japanese beetle, has managed to suck all the milkweed flowers to brown this year. More plants are springing up, and hopefully will bloom after the destructive beetle has gone underground. Though the caterpillars look a bit ominous, they won’t bother your tomatoes or other plants, so don’t poison or otherwise kill them off, as Monarchs already face a number of obstacles to survival.
For more information on milkweed, the following came from the Monarch Watch Web site at www.monarchwatch.org
Monarch larvae appear to feed exclusively on milkweeds in the genus Asclepias and several other genera of viny milkweeds in North America. Milkweeds are perennial plants, which means an individual plant lives for more than one year, growing each spring from rootstock and seeds rather than seeds alone. In the Midwest, milkweeds were historically common and widespread on prairies, but habitat destruction has reduced their range and numbers.
Milkweeds belong to the family Asclepiadaceae, derived from Asklepios, the Greek god of medicine and healing. Though most members of the genus Asclepias are tropical, there are approximately 110 species in North America known for their milky sap or latex contained in the leaves. Most species are toxic to vertebrate herbivores if ingested due to the cardenolide alkaloids contained in the leaves and stems.
When Monarch larvae ingest milkweed, they also ingest the plants’ toxins, called cardiac glycosides. They sequester these compounds in their wings and exoskeletons, making the larvae and adults toxic to many potential predators. Vertebrate predators may avoid Monarchs because they learn that the larvae and adults taste bad and/or make them vomit. There is considerable variation in the amount of toxins in different species of plants. Some northern species of milkweed contain almost no toxins while others seem to contain so much of the toxins that they are lethal even to monarch caterpillars.