Friday, July 21, 2006
Swamp Milkweed Leaf Beetle
Name: As its name suggests, the Swamp Milkweed Leaf Beetle, Labidomera clivicollis, is a member of the very large Leaf Beetle family, or Chrysomelidae, in the insect order Coleoptera, the Beetles, which is the largest order of creatures on the planet in fact, there are more species of beetles than there are species of plants. The leaf beetles comprise one of the most prominent families of beetles; they are often round or oval shaped, and frequently among the most colorful beetles we encounter.
When and where seen: Found the first week of July at Foss Farm on Common Milkweed (Asclepias syriaca) near the road about halfway to the community gardens. This beetle prefers Swamp Milkweed (A. incarnata), a less robust milkweed than the common species, with a somewhat smaller and brighter flower cluster, and found in wetter habitats. But our beetle is also a frequent visitor to Common Milkweed, as in this case. The beetle can be found on both the flowers and the leaves of the plant. Its range includes most of North America.
Identification: This is a stout, almost hemispherical beetle, though it is longer than it is wide, about 9 to 11 mm long (3/8"to 1/2"). The elytra — the pair of large, hard wing covers on the back of the beetle — are orange or yellow, with large black spots, often forming an X, across the middle. The head and the pronotum (the shield-like part behind the head and in front of the elytra) are black or deep bluish or greenish.
The prominent orange-yellow coloration is a warning to potential predators that this beetle will taste bad. As part of its diet — and the reason it is found on milkweeds — it ingests some of the milkweed's toxic "milk," the gummy white latex exuded by the plant's leaves and stems. Other orange-colored insects also advertise themselves as unpalatable because of the toxins they have ingested from the host milkweed. The best known, of course, is the Monarch butterfly (Danaus plexippus; profiled in the Biodiversity Corner August 1, 2003); others include the caterpillar of a moth, the Milkweed Tussock Moth (Euchaetes egle); a true bug called the Small Milkweed Bug (Lygaeus kalmii); and another hard-to-miss beetle, the Red Milkweed Beetle (Tetraopes tetrophthalmus; profiled in the Biodiversity Corner July 15, 2005). Even the larvae of some of these insects are bright orange, including those of the Swamp Milkweed Leaf Beetle. A study published in 1979 concluded that at least eleven species of insects feed exclusively on Common Milkweed.
Life cycle: Adult Swamp Milkweed Leaf Beetles are usually first seen in mid-summer (though some that have overwintered may be observed already in the spring). As already mentioned, they feed primarily on the foliage of milkweeds, and sometimes on the flowers as well. Elongated yellowish eggs are glued to the underside of the milkweed leaves. When the orange-colored larvae emerge, they grow quickly, also feeding on the milkweed, and eventually they drop to the ground to pupate. At least some of the new generation of adults that emerge from the pupae then overwinter, often hibernating among the woolly basal leaves of the Common Mullein plant (Verbascum thapsus), and then starting the life cycle anew.
References: National Audubon Society Field Guide to Insects and Spiders, by Lorus and Margery Milne; A Manual of Common Beetles of Eastern North America, by Elizabeth S. and Lawrence S. Dillon; Field and Roadside, by John Eastman; http://BugGuide.net.
Anyone can write the Biodiversity Corner about any living thing, animal or vegetable, that exists in the wild and was seen in Carlisle. Or tell me what you've seen, or send me a photo, and I will write the column. Send the information to firstname.lastname@example.org or to Kay Fairweather, 392 School Street..
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