Wednesday, August 31, 2011

Monarch cats

While on a trip into a remote part of Hocking County today, we came across a patch of common milkweed, Asclepias syriaca, that was loaded with monarch butterfly caterpillars. It was necessary to stop and admire the colorful little larvae, and make a few photos. I suppose monarch cats are one of the best known and most familiar of the some 2,500 species of caterpillars that ply their trade in Ohio.

As is typical, further inspection of the milkweed foliage revealed other tubular units hiding here and there. Monarch caterpillars are very host-specific, snacking only on plants in the Asclepiadaceae, or milkweed family, with occasional forays onto very closely related dogbane plants. Milkweeds are pretty nasty when it comes to edibility and you'll not want to garnish your salads with the stuff. The thick white sap which flows freely if you cut or a bruise a plant, is loaded with cardiac glycosides. You or I would get very sick if we ate  this stuff, but the caterpillars have evolved an immunity.

Monarch caterpillars are relatively fearless, because they are chemically protected. Predators quickly learn the folly of dining on a caterpillar that is loaded with poisons. But even the monarch cats can apparently find the toxic sap a bit much, and sometimes resort to the trick shown in the photo above.

This caterpillar has chewed through most of the leaf petiole - the short stem that attaches the leaf to the main stem. By doing so, it has largely shut down sap flow into the leaf that it is feeding upon, and thus reduced the volume of poisonous milky sap that it will ingest.

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