The large milkweed bug, Oncopeltus fasciatus, feeds on the seeds, leaves and stems of assorted milkweeds. Here, at least three instars, or developmental stages, of the bug have gathered on Asclepias asperula, the antelope horn milkweed: adults, near-adults with little black wing buds, and the very small, mostly orange nymphs.
Milkweed bugs not only resemble monarch butterflies, they’re just as distasteful to predators because of toxic compounds contained in the milkweed sap they ingest. The easily recognizable orange-and-black color scheme common to these nasty-tasting insects, called Müllerian mimicry after German naturalist Fritz Müller, offers the bugs a degree of protection.
In late summer or early autumn, the number of milkweed bugs in coastal Texas begins to burgeon as result of another similarity to monarchs. After spending the summer in northern states and southern Canada, the bugs begin to migrate toward southern states and Mexico, where milkweed still is available. According to entomologist Michael J. Raupp:
Their annual migration south is triggered by shortening day length, cooling temperatures, and declining quality of milkweed plants as food. Titers of a glandular product called juvenile hormone signal the milkweed bug’s ovaries to take a “time-out”, and trigger flight behavior that transports the milkweed bug to warm southern lands where milkweeds grow.
Once the southward migration is complete, juvenile hormone levels rise, ovaries are switched on, and reproduction resumes. In spring, the migratory pattern reverses and generations of large milkweed bugs leap-frog their way northward to colonize milkweeds as far north as Canada.
Clearly, the milkweed bug needs a better press agent. Its migratory habits are fully as impressive as those of the monarch, but most people tend to prefer a butterfly over a bug. Still, milkweed bugs, interesting and handsome in their way, are worth seeking out where autumn milkweed continues to thrive.
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