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Milkweed Leaf Beetles

Milkweed Leaf Beetle Survival Mechanism

Many insects use splashy colors and color patterns to defend against being eaten.  (This practice is called “aposematism” from the Greek for “away” and “sign.”) If you spend time in a milkweed patch, you’ll notice that several of the insects you see have bright orange and black coloration.  Milkweed contains defensive chemicals known as cardiac glycosides and Monarchs as well as several other insects (many of which are black and orange) that feed on milkweed can tolerate them and store these chemicals as a defense. When avian predators consume a Monarch butterfly containing these chemicals, a bird suffers digestive upset.

Once a bird has gotten sick after eating a poisonous black and orange insect such as a Monarch, it tends to avoid any and all insects with similar coloration, regardless of their toxicity or lack of it.  Milkweed Leaf Beetle larvae and adults do not absorb the cardiac glycosides in milkweed like a Monarch, so they have no toxic compounds in them and will not poison a predator.  Insect-eating birds don’t know this, however, and the beetles successfully deter predation through their coloration.

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