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| Milkweed Tussock cats - Kelly Bigbee |
Kelly Bigbee of
Fassnight Creek Farms is one of a number of volunteers at the Butterfly House who are raising butterfly larvae and host plants for the house. Milkweed is grown for Monarch Butterfly larvae but last week she sent a picture of a plant seen on the right resembling a Lutheran potluck.* It was covered with ravenous caterpillars of a different breed, the Milkweed Tiger or Tussock Moth,
Euchaetes eglei.
In the photograph above, a Monarch caterpillar is hanging below the leaf with a Tussock Moth on top. Chris Barnhart noted that the Tussock Moth seems to mimic the Monarch cat in both coloration and size when you account for the mass of hairs.
Since birds tend to avoid Monarchs because of the milkweed toxin in their bodies, could this be a case of Batesian mimicry, using its similar appearance to warn off predators by pretending to be a toxic Monarch cat? Actually it is even more complicated. The Milkweed Tussock Moth also stores the
milkweed cardiac glycoside toxins. This is an example of Müllerian mimicry, where two or more insects with the same toxic chemicals display similar color patterns to warn off predators.
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| Gregarious Tussock Moth caterpillars stripping a milkweed leaf - Chris Barnhart |
These Tussock Moth caterpillars are gregarious, with multiple individuals
feeding on the same leaf. They can afford this exposure because if one
of their siblings is eaten, the bird will soon be too busy upchucking to
come back for seconds.
Digitalis and Digoxin were cardiac glycoside drugs used in the past for congestive heart failure. Nausea was a warning that the drug levels were too high. Birds have the same reaction as shown by Dr. Lincoln Brower in studies of a naive bluejay fed Monarch butterflies.
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| After eating a monarch butterfly- Dr. Lincoln Brower |
There is a lot of variation in the amount of toxicity in different milkweed species as well as individual plants. The milkweed species in Monarchs' Mexico overwintering area is much less toxic. Also predators vary in their sensitivity to the glycosides as well as their strategies for reducing their exposure. This and other details are detailed in
The World of Insects.
*
Observation made by Barb's sister who is a connoisseur of Lutheran potluck suppers.
