Advice from a Caterpillar

While Alice was adventuring in Wonderland, she met a hookah-smoking caterpillar who asked her unpleasant questions and gave her advice.  The caterpillars we run into can give us advice too. 

Monarch Butterflies (Danaus plexipus) feeding on Aster sp. in the
North Carolina mountains

Two common butterflies in the Southeastern United States advise their predators, and us, how dangerous they are.  Monarch Butterflies (Danaus plexipus) are the most charismatic of our butterflies.  They are large, brightly colored and undertake epic migrations from Mexico to Canada and back.  If you plant milkweed in your garden you will probably have Monarchs.  They lay their eggs on the milkweed (Asclepius sp.) leaves.  

Butterfly Weed (Asclepius tuberosa), a species of
Milkweed

The eggs hatch into caterpillars with alternating white, yellow and black stripes.  Monarch caterpillars are not trying to hide.  That is because the milkweed upon which they feed is laced with cardiac glycosides, a potent toxin.  

A caterpillar of the Monarch Butterfly
on Tropical Milkweed (Asclepius curassavica)

Cardiac glycosides, as the name suggests, interferes with heart function that can result in heart failure. Cardiac glycosides also interfere with the digestive tract and muscular system of mammals that ingest them.  Not only that, these molecules taste bad.  The target of cardiac glycoside is a protein pump found in the membranes of all animal cells called the sodium-potassium ATPase.  This large protein maintains the proper balance of sodium and potassium ions in cells.  This is particularly important in muscle and nerve cells.  Monarch caterpillars take in the toxin when they eat milkweed leaves and store it in their bodies.  This makes them less palatable to predators like birds or mice.  The adult Monarchs retain cardiac glycosides after metamorphosis and this protects them from their bird predators.  A good question is; Why don’t the cardiac glycosides kill the monarchs?  It turns out the Monarchs carry a mutated version of the gene for the sodium-potassium ATPase that imparts  resistance to the effects of cardiac glycoside.  So the caterpillars can eat toxic milkweed with impunity, retain the toxin, advertise their toxicity with bright colors and patterns and pass this protection on to the adults. 

A Gulf Fritillary butterfly feeding on Lantana camara.

Another caterpillar giving advice is that of the Gulf Fritillary (Agralus vanilla).  The Gulf Fritillary butterfly, like the Monarch is large with a distinctive pattern of vivid colors.  When seen from above the Gulf Fritillary butterfly is bright orange with bold black spots.  The underside of the butterfly wings are brown and orange with large white spots.  

A Gulf Fritillary seen from below showing large white
spots on an orange and brown background.

Gulf Fritillaries lay their eggs on leaves of Passionflower vines.  The caterpillars hatch and eat the Passionflower leaves. Passionfllowers have chemical defense molecules called cyanogenic glycosides that protect the plant by releasing cyanide.  When a herbivore eats a Passionflower leaf they are poisoned by the cyanogenic glycosides.  But even cyanide cannot deter Gulf Fritillary larvae from eating the leaves.  The caterpillars are bright orange with black spines as if they are daring a bird to eat them.  Like the Monarch, its caterpillar is conspicuous and downright threatening.

Yellow Passionflower (Passiflora lutea) a host plant for
Gulf Fritillaries

Gulf Fritillary caterpillar on Passaflora sp.

Both the caterpillar and butterfly are protected by toxins from the Passionflower.  The adult butterfly also releases noxious chemicals from a gland in the abdomen to further deter predators.    

As a rule, boldly colored insects are toxic and should not be eaten.  I knew a young man that wanted to taste everything he could in the woods.  One day he found a Lady Bug and popped it in his mouth.  He immediately spit it out and spent the next half hour trying to get the horrible taste from his mouth.  The Lady Bug, with its red color and black spots was giving advice.  So are some of the caterpillars. 

An Asian Lady Beetle (Harmonia axyridis) aka Lady Bug
warning us not to eat it.