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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Swallowtail trifecta: Black swallowtail, Papilio polyxenes, Spicebush swallowtail, Papilio troilus, and Giant swallowtail, Papilio cresphontes

 

Late instar larvae of the black swallowtail are breathtaking.

 

In a previous episode we worried about the fate of the beleaguered monarch butterfly, whose population east of the Mississippi is threatened by habitat destruction and degradation throughout its native range. However, all news is not bad news for butterflies. This week I was delighted to spot numerous caterpillars of three marvelous native butterflies, black, spicebush, and giant swallowtails.

While weeding around a newly transplanted sassafras tree, I noticed small nibbles and curious folds on several leaves. Closer inspection of these folds revealed small larvae of the spicebush swallowtail that could have passed for bird droppings. Hiding inside rolled or folded leaves is a common disappearing act used by caterpillars of many species to escape the eyes and beaks of hungry birds. If this ruse fails, then looking like a bird dropping is probably a clever way to avoid being consumed because, well, what self-respecting bird wants to eat bird poop?

 

 

Here in the folded leaf of sassafras, a spicebush swallowtail caterpillar escapes the eyes of hungry predators.

Large snake-like eyespots on the body might give a hungry bird second thoughts about attacking this caterpillar.

 

As the spicebush caterpillar grows and develops, it switches to plan B and takes on the appearance of a serpent with two large eyespots on its thorax. These large eyespots strongly resemble a reptilian eye and it is believed that this disguise helps the caterpillar deter predators such as birds. Why? Think about what eats a caterpillar, often a bird; and what eats a bird, often a snake. So if you are a caterpillar, one way to scare away birds is to resemble a snake and this is precisely what the spicebush swallowtail does.

 

 

A closer look at my herbs revealed several black swallowtail larvae.

 

The second swallowtail in my trifecta was the gorgeous black swallowtail. Due to the unrelenting ravages of white-tailed deer in my suburban neighborhood, I have abandoned any hope of growing a garden in my yard. My tomatoes and herbs reside instead in small planters nestled on a deck very near my front door. While watching my parsley grow last week, I detected several missing leaves among the shoots of green. Among the fronds were a half dozen small larvae of the black swallowtail.

Black swallowtails deposit eggs on many plants, including dill.

 

 

 

Like their cousins on sassafras, these too use the “I am a bird dropping” defense to avoid predation. However, as they grow ever larger, their outward appearance transforms into a dazzling series of vertical black, yellow, and green bands encircling their body. These bands may help to break up the cylindrical pattern that identifies them as tasty caterpillars to a hungry bird. The banding pattern may also serve as a warning that they taste bad by virtue of nasty chemicals they may sequester from plants on which they feed.

 

 

Tiny black swallowtail larvae have large appetites for parsley.

Larvae of the giant swallowtail eat hoptree and other members of the rue family, including citrus. They retain the appearance of bird droppings throughout the larval stage.

To complete the swallowtail trifecta I ventured ten miles northwest of the historic town of Hancock, Maryland, where the hoptree grows on the limestone outcroppings along the Potomac River. On the leaves of this curious native tree, dozens of giant swallowtail caterpillars munched away. Hoptree is a northern member of the citrus family. Citrus and other members of the Rutaceae serve as hosts for giant swallowtail caterpillars that sometimes become abundant enough to be pests in orange orchards in Florida. The bird-dropping defense is employed throughout the development of the giant swallowtail from the smallest caterpillar to the largest.

 

This bevy of beauties has one more trick up their sleeve, or should we say just behind their heads. The caterpillar houses a specialized structure called the osmeterium in a special pouch on the dorsal surface of its thorax. Usually, this forked orange appendage is tucked beneath the skin out of sight. However, when the caterpillar is threatened, it extends the osmeterium in the direction of the attacker. This glandular organ is coated with foul smelling chemicals reminiscent of rancid butter. The disturbing visual and olfactory display might be the final deterrent to hungry predators wanting to dine on these intriguing caterpillars.

 

 

If stinky fluids from the osmeterium don’t dissuade an attacker, regurgitating the last meal might.

This week is an excellent time to go and inspect your spice bush, sassafras, parsley, dill, or hoptrees. If you are lucky, you may discover tricky swallowtail caterpillars devouring your plants and doing their best not to get eaten. Perhaps 2015 will be a very good year for some of our native butterflies.    

References

We thank Mike, Lisseth, and Dr. Shrewsbury for inspiring this episode. Thomas Eisner’s delightful book, “For Love of Insects”, and David Wagner’s wonderful guide, “Caterpillars of Eastern North America”, were used as references for this episode.