Transitions, Variations, and Life & Death

Top Photo: Tobacco hornworm on tomato plant.

The tobacco hornworm, or Carolina sphinx, and tomato hornworm, or five-spotted hawk moth, both use nightshade as a food plant including tomato and tobacco plants. And, they’re both subjected to attack by a tiny parasitoid wasp called a braconid wasp.

With the help of her ovipositor, the minuscule wasp lays eggs just under the skin of the caterpillar. The eggs hatch and the larvae begin eating the caterpillar from within. When the time is right, the larvae chew out through the caterpillar’s skin, attach themselves with silk to the caterpillar and spin oval shaped cocoons. They pupate inside the cocoons.

The wasps pupate within the silken cocoons.

In about a week, each transformed wasp chews around the top of its cocoon, leaving a small piece attached that acts like a hinge, pops the lid, and crawls out of the cocoon an adult wasp. After a brief drying out period, they’re ready to start the process all over again.

It’s quite a sight to see them all emerge. I watched it happen several times, videotaping it each time.

Over the past week or so, monarch butterfly larvae, or caterpillars, have been munching on tropical milkweed planted in the garden next to Sprout Cafe.

Tropical milkweed.
Monarch caterpillar eating the milkweed’s leaves

The caterpillars are familiar to most people and are fairly easy to locate. They only eat milkweeds, and the semi-circular holes on the edge of the leaves or entire leaves missing, frass (caterpillar poop) left on top of the remaining leaves, and bright yellow, black, and white markings on the insects should help the naturalist find the caterpillars without too much trouble.

If you can’t find them, you’re either not looking, or they’re not on the plant.

I would check each and every milkweed plant I pass at this time of year, whether it swamp milkweed, butterfly weed, common milkweed, or as here, tropical milkweed. The butterflies zero in on all varieties in the genus Asclepias. It’s what they do.

Leaf with frass and chew marks from caterpillar.
Big, with yellow, black, and white markings.

Robberflies like to sit motionless next to stands of flowers. Flowers attract butterflies, bees, wasps, flies, and other insects for their nectar, seeds and foliage. The robberflies are waiting in ambush for those insects. They belong to a family of fierce predators. Their flight is direct and swift and their voracity is legendary.

Red-footed robberfly.

Robberflies vary in shape and size but one you may notice more often than others, perhaps because of its size, is the red-footed robberfly or cannilbalfly. It’s a large robberfly and seems to boldly go where lesser robberflies dare not.

The two butterflies below are female eastern tiger swallowtails.

Eastern tiger swallowtail, yellow female.
Eastern tiger swallowtail, dark morph female. If you look carefully you can faintly see the black “tiger” stripes on the wing on left (right wing). Compare with the one above.

I’ve included many photos of eastern tiger swallowtails on this blog in the past. But I haven’t mentioned much on identification, for good reason. Locally, they’re unmistakable. Big yellow butterflies with black “tiger” stripes is pretty much all you need to know to identify the eastern tiger swallowtail.

That holds true for the males of the species. The females, however, can be yellow and black striped, entirely black, or somewhere in between. Supposedly, the further south you go in the country the more dark morph individuals there are, or so I thought.

The dark coloration in this species is believed to be Batesian mimicry. Through natural selection, the dark females are said to be mimicking another species of butterfly called pipevine swallowtail which is distasteful to predators. The pipevine swallowtail is essentially an all black species with a dusting of blue on the hindwing. Pipevine caterpillars eat pipevine which is toxic. This is much like the relationship between the monarch butterfly and the viceroy, the monarch being toxic from eating milkweed and the viceroy spared being eaten by birds or other creatures because it has the same orange and black coloration as the monarch. Predators avoid it.

Apparently, where the pipevine swallowtail is common, the dark female tiger swallowtails are too.

Twenty some years ago I casually counted all the female tiger swallowtails I saw in one morning at a particular site, a flower garden aflutter with butterflies. That was here in central North Carolina. I counted roughly 50% dark morph females. I recently read that one person in south Florida observed about 10% dark females at their location. Another naturalist encountered 90% dark females in south Georgia. Interesting.

I started looking around on the internet and found a paper that indicated the number of dark females is lowest at the limits or edges of the species geographic range, the southernmost part of Florida in the south and Massachusetts to Minnesota in the north. They’re scarce in the far north and far south (at least along the Atlantic Coast) and variously common in between. The report included a map indicting the same. One quote from that paper will give you an idea of how definitive the demarcation line is, “north of a certain latitude, about 41°30′ on the Hudson River, and 42°30′ in Wisconsin, all the females are yellow.

Too, it’s interesting to look at a range map of pipevine swallowtail (click here and scroll down to see map). Pipevine does not occur in southern most Florida where the dark morph female eastern tiger swallowtails were reported to be about 10% of the female tiger swallowtail population, nor does it appear to occur above the line of demarcation for dark morph female eastern tiger swallowtails in the north.

If you’re interested in reading about the phenomenon, click on the link below which also includes a map showing the numbers of dark tiger swallowtails and where they occur.


Speaking of color variations, look at the photos below.

Green treefrog.
Brown, green treefrog.

Both photos are of green treefrogs. The green individuals are typical. The brown frog, although a green treefrog, is not as typical but not uncommon. This, however, is not a variation in color within the species as with the butterfly above. It has more to do with temperature, activity or mood. That same brown frog may look more like its two green neighbors on a different day, or even later the same day.

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