The Carlisle Mosquito Online

Friday, November 2, 2007

Features

The care and feeding of cecropia caterpillars

This is the fourth instar, worth a look on the Mosquito web site for its "crayola colors" of blue, yellow and red knobs on a greenish-blue body. Check it out!
This is the time of year when you can find cocoons of the giant silkworm moths. Marty Schafer and Jan Sacks of Acton Street found a very large cocoon, probably a cecropia, in their garden on October 4. It was in tall grass near a pear tree. The cecropia moth featured in the Biodiversity Corner on June 22 laid a small cluster of eggs on a piece of cardboard. I kept watch on the eggs in case they hatched and this is the story of their progress to cocoons.

Cecropia eggshells
Timeline

All eight caterpillars hatched on the same day. Six made it through the five instars and formed cocoons. (Instar is the term for caterpillar stages between molts.) This process from egg to cocoon took three months for the group, but individuals developed at distinctly different rates. I called the leader Primo and the laggard Sextus. (Septimus and Octavius died very young. )

Food

As a species, Cecropias eat a variety of different leaves including poplar, willow, birch, maple and fruit trees. As individuals they eat the leaves where the eggs were laid. In captivity, once started on a type of food, they expect the same for the rest of their lives. I tried them first with maple since I have plenty of it but they showed it disdain. I didn't want to sacrifice my plum tree leaves, so I tried them on crab apple. They all moved off the maple and began a long munching marathon on crab apple leaves. If the leaves got a bit dry, the caterpillars would go walk-about on a search for something fresher. They get all their moisture from the leaves and can dehydrate very quickly and would die if they got lost on their walk-about. I started putting the crab apple twigs in cups of water, with a cover, which is necessary to prevent the caterpillars from falling in or crawling in and drowning. (They do not expect their twigs to be in a pond and have not evolved to swim.)

In the first instar stage, they would often feed on the same leaf and then line up in straight rows side-by-side when resting. The fourth and fifth instars had huge appetites and had to be resupplied with leaves twice a day. By the middle of August each caterpillar was about four inches long, so with six of them I had the equivalent of a two-foot-long, very hungry caterpillar to satisfy.

Change masters

Everyone knows of the huge change that takes place from caterpillar to moth in metamorphosis but the change in the caterpillars themselves is also remarkable. The hatchlings were about a quarter-inch long, black, and covered with black bumps and black bristles. After their first molt, they were almost three quarters of an inch long and the body color was now yellow, but the knobs and bristles were still black. The third instar still had the yellow body, but four of the knobs were bright red and some of the others were yellow.

The fourth instar was a crayola marvel, with four rows of blue knobs, two rows of yellow knobs down the center back and four very large red knobs just behind the head. The body of the caterpillar was a greenish-blue and the feet were blue.

The final instar had the same general color scheme but was a little less vivid and the red knobs had changed to orange. Each knob had a ring of black bristles. Some caterpillars with bristles sting but not the cecropia.

Cecropia caterpillars, day one.
Molting

I had never watched a caterpillar molt before and found it fascinating. In the early stages I didn't see it happen but when the caterpillars were large, they telegraphed the event by not eating for two days prior to the molt.

At first I thought Primo was sick and moved him (maybe her) to separate quarters. After two days I noticed he had a very firm grasp of a twig and was twitching a lot. The twitchings soon became an orderly wave of contractions passing along the whole caterpillar body. Then, yellowish stress lines showed up all along the body. Next, the legs pulled inside the skin and left the caterpillar clinging to the twig by just the shrivelled leg skins. Eventually the skin split near the head, and Primo dragged himself out.

The final step was when the old scales covering the face dropped off. The time from the stress lines showing up to the completion of the molt was about 15 minutes. The shed skin that used to cover a caterpillar over three inches long had shrunk to about half an inch. Before starting to feed on leaves again, Primo ate his old skin.

Cocoon making

The prelude to cocoon-making is an extreme purge of body contents not needed in metamorphosis. Again with Primo being the first to exhibit the behavior, I thought he was ill. He first eliminated a copious amount of fluid and then a few clumps of partially digested leaves. He didn't look well and was restless. After about seven hours, he started pulling threads of white silk between the twigs I had left for him. The silk was coming from a "spinneret" near his mouth. A day later he had completely enclosed himself in silk but not so densely that I couldn't see him. In another day the silk was dense and had hardened and darkened to a pale rusty brown.

The process was more or less the same for each caterpillar, but with Quintus, I noticed that the fresh silk had a very strong smell of human body odor (it wasn't coming from me). The smell went away went the silk was older. When Sextus finally made his cocoon — three weeks after Primo —- the fresh silk had a sweet fruity smell like raspberries.

Second and third instars.
Sounds

These caterpillars were noisy eaters. Even with the radio on and sitting across the room from my caterpillar zoo, I could hear the rasping sound of each bite being taken. The caterpillars also made sounds inside their cocoons. When the cocoon was still new but dense enough that I couldn't see into it, I could hear scratching sounds for a day or two. I don't think it was "Help, I've changed my mind," but more likely some kind of positioning for pupating.

Care of cocoons

This is reputed to be the trickiest stage in raising giant silkworm moths. There are two major concerns. One is to prevent the moth from emerging too early before it would have any potential mates and before there are leaves for its caterpillars. Emergence from the cocoon is triggered by changes in temperature and day length. Older books suggest keeping cocoons in the fridge, but modern frost-free fridges cause dehydration and kill the pupae.

The other concern is protection from predators. In the wild, cocoons high in trees are subject to attack by downy woodpeckers. Gilbert Waldbauer, author of The Birder's Bug Book, has seen "downy woodpeckers cling to two-inch-long cocoons of the cecropia moth as they pierce the tough, silken, double walls to get at the large and succulent overwintering pupae." Deer mice and opportunistic omniverous raccoons will open cocoons nearer the ground and eat the pupae.

My cocoons will spend the winter in separate wire mesh enclosures in my unheated garage. I want to keep them separate so I can tell which one came from which cocoon. Each enclosure will have something for the new moth to hang from while its wings expand and take on their proper shape. If all goes well, the new moths will emerge in June 2008.

Summary

This was a very interesting project that I would recommend to anyone who hasn't done it before. It was a little demanding during the heavy feeding phases, but not so bad that friends who mind my dog when I'm away weren't willing to take on the caterpillars too.

The caterpillar has begun to molt between the fourth and fifth instar. Its skin has split and it will soon be shed.



This is the fifth instar, in which the red knobs have changed to orange and have spouted black bristles.

Sextus makes its cocoon out of white silk coming from a spinneret near its mouth.

The final cocoon is dense, hard and evolved into a pale rusty brown color.

 


2007 The Carlisle Mosquito