Cannibalism by newly-hatched caterpillars

Cannibalism is the largest cause of mortality in our program

Hatching caterpillars normally eat their own eggshell just after hatching.  The shell is made of chitin. Caterpillars need a good supply of chitin, because their skins are made of it; they expend chitin each time they shed their skins.

There's a second reason caterpillars benefit from eating a nearby egg.  Wild females usually lay only one egg per plant.  It's in the caterpillar's interest to eliminate competing caterpillars on the same plant.

But our captive females lay many eggs per plant, up to 20 per leaf.  When eggs are close to one another (photo), the caterpillar that hatches first may partially eat eggs nearby, killing the caterpillar inside.  It's possible to lose 10%-25% of eggs during hatching if many eggs are close to one another.

You can reduce this mortality by one of two methods...

1. Transfer the caterpillars to a fresh plant as soon as they hatch, or...
2. 
   Cut out each egg on a tiny square of milkweed, about 1/16" on a side.  Pick up the square by pushing a pin into the corner of the leaf square, and transfer it to a white ice cube tray, one egg per compartment. At hatching time (4 days after laying), watch for dark caterpillars crawling around the white tray.  After 6-8 hours of wandering on the tray, the tiny caterpillars may try to escape by "rappelling" down from the tray on their silk line, so check for caterpillars periodically.  Transfer them to fresh milkweed.  Label the tray with the date eggs were laid, no you know when to expect hatching. The wine glass makes it harder for caterpillars to escape.

Method two has the advantage of avoiding having leaves curl up before eggs hatch. Cutting eggs out may seem difficult, but it's not as hard as it sounds.  It's much easier than finding caterpillars hidden in dried and curled up leaves.

How to spot cannibalism

If most of your eggs are just empty eggshells, then a spider probably found and ate the eggs at night--especially if you didn't protect the stem with water in a saucer.  For more evidence of spider predation, look for a spider hiding under the table.  The spiders with very long legs that wait high in the corners of rooms aren't the kind that eat eggs.

Cannibalism looks different.  You have many hatched caterpillars, but some eggs have broken eggshells with a black spot on top. That black spot is the head capsule of a caterpillar that died inside the shell. 

In the photo, note the broken eggshell (upper center) containing a dark mass-- the body and/or head capsule of the dead caterpillar.

Usually, the dead egg will be found next to an empty and partially consumed eggshell (lower left) from which the cannibal caterpillar hatched (lower left in the photo).

Additional evidence for cannibalism is if you notice one caterpillar is noticeably larger than others.  He got fat eating other eggs.

Cannibalism isn't entirely negative, because it gives some caterpillars a healthy first meal.