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The predator from within...

If you have a young 2nd- or 3rd-instar caterpillar that repeatedly falls off its food plant despite being awake and responsive to touch, or that remains stationary for several days and shows no interest in eating, it may have been infected by a parasitoid wasp. "Parasitoid" rather than "parasitic" because parasites don't generally kill their hosts. These wasps do.

The young caterpillar shown below played host to the larva of a type of parasitoid wasp (perhaps a species of Apanteles), which devours a substantial portion of the caterpillar's innards.

The larva then emerges from the side of the caterpillar's body and immediately starts spinning itself a cocoon from its own silk. Within a week or two, a tiny wasp (pic below) will emerge from the cocoon.

These wasps had been incorrectly identified on this page as Trichogramma wasps, but the latter are far smaller and have shorter antennae and stubbier wings and do their damage to butterfly eggs rather than to caterpillars. [thanks to Jean in Portland].

The caterpillar shown above is probably dead. Although the wasp larva, while growing inside the caterpillar, reportedly spares the muscles and nervous system of the caterpillar so that the caterpillar will be able to hold onto the twig or leaf and also retain its reflex to move its head away from contact with another caterpillar, the caterpillar usually becomes entirely unresponsive within a few hours to a day or so after the wasp larva emerges.

Below is a video of a recently emerged wasp larva in the initial stage of spinning its cocoon.

The larva has spun silk strands around itself to form the outline of a cocoon, and then will laboriously fill in the gaps until a thick mat of silk entirely encloses the larva. This video shows 3 minutes of the process, but well over an hour was required to complete the cocoon. The caterpillar is usually situated immediately adjacent to the larva (see image above) as it spins, but in this case the caterpillar was moved away from the larva so the latter's spinning actions could be seen more clearly.

Look carefully at the caterpillar during the video and from time to time you will see very slight movements of its head (upper left of the caterpillar). It is not dead yet, but as noted above, will die within hours to a day or so. Best to view the video full-screen.

What follows is a series of images shot at intervals over about 1.5 hours as the larva spun its cocoon. You can see in several places that the larva has also used its silk to attach the cocoon to the fennel stem. Also note the flexibility of the larva. It can bend itself into a tight "U" shape as well as turn itself around within the cocoon as needed to work on opposite ends of the cocoon.

wasp larva spinning cocoon

wasp larva spinning cocoon

wasp larva spinning cocoon

wasp larva spinning cocoon

wasp larva spinning cocoon

wasp larva spinning cocoon

wasp larva spinning cocoon

wasp larva spinning cocoon

At the above stage, the cocoon appears outwardly to be complete, but motion could still be detected on the inside, so presumably the larva was still coating the inside with silk.

Below is another dead or near-dead caterpillar with its wasp cocoon. Notice that the surface of the fennel frond is well-silked, presumably by the caterpillar.

pic of wasp cocoon

Below is a picture taken through the low-power lens (described in tools) of 2 wasps and the cocoons from which they emerged. That's a standard-sized paperclip next to them, so you can see that they are very small. Not the sort of wasp that we worry about encountering.

You might wonder how insects manage to escape from their cocoons once they are ready for adult life. According to various web sources, they either cut their way out or they secrete fluids with enzymes that soften the cocoon sufficiently to allow escape.

Notice the wonderfully long antennae!

Point-of-view is important in deciding our alliance with one creature or another.

Do a Google "Images" search on 'caterpillar wasp' to find out lots more about parasitoid wasps. For example, this is an excellent page about these wasps (and much else about butterflies):