Last August we visited the beautiful tiger swallowtail butterfly. You may recall that it had turned into a remarkable chrysalis resembling a dead leaf. We placed the chrysalis in a terrarium and were anxiously awaiting the appearance of a beautiful swallowtail butterfly when events took an unexpected turn last week. Imagine our chagrin when, instead of a swallowtail butterfly, a rather feisty looking wasp emerged from the chrysalis. You see, unbeknownst to us, the larva of our butterfly had been visited by a parasitic wasp in the latter days of last autumn. The female wasp likely grappled the caterpillar before stinging it and depositing an egg within. The fascinating part of this story is that the parasite inside the swallowtail did not immediately kill the caterpillar. This clever parasite waited for the caterpillar to turn into a pupa and passed the harsh winter safe and snug inside the chrysalis of its host. It was not until the return of spring that the parasite completed its development and emerged as a gorgeous ichneumon wasp.
Parasitoids with this unusual type of delayed development within a host are called koinobionts. Many species of koinobionts synchronize development with that of their host by responding to changing levels of hormones produced by their host during growth and development. The appearance of the ichneumon was well timed because very soon other swallowtail butterflies complete their metamorphosis, emerge from a chrysalis, feed, mate, and lay eggs. After eggs of the swallowtails hatch, a fresh supply of caterpillars will be available to serve as hosts for the next generation of ichneumon wasps. Our good fortune in witnessing the emergence of the ichneumon from the chrysalis of its host was a rather rare event. If you would like to see other ichneumonid parasitoids, switch on your porch light on a warm spring evening, and see who arrives. Last week several pale orange ichneumons visited our light and we invited them in for a drink. A little honey and water seemed just the right tonic for these busy parasitoids. After they had their fill, we bid them adieu, and returned them to the wild. Last spring my flower bed was plagued by cutworms and armyworms. Perhaps, our hospitality will be returned by the ichneumons in the form of a koinobionic attack on these pesky caterpillars.
The fine reference “The Insects: an Outline of Entomology” by P.J. Gullen and P.S. Cranston was used as a reference for this Bug of the Week.