I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars…
~Charles Darwin, 1860
So strange are the habits of certain groups of wasps that they have caused many a person to look upon them in disbelief (and perhaps a little sense of dread or disgust). But, we owe them a great deal in terms of their ability to provide some biological control on many species of insects, including some that can damage our garden plants. Here are a couple of examples from recent weeks.
While walking around the NC Botanical Garden one day last week, I spotted this egg mass on a leaf. The general shape (like small barrels) and arrangement made me think some sort of true bug (Order Hemiptera), most likely a stink bug of some sort.
When I got in close with my macro, something seemed odd. I can usually tell when these eggs hatch because the top of the “barrel” pops open like a lid. But these eggs had irregular holes with something coming out. As I zoomed in on my image, the creature emerging from the egg did not look much like a stink bug to me. At home, I referred to one of my go-to references, Tracks and Signs of Insects and Other Invertebrates, by Charley Eiseman & Noah Charney. Sure enough, I found this quote…”An irregular hole chewed in the top indicates the emergence of a chalcid or platygastrid wasp parasitoid”. In looking at more images online, it looks similar to some of the species in the genus Trissolcus, but I would need to collect some of them and send them off to an expert to be sure.
The reason I saw the stink bug eggs was that I saw a pawpaw sphinx on a nearby branch and stopped to photograph it. I had permission to collect a few caterpillars, share them with Garden staff for their programs, use them at BugFest, and then return them.
It wasn’t until I got home and looked at my image that I saw the caterpillar had company – a parasitic wasp was on its side, most likely laying eggs.The following tidbits are from a great article on one type of parasitoid wasp that attacks sphinx moth larvae. Female wasps are believed to locate their hosts via chemical cues released by the plant when its leaves are being chewed by the caterpillars. When she lays her eggs inside the caterpillar, the wasp also injects a venom and a symbiotic virus. The virus prevents the caterpillar’s immune system from attacking and killing the wasp eggs. Special hormones are released that also inhibit the development of the larval host.
I was surprised to see wasp larvae emerging only two days after I took the photo above as I had always assumed the larvae fed inside the caterpillar for many days before exiting their host. Indeed, the article mentioned above stated that the usual time from egg-laying to emergence from the host cuticle is 12-16 days. I suppose the wasp in the photo is laying additional eggs…I doubt her offspring will have much to feed on when the time comes.
Looking at the emerging wasp larvae, I could see them already beginning to spin their silken cocoons. I, unfortunately, had to be get back out in the field, so missed an opportunity to sit and watch (and perhaps video) this amazing, albeit somewhat gruesome, process.
On the day of BugFest, we had another caterpillar with dorsal decorations. This species is so small that it can carry only a few parasitoid larvae. I include this image just to show what the completed cocoons look like. When the wasps emerge from the cocoons, usually in 3 to 8 days, it looks as though a lid has popped open at the end of the cocoon. It is, obviously, tough being a caterpillar (or a stink bug egg) when there are so many tiny wasps lurking out there, waiting to provide you with an alien surprise. And yet, they carry on, adapted to survive this and the many other hazards they encounter in their short, amazing lives.