Things That Make You Say "Hm"

Have you ever wondered what sort of things go through the mind of a naturalist?  No doubt it varies from person to person, as all such things do, so I can only tell you for sure the things that often have me entranced as I try to puzzle them out.  Here is a "for instance."

a mass of braconid wasp cocoons Microgastrinae.jpg

A couple days ago, while leading a school program, one of our volunteers found this fluffy white mass on the back of a dried leaf that had fallen from one of the trees along our path.  Now, I knew it was an egg or pupa case, and I knew I had seen it before, but I could not recall what species it belonged to or any details of its life history.

When we got back to the visitor center, and while the students took their lunch break, I grabbed my copy of "Tracks and Signs of Insects" by Eiseman and Charney, and searched for the answer I was sure was in there.  No luck.

And yet I KNOW I had seen (and identified) it before.

My go-to site for all insect ID is the terrific website BugGuide.Net.  If you haven't been there yet, you must do so soon.  These folks are terrific: you send them a photo of some mystery insect or insect part, and often within a couple hours you have an answer as to what it is. 

So, off I sent my photo, and sure enough, the answer soon came back: Braconid Wasp, subfamily Microgastrinae.

My work had only just begun.

This morning I have been scouring my insect books and the Web in search of details about this small ball of fluff.  Eggs?  Pupae?  Life cycle?  And while some information is quite readily available, much (to me) still remains a mystery.

For example:

There are about 2000 described species in the subfamily Microgastrinae.  Scientists suspect there are 5-10,000 species possible; that is a lot of unknowns.

The Microgastrinae are a subfamily of the Brachonid wasps, which are the second largest family of wasps in the world (right after the Ichneumonids, just in case you were wondering).  Over 17,000 species of Brachonids are recognized today, but again, it is believed there are thousands more that we know little to nothing about.

Brachonids are parasitic wasps.  In general, they lay their eggs on the host (in the case of the Microgastrinae, the hosts are the caterpillars of moths and butterflies), the eggs hatch, consume the host, pupate, and then the adults fly off to continue the cycle. 

If you have ever seen a tomato hornworm in your garden that is covered with small white ovals all over its back, you are seeing the pupal cases of a Microgastrinae.  The larvae ate the caterpillar's "blood" (hemolymph) and internal organs, then tunneled out of its dying body to spin silky cocoons on this back.  This is a good thing for you and your tomatoes - not so good for the caterpillar.

Anyway, this was all familiar territory for me -- I have seen many a hornworm covered with cocoons, and plenty more photos of other species of caterpillars similarly parasitized.  But the fluffy mass that is currently sitting on my desk has no (apparent) caterpillar host associated with it.  Why?

I continue to search the Web, but even BugGuide has let me down - there are photos a-plenty, but mostly all it says about them is "Brochonid Wasp - Microgastrinae" - no life histories.

The best I can come up with at this time is that this is a collection of Microgastrinae pupae (and if I teased apart the mass I would surely find all the little oval pupal cases), and perhaps the desiccated husk of the host caterpillar has fallen away, since by now its purpose is fulfilled and it is obsolete.

Knowing me, however, I shall continue to puzzle about this until one day I have found THE answer.