Giving Wasps Their Due

I often see pie charts like this one suggesting that about a quarter of all insect species are beetles. Suspiciously, other sources (e.g. here) say beetles represent about a quarter of all animal species, and Wikipedia goes so far as to say beetles constitute “almost 25% of all known life-forms”! (they may have meant to say “animal life forms”, but a pie chart here does show beetles representing about 20% of all organisms). The first statement is probably closest to the truth, but I suspect that it is based on numbers of described (named) species, and I can’t help but wonder if this figure is a result of a disproportionate number of people studying beetles. I’ve lost track of how many undescribed moths, flies, and wasps I’ve found in just the past few years; the moths and flies belong to groups that are studied by just one or two people in North America, and the situation is even worse for parasitoid wasps and gall wasps: for most of the wasps I’ve reared, I can’t even find a specialist to examine them and determine whether they match any described species.

According to Wikipedia, there are about 400,000 described beetle (Coleoptera) species in the world, with estimates of total species ranging from 850,000 to 4 million. There are about 180,000 described moths and butterflies (Lepidoptera), over 150,000 wasps, ants, bees, and sawflies (Hymenoptera), and about 150,000 flies (Diptera). I haven’t seen an estimated total number for Lepidoptera, but there are said to be one to three million Hymenoptera species and about a million Diptera species.

It seems like every insect has a set of more or less host-specific wasps that parasitize it. Even parasitoid wasps have wasps that parasitize them. So it’s hard for me to believe that there aren’t more wasp species than anything else. I looked around for estimated numbers for the major parasitoid groups, and found that the family Braconidae has roughly 12,000 described species, with 50,000 being a “probably highly conservative” estimate of the total number in the world. The related family  Ichneumonidae has 24,281 described species  and “probably includes more than 100,000 species” total. According to BugGuide.net, in North America it is estimated that there are 3000 species in this family alone remaining to be discovered and/or named, the same as the estimate for the entire beetle order (there are currently about 25,000 named beetle species and 5000 ichneumonid species in North America). The superfamily Chalcidoidea has about 22,000 described species, with an estimated total of more than 500,000 according to BugGuide.

With a thousand or so described species, the subfamily Platygastrinae (Platygastridae) is a relatively small group of wasps. (The wasps themselves are small, at a millimeter or so long.) As far as is known, all of them parasitize gall midges (Cecidomyiidae). As of 2014, there were 6203 known species of gall midges, with the number of species still unknown being “inestimable”.* It seems to me that the true number of platygastrines is therefore likewise inestimable, and I suspect that Peter Neerup Buhl’s report on the eight sets of platygastrid specimens I sent him last year is representative of the current state of knowledge of this group. As I mentioned here, he identified one of the wasps as Metaclisis floridana and another as a Synopeasnot identifiable to species because it was a male, but possibly S. pubescens; neither M. floridana nor S. pubescens had ever been reared before. The remaining six were all species of Platygaster, a genus with about a hundred known species in North America. Of these, one keyed to P. baccharicola but its antennae were broken off and it couldn’t be identified with certainty; two were males and therefore unidentifiable; and the remaining three didn’t match any known species. Our paper describing them was published last month**, so I can now introduce them to you:

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Platygaster pruni is a parasitoid of Contarinia cerasiserotinae, which forms galls on black cherry (Prunus serotina) that look like this:

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The larvae of Contarinia cerasiserotinae pop out of their galls in late May, burrow into the ground, and emerge as adults the following spring.

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However, in my one attempt to rear this species (from galls collected in Leverett, Massachusetts), all that emerged the following spring were a male and female of Platygaster pruni and this as yet unidentified eulophid:

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The other two new species were reared from gall midges that are themselves likely undescribed. The host of the first has never been reared (adults are unknown), but its larvae are common in western Massachusetts. They form leaf spot galls on wild oats (Uvularia sessilifolia), which are sometimes already empty by late May…

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…but sometimes larvae are present as late as early July. The whitish larva is only visible on the lower leaf surface.

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The larvae of this species likewise exit their galls and burrow into the ground, presumably emerging as adults the following spring.

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The four females and two males of Platygaster uvulariae all emerged on May 18.

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The last species came from the same site on Nantucket where I found Megaselia nantucketensis and is likewise known from a single specimen. Its host midge develops in little blisters on grape leaves; the blisters project from both leaf surfaces and have a little central tuft on each side.

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The larvae exit their galls in the first half of August, once again burrowing into the ground and overwintering.

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I did manage to rear two adults of this species, but they were in poor shape when I discovered them on May 15.

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Ray Gagné confirmed that they belong to the genus Vitisiella, and probably a new species, but the genus needs to be revised before that can be said with certainty. A third midge emerged on June 5 and was in good shape when I found it, but it turned out to be an inquiline (not the gallmaker), Lasioptera vitis.

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The single female of Platygaster vitisiellae emerged the same day.

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Hard to estimate how many Platygaster species there might be when every specimen I rear is either unidentifiable or a new species!

* Shortly after I posted this, Jason Dombroskie alerted me to this brand new paper (published just three days ago), which used DNA barcoding to estimate insect diversity in Canada. The estimate for gall midges is 16,000 out of 94,000 species, which they extrapolate to 1.8 million cecidomyiid species out of ten million insect species worldwide. Addressing the very point I was making at the beginning of this post, they state that if their estimates are accurate, “the global species count for this fly family may exceed the combined total for all 142 beetle families.”

** Buhl, Peter N. and Charles S. Eiseman. 2016. Three new reared species of Platygaster (Hymenoptera, Platygastroidea: Platygastridae) from the United States. International Journal of Environmental Studies, DOI: 10.1080/00207233.2016.120132

About Charley Eiseman

I am a freelance naturalist, endlessly fascinated by the interconnections of all the living and nonliving things around me. I am the lead author of Tracks & Sign of Insects and Other Invertebrates (Stackpole Books, 2010), and continue to collect photographs and information on this subject. These days I am especially drawn to galls, leaf mines, and other plant-insect interactions.
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