Wild Side: Ode to wasps

Like most insects, they are often misunderstood.

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Parasitic wasps, like this member of the family Cynipidae, are as diverse as they are important. — Matt Pelikan

Amid all the gloom that currently dominates the national media, the New York Times ran a real day-brightener (for me, at least) on Feb. 17. Writing for the “Trilobites” science series, Sabrina Imbler summarized recent research on “A Parasitic Wasp Unmasked: One Species Is Actually 16 Species.” 

What a delightful headline for an insect enthusiast! The article Imbler reported on appeared earlier this year in the journal Insect Systematics and Diversity. It’s heavy going, but you can read it here: academic.oup.com/isd/article/6/1/8/6528936

In that article, a team of entomologists led by Sofia Sheikh at the University of Iowa presents research on a tiny, elegant wasp called Ormyrus labotus. This metallic green wasp is a member of the family Ormyridae, a group with about 20 North American species that specialize in parasitizing insects that protect their offspring by forming galls (essentially hijacking plant tissue to form protective lumps around their eggs). O. labotus attracted the researchers’ attention because of the large number of host species (“more than 65”) it has been documented parasitizing.

A parasitic lifestyle in a wasp is not in the least surprising. Parasitic wasps lay their eggs in, on, or near an appropriate host (an arthropod of another species), and the wasp larva develops inside the host, generally killing it in the process. It’s a common arrangement: There are hundreds of thousands of parasitic wasp species, across a large swath of wasp families, and the parasitic lifestyle is the rule rather than the exception among wasps.

No, it’s the ecumenical proclivities of O. labotus that are unusual. When you think about it, having your larvae develop inside another species poses enormous challenges. First, you need to find an appropriate host, which means being able to find and recognize the right kind of habitat and setting. Is your host a wetland species, or does it associate with dry habitats? Does it live in treetops, and if so, trees of what kind? Does it prefer to hang out on leaves, twigs, or bark? You get the picture. The complex behavior a parasitic wasp uses to find its host has been programmed genetically, because wasps don’t get to go to school.

Finding your host, challenging as it is, may be the easy part of the process. Once it has found a host, the parasitic species needs to have a strategy for getting an egg, or at least a larva, inside the host. This won’t be easy, because the host will object if it can, and in any case it’s going to be protected by its exoskeleton, and perhaps also by hairs, spines, noxious chemicals, or other defenses. And once a larva is growing inside a host, it has to be able to resist the efforts of the host’s immune system to reject it. Again, the necessary behavioral or biochemical strategies the parasite uses have to evolve and be encoded in genes.

Because of these challenges, parasitic wasps are typically picky about their hosts. Parasites of gall-forming insects face especially great challenges, since galls provide excellent protection for the eggs or larvae they contain. So the team studying O. labotus collected large quantities of the many gall types O. labotus is known to parasitize, raised the contents of the galls to maturity in an incubator, and then examined the insects — a mix of gall-forming species, O. labotus, and many other types of parasites — that emerged. 

The team conducted genetic analysis on the O. labotus they found, correlating genetically distinct types with host species, and found that O. labotus is actually not a generalist parasite: It’s a cluster of at least 16 specialized species, impossible to distinguish by physical appearance but clearly separated by genetic markers, as well as well as by the hosts they use.

These results, which make one suspicious of all other apparently generalist parasites, are actually not that surprising, given the challenges of successful parasitism. And while I don’t know if O. labotus occurs on the Vineyard (the genus, at least, does occur in Massachusetts), this elegant study highlights several points that are important to anyone who cares about Vineyard biodiversity.

One take-home is that parasitic wasps are incredibly diverse and incredibly important in ecological terms. Many or most of our insects host at least one kind of wasp parasite (even parasitic wasps are parasitized by other wasps). Parasitism is a crowning achievement of evolution, a vast network of relationships providing essential regulation of insect populations.

Another point is that wasps — indeed, insects generally — are poorly understood. Sure, modern genetic methods and information systems have revolutionized the study of nature. But what we don’t know hugely outweighs what we do. The diversity and complexity of the natural world defies comprehension, and as our habitats dwindle and fragment, diversity that we don’t even yet know about is at risk.