Wild Side: Cynipid wasps

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This tiny (5mm) gall wasp, or cynipid, was spotted in Vineyard Haven last week. - Matt Pelikan

In the roughly quarter-century I’ve been seriously studying insects, I’ve grown used to oddness among our six-legged neighbors: bizarre body structures, peculiar life histories, ecological relationships that defy belief in their complexity. I’m not easily surprised.

But cynipid wasps? Those things are strange!

Perhaps the first illustration of this conclusion is the fact that, although the tiny wasps in this family are plentiful and diverse (there are some 800 species in North America), I’d never actually laid eyes on one until last week. The fact that this encounter took place in mid-January seems consistent with how these insects insist on strangeness.

In contrast with the wasps themselves, the effects of cynipids are easy enough to see. Known familiarly as gall wasps, cynipids have evolved an astounding knack of hijacking the tissue development of plants. The site where a cynipid parks an egg in or on a plant balloons into a distorted growth inside which the egg matures.

Known collectively as galls, these growths — effectively tumors induced by the insect — vary in form from subtle thickening of a stem to colored dots on a leaf, to lumps the size of a golf ball, depending on the wasp species that forms them. Some gall wasps specialize in invading the galls formed by other species and maturing as “guests.”

As a family, cynipids have a marked preference for oaks, used as hosts by the majority of gall wasp species. One cynipid, Callirhytis ceropteroides, has achieved infamy on the Cape and Islands for infecting and sometimes killing black oak trees.

Laying its eggs in the twigs of this abundant tree, callirhytis (with the assistance, possibly, of some not-yet-identified fungus or microbe) produces galls that appear as knobby thickening on the twig and block the tubes through which the oak transports water and nutrients to and from the leaves. On badly infected oaks, the leaves die and cannot regrow, and in a season or two, the entire tree may expire. Usually, however, wasp galls have little apparent effect on host plants.

The mechanism by which cynipids cause gall formation has not yet been fully described. It may depend on interaction with viruses or other microbes. Amazingly, given how complex gall-related interactions must be, other insects, notably some kinds of flies, form galls in other plants. So this bizarre bit of biology has clearly evolved more than once.

My discover of an adult cynipid ambling about in January leads to more strangeness. Many cynipids, it turns out, alternate between a generation that reproduces in the conventional way, with males and females mating, and a generation that consists solely of females, which produce eggs without fertilization — a process known as parthenogenesis.

Adults of the two generations often look different from each other, and in some cases the all-female generation is active in — you guessed it — winter. It makes a certain amount of sense: emerging as a self-sufficient egg-producing machine, a female of this generation wouldn’t even need to move much. She could begin laying eggs right where she emerges as an adult, and wouldn’t even need to fly. (Some species are flightless in this state, and given the weak and incompletely formed wings on the cynipid I photographed, I suspect it belonged to one such species.)

Cynipids range in size from tiny to tinier — from about eight millimeters down to about one millimeter in length — which makes them easy to overlook. They are often oddly formed: Many have a hunch-backed appearance, others have large, bulbous abdomens. Given that most cynipids reproduce by forming galls on oak trees, I presume that many species spend their entire lives up in the tree canopy, accounting for the fact that they aren’t often encountered.

So my cynipid, which I haven’t been able to identify even to the genus level, was likely a flightless (or at best weak-flying) female, producing eggs without benefit of male companionship until something caused her to drop out of a tree and onto the porch railing where I found her.

As is invariably the case with insects, cynipids are embedded in an ecological web that governs, and usually limits, their abundance. In particular, this family is susceptible to parasitism, largely by other types of wasps that recognize galls or the sites where cynipid eggs have been deposited. Eggs of the parasitic species, laid nearby, hatch into larvae that lethally parasitize the gall wasp eggs or larvae. (There are even other specialized insects that parasitize the parasites!)

The case of Callirhytis ceropteroides, the gall wasp responsible for so many dead oaks on the Vineyard, is not well understood. Perhaps it was a newly arrived species in our region, getting established before its associated parasites got here to keep it in line. In any case, the outbreak seems to have subsided, and this wasp may have joined the unknown number of its family members that occupy the Island, carrying on their bizarre lifestyles largely out of sight of humans.