A thripsish tale

Originally published 13 March 1989

Accord­ing to the news, Ver­mont maple syrup pro­duc­ers are run­ning scared of the pear “thrip.” This lit­tle insect defo­li­ates maple trees, which is not good for the sap.

First, let’s get one thing straight. There’s no such thing as a thrip. The sin­gu­lar of thrips is thrips. The plur­al of thrips is thrips. As Dr. Seuss might say, “One thrips. Two thrips. Red thrips. Blue thrips.”

But then again, the thrips that afflict maples are nei­ther red nor blue, but black­ish or whitish, depend­ing on the stage of their life cycle. Dark adult thrips spend the win­ter under­ground, emerg­ing in ear­ly spring to feed on buds, blos­soms, leaves, and young fruits of var­i­ous trees, includ­ing pears and sug­ar maples. They deposit eggs in leaf­stalks. Pale young thrips emerge from eggs, eat, get fat, then fall to the ground, where they take up res­i­dence in the soil and lie dor­mant for the sum­mer. Pupa­tion takes place in late fall, where­upon thrips go back to sleep for the dura­tion of the winter.

A sim­ple life, most­ly spent snooz­ing, by an almost invis­i­bly small insect with a Dr. Seuss sort of name. But in spite of what you now have rea­son to think, this essay is not about thrips. It’s about humans, about curios­i­ty, and about the Dr. Seuss in all of us. Allow me to go fur­ther along my thrip­sish course before I reveal the moral of the story.

Exploring with a microscope

One of my favorite pas­times — in sea­son — is to plunk a wild­flower onto the stage of a dis­sect­ing micro­scope and go explor­ing. No Dar­win in the Gala­pa­gos or Hum­boldt on the Ama­zon ever sur­veyed a rich­er fau­na than what exists micro­scop­i­cal­ly on almost any plant. A dan­de­lion can be a trop­i­cal for­est for the nat­u­ral­ist armed with even a mod­est degree of opti­cal mag­ni­fi­ca­tion. With­in the sta­mens and corol­las of every mead­ow weed lurks an aston­ish­ing cat­a­log of beasts.

Now I would­n’t know a pear thrips from a wheat thrips or any oth­er kind of thrips (there are hun­dreds), but I know a thrips when I see one. Adult thrips have four fringed wings, like tiny oars with eye­lash­es, and often I’ve seen one skulk­ing about in a flower-head. Mag­ni­fied 10 times even the wimp­ish­ly-named thrips presents a for­mi­da­ble aspect. Eye-to-eye with one of these mon­sters you begin to under­stand how a thriv­ing throng of thrips could trash a maple.

No soon­er had I read about thrips in Ver­mont than I came across anoth­er thrips sto­ry in the jour­nal Nature. “Fac­ul­ta­tive vivipar­i­ty in a thrips,” it was called, by Bernard Crespi of the Uni­ver­si­ty of Michi­gan. Crespi has dis­cov­ered a species of thrips in which females lay eggs (ovipar­i­ty) or give live birth (vivipar­i­ty) or both. Oviparous off­spring are female, and vivip­a­rous off­spring are male. Dur­ing a giv­en bout of repro­duc­tion it’s all one way or the oth­er, but long-lived females can change modes and some­how the ratio of sex­es bal­ances out. Crespi’s switch-hit­ting female thrips are appar­ent­ly the only known ani­mal capa­ble of “choos­ing” her mode of repro­duc­tion in response to sub­tle sig­nals from with­in or without.

Lit­tle puz­zles like this are the delight of evo­lu­tion­ists. How and why did such bizarre behav­ior evolve? What is the adap­tive val­ue of being able to switch repro­duc­tive modes? Why is being born live good for males but not for females? Crespi posits plau­si­ble expla­na­tions, but I won­der if an expla­na­tion by nat­ur­al selec­tion is real­ly nec­es­sary. Thrips run the gamut of repro­duc­tive strate­gies. Some lay eggs, some give birth to live young, and one, at least — Crespi’s thrips — has it both ways. Nature seems to have a niche for almost every­thing. Not even the wildest prod­uct of Dr. Seuss’ imag­i­na­tion — the Moth-Watch­ing Sneth, for exam­ple, or the Grick­i­ly Grac­tus (that lays eggs on a cac­tus) — is stranger than crea­tures that actu­al­ly exist. The egg-lay­ing/live-birthing Flip-Flop Thrips is a case in point. So is the Syrup-Sip­ping Thrips of Vermont.

And now I’m warm­ing to my sub­ject. The nat­u­ral­ist Don­ald Cul­ross Peat­tie wrote: “The world, from a wee­vily point of view, seems to exist for the wee­vils.” He might as well have said that sug­ar maples, from the pear thrips’ point of view, exist for pear thrips. And, of course, he’s right: it’s all a mat­ter of point of view. I’ll draw my moral like this: From the human point of view, thrips exist for humans.

Following curiousity

Why does a biol­o­gist like Crespi spend all that time with nylon-mesh bags filled with dead oak leaves and live thrips, count­ing off­spring, sex­ing baby thrips, and deter­min­ing repro­duc­tive mode? OK, some thrips are agri­cul­tur­al pests, and one can argue — espe­cial­ly when apply­ing for research grants — that study­ing the repro­duc­tive behav­iors of thrips might have an eco­nom­ic pay­off. But I doubt if such prac­ti­cal con­cerns have much to do with Crespi’s moti­va­tion, or that of any oth­er ento­mol­o­gist. He’s doing in a seri­ous, sci­en­tif­ic way what I do more casu­al­ly with my micro­scope — fol­low­ing his curios­i­ty into the bizarre Dr. Seuss­ian world of bio­log­i­cal diversity.

There are more than 10 mil­lion species of life on this plan­et, and we are the only one insa­tiably curi­ous about all the rest. If some­where there’s a bird that watch­es moths or lays eggs on a cac­tus, we want to know about it. Not even the lowli­est insect escapes our inter­est. Why else did I write this thrip­sy essay? Why else did you read it?