As you’d expect with cold-blooded creatures, insect numbers and diversity follow a distinct annual arc, building to a summertime peak from a winter nadir. The pattern varies somewhat depending on what group you’re talking about: Butterfly diversity peaks in mid-July on the Vineyard, while katydids don’t hit their peak until early September. Moreover, insect life never truly disappears: Even discounting dormant overwintering insects, which are usually immature forms rather than adults, there are a few species specially adapted to maturing and reproducing in the dead of winter. But as a general rule, summertime is insect time.
Except, of course, when it’s not. I spent a good portion of our recent, unseasonably mild Thanksgiving and Black Friday hunting for insects in my Oak Bluffs yard. The initial results were uninspiring; I found a couple of grasshoppers I know would be there, heard a ground cricket, spotted a tiny moth of some kind darting out of the frost-blackened wreckage of a peony plant.
And then, at random, I kicked the lower branches of a red cedar tree, rousing a startling crowd of insects into flight. The sheer numbers and apparent diversity of these insects would have grabbed my attention even in July; in late November, I was astounded. For the next couple of hours, I staked out the cedar, camera in hand, trying to figure out what was there and what it liked about cedars.
My first discovery was that virtually all these insects were flies, in the broad, biological sense: members of the astonishingly diverse order Diptera. Between direct observation and examination of photographs, I was able to make some headway narrowing things down. A few species — several syrphids or “hover flies,” fat, bluish blowflies in the genus Calliphora — I recognized outright. A few others I could place into families (a winter crane fly; a mosquito; a couple of members of Tachinidae, a family of parasitic flies; and a host of fruit flies, there by the hundreds and easily the most abundant type). Still others, in the annoying manner of flies, were too generic-looking for me to ID as anything more specific than “flies.”
In all, more than 20 species were hanging around the cedar, or on similar cedars elsewhere in our yard; finding just a dozen fly species here in July would impress me. Many of them appeared to be using the cedar simply as a convenient roost site, which makes sense. As evergreens, the trees retain their green leaves, making concealment easy. (Many flies that I tried to follow seemed simply to disappear as they landed on the underside of cedar leaves.) Also, the finely detailed structure of a cedar leaf undoubtedly offered a lot of nooks and crannies of exactly the right size for, say, a fruit fly to hide in.
But the real draw turned out to be robust clusters of blackish “berries” that had ripened on some of the cedars. (Botanically speaking, these are actually cones, like pinecones. But in practice they look and feel like berries, so I’ll use that term.) Large numbers of flies were perching on berry clusters, and close observation showed these flies running their sponge-like mouthparts over the surface of berries or over spots where damage allowed berry juice to leak out.
I tasted a couple of berries, confirming what I had once read: When ripe, red cedar berries may be as much as 30 percent sugar. Inside each black berry were two or three seeds, each about the size of a sesame seed, along with a juicy pulp that tasted strongly of pine and sweetness. They tasted pretty good!
These insects, in other words, had zeroed in on a rich food source, about the only meal that was available in a yard where nearly all the flowers had been killed by frost. The sugar stored in the berry juice provided a rich energy source to sustain these insects, many of which had outlived the other adults of their species, nearly into December.
Some kinds of birds, such as robins, starlings, and cardinals, are also huge fans of cedar berries, drawn by the same nutritional resource my flies had responded to. Indeed, the sugary juice probably evolved as an enticement to birds to eat the seeds, depositing them elsewhere in droppings and thereby helping the cedars spread to new locations. But I had never considered insects to be potential beneficiaries of this bounty.
In retrospect, there are no surprises here, except perhaps my own obtuseness. The cedars produce something insects can make use of. And insects seized the opportunity, which is what insects do. I doubt it will be long before cold weather finishes off most of these flies. But in the meantime, they’ve found the means to extend their lives.