Wild Side: A snappy fly

It can help you in the garden.

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A male Trichopoda pennipes; you can see the distinctive comb of hairs on the right hind leg. —Matt Pelikan

Regular readers will know that I have a certain fondness for flies. While some flies are justifiably detested by humans — blowflies and mosquitoes, for example, both of which carry disease — the order Diptera in general is rich with attractive, interesting, and useful insects. One of my favorites, a pretty little job called Trichopoda pennipes, is on the wing right now.

The orange abdomen, pigmented wings, and patterned thorax of this insect would be enough to make it a favorite. But there’s more! The hind legs of this species are adorned with a remarkable feature: A comb-like row of erect black hairs, which give this fly its name (“trichopoda” means “hairy foot”). Oddly, I’ve never learned what function those hairs serve. But they’re an appealingly quirky touch.

Trichopoda can be found, off and on, through the entire warm season on the Vineyard; my own records combined with those on the citizen science website bugguide.net suggest that trichopoda may have as many as three clearly separated generations annually, with adults active in spring, midsummer, and early fall. For me, anyway, that last window — late September and the first week or two of October — seems to offer the best chance for finding this elegant fly.

Like all flies in its family, Tachinidae, trichopoda has parasitic larvae. And as is often the case, that life history results in a lot of interesting biology. (There are parasitic flies in other fly families, as well: Most bee flies, for example — the family Bombyliidae — are parasitic. But tachinids have refined parasitism to an art form, and this method of reproduction is a fundamental characteristic of the family.)

While the pattern can be quite different in other flies, here’s how larval parasitism works for trichopoda. After mating, a fertile female fly pokes around vegetation, looking for one of a short list of true bugs (in the order Hemiptera) that furnish the preferred hosts of trichopoda. These first-choice targets include several kinds of leather bugs, leaf-footed bugs, and stink bugs. But at least in our region, researchers suggest that by far the favorite host is the squash bug, Anasa tristis — an agricultural pest.

With a victim located, the female fly lays one or more eggs on the bug, usually on its underside. (Don’t ask me to describe the gymnastics involved; I’m relying here on published accounts, not on my own observation!) When the eggs hatch, the tiny larvae they contain promptly drill into the body of the bug, settling in to grow up eating the bug’s juices and tissue. If more than one egg has been laid on the bug, the larvae evidently compete among themselves, with only one surviving.

As it nears maturity, the larval fly busts its way out of the bug, a process that leaves the bug dead and the larva on the ground, where it burrows into the litter, emerging later as an adult fly. Larvae of the last generation of the year overwinter without maturing; they’ll finish their transformation to adult in the late spring, about the time that adult bugs start to be available as hosts for the next generation of flies.

Oddly, trichopoda, which is widespread in North America, has different host preferences in different regions. Western ones prefer stink bugs to squash bugs, and indeed they have reportedly evolved the knack of zeroing in on the same scent chemicals, or pheromones, that the bugs themselves use to govern their own reproductive activity. One intrepid researcher, thinking that squash bug–loving trichopoda might be welcome in California’s Central Valley, where squashes of various kinds are among the many valuable crops grown but often facing damage from squash bugs, transported some eastern trichopoda to California and released them.

Sure enough, the flies remained true to their ancestry, behaving like Eastern flies and targeting squash bugs. Introduced populations in the far West now serve as a moderate sort of biocontrol, helping protect squash crops. Moreover, since parasite-host relationships usually require elaborate behavioral and biochemical specialization, biologists now suspect that Trichopoda pennipes is actually a cluster of closely related species, separated by geography and host preference.

In any case, if you grow squash, be aware that trichopoda is your friend. But there are other good reasons to welcome this fly into your garden. For one thing, there’s the obvious point that this is a very snappy-looking insect (with leg hairs!). For another, like adults of many fly species, mature trichopoda feed on pollen from flowers. As a result, they probably function as pollinators, assisting plant fertility. I doubt these flies are particularly effective pollinators, but they surely make a contribution.

At this time of year, I find trichopoda almost exclusively on goldenrod (though I could easily be overlooking them in other situations). Despite their bright coloration and reasonable size (a big one can be close to a centimeter long), these flies are not all that conspicuous. Their flight is lazy rather than darting, and their movements on a flowerhead deliberate rather than frantic. And they seem to spend a good portion of their time resting between flower visits, perched on a leaf or twig.

If you look for them, though, and know what you’re looking for, this is not a difficult insect to catch up with. It’s worth the effort, and might change how you think about flies.