This column is about some of the unsung heroes of the insect world. While you’ve almost certainly seen one, you’ve likely never heard their name. But without their role in controlling numbers of other insects — and I mean this quite literally — human society would have a hard time functioning.
I mean the myriad members of the fly family Tachinidae (most entomologists pronounce it “tak-IN-adee”). Tachinids, as they’re called, are numerous, diverse, and highly variable flies. Some 10,000 species have been described worldwide, and many tens of thousands more are surely out there, waiting for someone to notice their existence.
Flies, including tachinids, are hard to identify; pretty much all of the dozens of fly families have some members that look like your basic, generic fly: gray, hairy, and sporting no traits that stand out as distinctive. While there are certainly many distinctive flies, even experts can have trouble assigning a photographed fly to a particular family. Identification often relies on viewing tiny anatomical details under magnification.
But if I had to pick one feature as a mark of the family Tachinidae, I’d point to their bristly butts: The abdomen of a typical tachinid is well-endowed with strong bristles, and many times this trait is enough to put an unknown specimen into this family. (Actually identifying a tachinid to species, once you’ve recognized its family, is another, often harder task. But students of the insect world get quite comfortable with tentative or imprecise identifications.)
But despite enormous variation in the size and appearance of tachinids, members of this family share one crucial life-history trait that gives this group its importance: The larvae of tachinids (I’m trying to avoid the word “maggots”!) are parasites on other species of insects. From an egg laid on or near a suitable host, a tachinid larva hatches and, if it’s lucky, succeeds in burrowing into its host. Maturing inside the unfortunate victim, the tachinid develops by eating the living tissue of its host, and when mature, it eats its way out, finishing its maturation in another location but leaving behind a dead host.
Within this enormous family, there are countless variations on this basic theme. Some tachinids are highly host-specific, using just a single species or genus as their larval host. Others are quite ecumenical in their tastes, laying eggs on a wide range of victims. Immature flies use different methods of entering their hosts: some chew their way in, while others may be ingested (even while still in egg form) as their hosts-to-be feed. Many adult tachinids, much more genial than their larvae, visit flowers to eat pollen (helping pollinate the flowers they visit as a side effect).
Because a single female tachinid can lay dozens of eggs, and each egg is a potential assassin, these flies can exert a powerful downward pressure on the populations of their hosts. And a sort of feedback loop can amplify this effect over the course of a few generations: If a host species increases in number, more Tachinid eggs find their target, producing a much larger next generation of flies. Often with just a generation or two, the flies have parasitized their excessively abundant host back down to normal numbers.
While many types of insects (and even, occasionally, other types of arthropods such as millipedes) can host tachinids, as family these flies focus mainly on Lepidoptera — that is, on moths. Many moths have larvae that eat plants of concern to people (crops and ornamentals); and many moths are capable of staging huge population explosions (think of gypsy moths or the winter moths that have helped defoliate large tracts of Vineyard woodland in recent years). Tachinids are not the only things that control such outbreaks, but they can be among the most effective. Without the regulating role of these flies, human agriculture and horticulture would be far more difficult undertakings.
Tachinid flies are sometimes deliberately deployed as “biological controls” of problematic insects such as invasive moths introduced to new regions, away from the effect of the predators of their native range. While a few ecological catastrophes have resulted from well-meaning introductions of tachinids, with the flies decimating desirable species once they’ve brought their original target under control, biologists have learned a great deal from such situations. As more and more invasive insects get established around the world, the use of tachinids to control them will likely be an increasingly frequent human response. One tachinid, Cyzenis albicans, is currently being used (with promising results and no discernible bad effects) to control the invasive winter moth in our region.
I have no real sense of how many tachinids live on the Vineyard — certainly hundreds of species, maybe many hundreds. But members of this family turn up everywhere I go. You’ve probably mistaken some for common house flies. But in doing so, you’ve overlooked an important ally.