Little more than a month ago as I wandered the eastern shore of White Rock Lake on a gorgeous spring morning, I came across a familiar site.

Eastern tent caterpillars (Malacosoma americanum) had invaded a sapling and built a respectable web.  Many of the ravenous beasts crawled along the small limbs while a few meandered about the outside of their silken tent.  The true horde, though, remained inside.

As I took a closer look, I couldn’t help but find myself enthralled with the spectacular beauty of these larvae.  Destructive though they may be, they offer a mesmerizing splash of colors and designs, a singular nod to nature’s fabulous handiwork.

I then forgot about the web and its inhabitants, and during subsequent walks I neither went back to check on them nor looked for additional colonies in other trees and other places.

Then yesterday I took special note of the growing abundance of a large fly, one of which kept visiting me on the patio in early evening.

After some investigation, I discovered this species is known as the friendly fly (a.k.a. government fly or large flesh fly; Sarcophaga aldrichi).  And it’s a predator or, to be more precise, a parasitoid.

Friendly flies are the single most important biological control mechanism for tent caterpillars.  Adult flies deposit live maggots on tent caterpillar cocoons, after which the maggots bore into the cocoons and feed on the pupating larvae.  This means the fly population grows only as the caterpillar population shrinks (or grows, depending on how you look at it), and a large outbreak of tent caterpillars means a subsequent increase in friendly flies.  Or so one would hope.

My fascination with the fly stemmed entirely from my fascination with insects, so it was only later that I realized the buzzing buddy really was a friend.  Non-biting and a pest only insofar as it swarms and lands on anything—like people and food—these large insects annoy us only because their numbers grow in direct relation to the service they provide: control of a defoliating monster that can cause significant damage.

[of special interest is that none of the fly images are macro shots]