Beelike flies hunt hosts for their eggs

By Jeff Mitton

I find myself ineluctably drawn to the flowers of thistles and milkweeds.

The flowers are large, bright, and showy, but more than that they usually attract a host of insects, including butterflies, scarab beetles, longhorn beetles and assassin bugs. Several times, on both thistles and milkweeds, I have found beelike tachinid flies.

When I first encountered them I thought they were bees. They are about the size of a honeybee, are relatively clumsy and their abdomens are orange and black. But closer inspection revealed that the surface of the thorax is orange, that it has a light layer of short golden hairs and a series of long, stout bristles arranged in transverse rows. The legs are long and the mouthparts are not those of a bee. The yellow palps form a long pole that projects straight forward, while the black proboscis is a hollow tube that sticks straight down.

The beelike tachinid fly, Adejeania vexatrix, is a native to western North America and is distributed from Mexico to British Columbia.

It is not abundant, but neither is it uncommon. But I was unable to find any papers written about its life cycle, or behavior, or its hosts. An expert on this group of flies confirmed that this species was essentially unstudied.

Fortunately, the beelike fly is a member of a family of more than 1,500 species and, because some have substantial impacts on agricultural systems, we know a fair amount about them.

All species in the family Tachinidae are endoparasitoids, meaning that the larvae grow within a host, eating it from the inside and ultimately killing it.

Females of some species have sharp ovipositors so they can inject an egg into a caterpillar. Other species glue their eggs to the outside of a caterpillar or an adult beetle. When the egg hatches to release the larva, it drills into its host and begins eating. Other species deposit eggs near hosts so the larva must pursue its host and then gain entry. All hosts are insects and the most common hosts are caterpillars of butterflies and moths, larval and adult beetles, and grasshoppers and crickets.

Three larval stages are passed within the host, which is moribund or dead when the third instar larva or maggot bursts out of its host (did you see the movie "Alien"?) and drops to the ground to pupate in the soil. Some species have one generation per year, others have several.

Adults drink nectar from a variety of species of flowers and they hunt for insects to lay eggs in, on or near. Because an average female lays 1,000 to 2,000 eggs, a female needs to find many appropriate hosts.

Some tachinid species can detect pheromones or chemical signals and they follow these scents to locate their hosts. So some unsuspecting insects release pheromones to attract mates but are attacked instead by tachinid flies; all is fair in love and war.

Each time you see a beelike tachinid fly remember that a butterfly, moth, beetle, cricket or grasshopper died nourishing it. Some people believe that natural means humane, but it isn't always so.

Jeff Mitton, mitton@colorado.edu, is chairman of the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Feb. 19, 2010