Thursday, July 27, 2017

Chalcid wasps in homes

After identifying an unusual insect for a homeowner today, the thank you email ended with a bang. Because I was able to quickly identify her pest, which her PMP had incorrectly insisted was a "bee", she concluded, "[I guess] it's best I change pest control companies."

Brachymeria podagrica on window screen
Could you identify this insect from this picture? Brachymeria
 is a chalcid wasp parasitoid that attacks filth flies,
like those that feed on carrion.
Ouch... I hate to hear that.

Admittedly the insect was an obscure critter. I'm guessing that not one in 100 PMPs has ever heard of a chalcid (CHAL sid) wasp before. But chalcid wasps are common natural enemies of many insect pests. Identified by their small size and giant hind femurs, the Chalcididae family makes up one of the dozen or so "parasitoid" wasp families within the bee/wasp/ant order Hymenoptera.

Parasitoid wasps are certainly one of the most fascinating and wonderful, yet horrifying, of all creatures.  So seemingly cruel in its behavior that theologians and biologists argued over the last 200 years whether the mere existence of insects like the ichneumon wasp (a cousin of the chalcid wasp) served as proof against the Christian belief in a loving Creator-God.*

Parasitoids are parasite-like predators. Like a parasite, they grow up feeding on or in a single host. But unlike true parasites, which weaken but rarely kill, parasitoids invariably kill their hosts. Parasitoids begin their lives as an egg laid by their mother on a soft part of a host's anatomy. Upon hatching the parasitoid larva burrows into the body cavity of its host and begins feeding. The larva knows instinctively to begin with the non-essential parts, prolonging the life of its victim as long as possible. Eaten alive from the inside, ultimately the host dies. Ugh.

It does sound cruel, but parasitoids are also one of nature's most effective population control agents. Without them, crops would vanish under billions of caterpillars.  Flies would breed unchecked. Even spiders would be more abundant than they already are.  Parasitoid wasps possess some of the world's sharpest "noses" (actually antennae), able to sniff out prey even when the prey are vanishingly rare. They are also smart, with some species recently being trained to sniff out illicit drugs and even bombs on the battlefield. Gardeners and farmers, especially, reap the benefits of parasitoid services every day.

The key to identifying chalcid wasps is their tiny size (this is one 
of  the larger chalcids at 5 mm), reduced veination in the wings, 
and swollen hind femurs. This Brachymeria podagrica is further 
identified by its distinctive markings. Photo courtesy Graham
Montgomery via
Admittedly we in structural pest control don't have many chances to encounter parasitic insects in our daily work.  Most parasitoid wasps live peacefully out of sight in the natural world, ill at ease in our indoor environments. Occasionally, though, parasitoid wasps make an appearance in a home or business. For this reason, it's a good idea for PMPs to know something about these insects.

The species of chalcid wasp my homeowner encountered "swarming" in her attic this spring appeared identical to other similar wasp pictures I've received recently.  These turned out to be Brachymeria podagricaa parasitoid (primarily) of flies.  Their presence indoors suggests that the source could have been a dead animal full of fly larvae somewhere in the home--a theory backed up in this case by the homeowner's report of a foul stench several days before the little wasps appeared in the attic. Likely they were drawn to the smell of the carcass in search of their blow fly hosts.

When one, or a few, unusual insects show up overnight in a structure, they are often called "accidental invaders". Accidental invaders are chance occurrences, when an insect or spider accidentally enters through an open door or window, or unsealed crack. Such accidental entries occur on a regular basis in residential accounts, but usually with a variety of arthropods.  But when several (or dozens) of the same kind of insect appear inside a home, or when the same insects show up over many days, it usually means something is afoot. Insects always have a story to tell, and they never lie.

Chalcid wasps are not likely to enter an account over and over by accident. If you find chalcids indoors, get a sample and have them identified. Brachymeria podagrica suggests the possibility of wildlife or rodents; however other species of Brachymeria and other species of chalcids are known to parasitize beetle or moth larvae, and might be evidence of a stored product pest infestation.

And remember, if you're ever unsure of the identification of an insect, don't hesitate to bring it to your in-house entomologist (if you have one), or send to your state university or other reputable insect ID authority. And don't just call something strange a "bee" unless you know for sure that it is.

* An interesting discussion of the ichneumon wasp controversy can be found in Stephen Jay Gould's essay on Non-moral Nature in the book Hen's Teeth and Horse's Toes

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