Wasp Wednesday: More on Isodontia

Last week I showed a short video introducing you to a puzzling phenomenon in suburban areas of eastern North America. A quick review: Some homeowners in the U.S. and Canada, upon sliding open their windows, have been startled to discover the tracks filled with dry grass, dead insects, and live “worms.” Even the odd wasp has flown out. What is going on here?

Certain solitary wasps called “grass-carriers,” genus Isodontia, seem to have found window tracks make ideal nesting sites. Each female wasp makes her own nest, usually a series of cells along the length of the tunnel (though one species forms a communal brood chamber much like a bird nest).

After selecting a site, she goes in search of tree crickets or small katydids which she paralyzes with her sting and carts back to the nest. Once a cell is filled with victims, she lays an egg, and makes a partition of dry grass. Then she begins the provisioning process again, until the tunnel is filled with cells. She finishes the nest by plugging the entrance with dry grass, such that it looks as if someone has shoved a tiny broom into the hole, handle-first. Her job complete, she leaves permanently.

The larva that hatches from each egg grows by consuming the fresh prey insects. These are the “worms” that people find in the window tracks. After finishing its meals, the larva then enters the pupal stage, and an adult wasp emerges later (the following summer in northern climates).

Grass-carriers are not pests, and not aggressive towards people or pets. You will not get stung unless you physically grab a female. Males have no stinger at all.

My initial theory was that the wasps much prefer using natural cavities in dead wood, but few such natural resources exist in suburban settings. My revised theory is that the wasps may find window tracks to be a superior nesting site. Perhaps the space is “roomier” than conventional natural cavities. Maybe the parasitic flies and wasps that plague Isodontia have not yet caught on to the new nesting strategy, therefore giving Isodontia an advantage that yields more success in terms of offspring reared to maturity.

Please tolerate these wasps if you can, they are fascinating to watch. Otherwise, simply flushing the wasp and cleaning the track should discourage her from trying again.

Meanwhile, look for the adult wasps on wildflowers, especially sweetclover, sumac, and grape. They will also frequent aphid colonies, lapping up the “honeydew” secreted by the sap-sucking pests. Notice that these wasps usually have the wings splayed at rest, in contrast to other sphecid wasps that habitually fold their wings flat over their abdomen while sipping nectar.

The species shown in the images accompanying this article is Isodontia mexicana, very common over most of the eastern U.S. There are also five other species occurring in North America.