This hairy wasp with a yellow spot on each side of the abdomen digs through the soil to lay its lethal spawn on subterranean beetle grubs. White grubs, be very afraid when Scolia dubia comes to the garden.
Although this year has been tough on some charismatic butterflies, it seems to have been a great year for wasps. Earlier this year giant murder hornets made a sensational splash when they were discovered in the Pacific Northwest. And here in the DMV we have visited cow killers hunting bumble bees, great black wasps capturing katydids, cicada killers living up to their name, and tiny Aphidius wasps exiting the rear end of aphids. This week, let’s visit a pair of gorgeous and really cool wasps intent on annihilating dastardly beetle pests in our lawns and gardens.
The beetle pests these wasps focus on include larval stages (a.k.a. grubs) of green June beetles, Japanese beetles, Oriental beetles, Asiatic garden beetles, and other members of the scarab clan that dwell in soil and devour roots of our lawn grasses in addition to those of annual and perennial plants. As adults these beetles, known as scarabs, strip leaves and blossoms of hundreds of species of ornamental plants and wholesome vegetables. Abundant spring and summer rains conducive to grub survival have turned my flower beds into ideal breeding sites for these rascals. But in Mother Nature’s system of checks and balances, it is not unusual to see populations of predators and parasites rise shortly after populations of their prey increase. Over the past week or so, several folks have commented on hordes of dark winged wasps cruising back and forth a foot or so over their turf. Most of these sightings are digger wasps, members of a family known as Scoliidae. The digger wasp moniker stems from the impressive ability of these fierce fliers to locate white grubs beneath the surface of the earth, tunnel through the dirt, deliver a paralyzing sting, and deposit an egg on the skin of the grub. The hapless white grub is incapable of removing the egg, which soon hatches and the parasitic larva of the digger wasp slowly consumes its living victim. Glad I’m not a white grub. After completing its development during summer and autumn, the wasp larva spins a silken cocoon, pupates, and then passes the winter in the burrow created by the grub. Fresh, new wasps emerge as adults the following August.
Whether it’s small white grubs in the soil or zany green June beetle grubs that crawl on their back across the ground, these pests face an awesome grim reaper when Scolia wasps come to my garden. Video credit: Michael J. Raupp
Four yellow spots on the abdomen of Scolia nobilitata make it easy to distinguish from its cousin, Scolia dubia.
Several species of digger wasps can be found in the DMV and two in the limelight this time of year are Scolia dubia and Scolia nobilitata. Commonly known as the blue-winged digger wasp, Scolia dubia sports striking iridescent blue-black wings. Its body is black excepting the end of its fuzzy abdomen, which is reddish brown. Scolia dubia is easily recognized by one pair of bright yellow spots on either side of its abdomen. Its cousin, Scolia nobilitata, has smoky brown wings with two pair of yellow or off-white spots on its abdomen. These colorful beauties need energy to search for grubs and tunnel underground to find their victims. They can often be seen carbo-loading in preparation for the hunt on nectar-rich members of the mint family (Lamiaceae) such as mountain mint and spotted horse mint, and members of the aster family (Asteraceae) such as goldenrod. Lawns and landscape beds rampant with white grubs often attract squadrons of digger wasps flying in tight figure-eight patterns just above the ground, presumably hunting for prey. While they might appear scary, please understand that these wasps are not aggressive towards humans and that they are highly beneficial by virtue of the beat-down they put on white grubs. You can befriend these beneficial wasps by providing nectar sources, mints and asters, in your landscape and thereby invite them to hang around and find some pestiferous white grubs to serve as food for their offspring. As part of Mother Nature’s system of checks and balances, plant some flowers, sit back and relax on a warm autumn day and let them do their work.
Mints and goldenrods bring digger wasps like Scolia dubia to your garden on sunny days in late summer and autumn. After tanking up on energy-rich nectar, they will search for white grubs in your lawn and gardens, dig into the soil, and deposit eggs on hapless grubs. Upon hatching, wasp larvae devour the grubs and thereby help to rid your landscape of these noxious pests. Video credit: Marie Rojas and Michael J. Raupp
Acknowledgements
Bug of the Week thanks Marie Rojas for the awesome video of blue-winged digger wasps nectaring on mountain mint and Dr. Shrewsbury for providing the inspiration for this episode. The wonderful books “Destructive Turfgrass Insects” by Dan Potter and “Bees, Wasps, and Ants” by Eric Grissell were used as references.