In a previous episode of Bug of the Week, we met the dastardly Japanese beetle and its cousin the Asiatic garden beetle as they waged war on trees and shrubs in the landscape. Abundant summer rains in recent years have been conducive to the survival of larvae of these and other scarab beetles. These beetle larvae go by the name of white grubs. But in Mother Nature’s system of checks and balances, it is not unusual to see populations of predators rise shortly after populations of their prey increase.
White grubs like these may soon become victims of the blue-winged digger wasp.
Over the past few weeks several people have commented on hordes of dark winged wasps cruising back and forth a foot or so over their turf. Most of these sightings are of digger wasps, members of the wasp clan known as Scoliidae. The digger wasps 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. Yikes, these wasps make Hannibal Lecter look like Mother Theresa! After completing its development during summer and autumn, the wasp larva spins a cocoon of silk, pupates, and then passes the winter in the burrow created by the white grub. Fresh, new wasps emerge as adults the following August.
Several species of digger wasps can be found in the Washington metropolitan area, but the one in the limelight over the past several weeks is the beautiful Scolia dubia, often seen flying in a figure eight pattern just above the ground. This fairly large species is ~ ¾” in length with iridescent blue-black wings. Its body is black except for the end of its fuzzy abdomen, which is reddish brown. Scolia dubia is recognized by two bright yellow spots on either side of its abdomen. When not hunting for white grubs, adults can be seen foraging for nectar in wildflowers such as goldenrod and horsemint.
Spotted horsemint is a favorite nectar source for Scolia dubia in my garden.
According to the literature, these highly beneficial insects attack a variety of white grubs including green June beetle, Japanese beetle, and May and June beetles. Their presence above your lawn indicates that white grubs lie beneath your turf and you might want to keep an eye out for turf damage in the future. But 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. They are part of Mother Nature’s system of checks and balances in your landscape, so let them do their work.
Bug of the Week thanks Joann Pettinicchio for the wonderful image of Scolia dubia and Dr. Shrewsbury for providing the inspiration for this episode. The wonderful book “Destructive Turfgrass Insects” by Dan Potter was used as a reference, as were the following web sites: