By: Gerry Wegner Ph.D.
All too often cicada killers (Sphecius species) get the lion’s share of attention when the subject of digging wasps comes up. Yes, they’re large and impressive; but they’re not the only digging wasps worthy of notice by pest management professionals (PMPs).
A number of solitary wasp and bee species are known for excavating burrows and galleries in soil and sand in which to rear their young. Among these are the Sphex digger wasps, Bembix sand wasps and Bicyrtes sand wasps.
The aforementioned wasps are not social wasps like yellowjackets or hornets (Family Vespidae), but populations of a given species appear to be gregarious in that they nest in close proximity in favored habitats.
It’s not uncommon for as little as a few to as many as several dozen females to excavate burrows only inches apart in sand and sandy soil. Prime nesting habitats include dunes, golf course bunkers, gaps between garden and patio pavers, playgrounds, sand boxes and piles, waste water leach beds and bare areas of lawn in which the soil is high in sand content.
Sphex Digger Wasps
Sphex digger wasps (Hymenoptera: Sphecidae) tend to capture the attention of those who pass within several feet of nesting sites, due to their large size and formidable appearance. Wasps of this genus have long jaws and the abdomen has a distinct stalk-like waist. Despite their size — 1 to 1½ inches long — and intimidating nesting site activities, these wasps tend not to be aggressive towards humans. Stings are rare, although the wasps will hover and display alarm when their burrow entrances are being disturbed.
Although several species of Sphex digger wasps occur in North America north of Mexico, two species in particular have generated enough interest to warrant being assigned common names — Sphex ichneumoneus, the great golden digger wasp, and S. pensylvanicus, the great black wasp.
The great golden digger wasp can be found across the continental United States. It has a dense covering of golden hairs on the black head and thorax. The legs and forward half of the abdomen are reddish-orange, while the posterior half of the abdomen is black. The wings are black with a bluish tint.
If entrances to the burrows of this wasp are disturbed superficially, the females will locate and re-excavate the openings within minutes of returning to their nests. The vertical burrows descend several inches into the ground and several brood chambers or cells branch outward from near the deepest point.
The great black wasp, as its name suggests, is jet black and nearly 1½ inches long. The wings are bluish black. It occurs nationwide and has been observed to use existing subterranean cavities for its nests, in addition to those they excavate themselves.
For example, S. pensylvanicus nest entrances have been found at the bases of window wells and the paved rims of storm sewer catch basins.
Three other Sphex species that may be encountered by PMPs are:
1) S. habenus, a mostly black wasp with golden hairs on the head and thorax and smoky-tinted wings with a reddish to yellowish tint near the shoulders, inhabiting the southeastern U.S.,
2) S. nudus, having a black body with patches of short white hairs on the thorax, black and yellow (or red) legs and black wings, inhabiting the eastern U.S., and
3) S. jamaicensis, looking similar to S. ichneumonius except for having a completely red abdomen, found in Florida and the Caribbean.
Sphex digger wasp females excavate their burrows in early summer and then spend much of their time searching for prey for their larvae. Long-horned grasshoppers and katydids (Orthoptera: Tettigoniidae) are the preferred larval food. Once stung and paralyzed by the wasps, the grasshoppers are placed into the brood chambers and eggs are deposited on them, one egg per chamber.
Two to four grasshoppers are required for complete development of each larva. Following pupation, the new adults emerge from their cells between late July and mid-August in the northern latitudes, where there is one generation per year. Adults frequent nectar-bearing blossoms for their sustenance.
Bembix Sand Wasps
Bembix sand wasps (Hymenoptera: Sphecidae or Crabronidae, depending on reference) are represented in North America north of Mexico by nineteen species. Bembix species are mostly black with yellow or white abdominal bands — one pair of curved bands per segment that touch, or nearly touch, along the dorsal midline of the abdomen. Typically, the eyes are green and the legs mostly yellow. There may be yellow markings on the thorax also.
North American Bembix wasps measure ½ – ¾ inch in length, depending on species and sex (females are larger than males). The eastern sand wasp, B. americana, is the most photographed representative of this genus in the United States.
Female wasps capture flies with which to feed their larvae. The brood chamber portion of the burrow is provisioned with numerous flies for the development of each larva. Often the males (which, of course, cannot sting) fly back and forth, low to the ground, within several feet of the burrow entrances. Numerous males in flight over the preferred sandy nesting sites sometimes present an intimidating situation to the general public.
Bicyrtes Sand Wasps
Bicyrtes sand wasps (Hymenoptera: Sphecidae or Crabronidae, depending on reference) are similar to the Bembixsand wasps in size, anatomy and behavior. However, Bicyrtes sand wasps have the paired bands pointed or tapered and not touching along the dorsal midline of the abdomen; also, the eyes may be greenish, brown or black in color.
Six species occur in North America north of Mexico but the most encountered species nationwide is B. quadrifasciatus. As is the case with most other wasps of interest to pest management professionals, Bicyrtes wasps satisfy most of their nutritional needs from nectar sipped from blossoms.
IPM Measures
Since digger and sand wasps are beneficial and not aggressive, control should be implemented only when required by clients. Customers who are frightened by the mere presence of wasps near their residences should avoid planting nectar-bearing flowers and flowering shrubs close to entrances, porches, decks and sidewalks. Sandboxes should be kept covered when not in use.
Some lawn and planting bed situations likely to be favored as nesting sites for digger wasps can be sodded, re-seeded, mulched or covered with porous weed barrier, as appropriate, as a preemptive measure.
For sensitive situations, such as playgrounds and sandboxes where children will return to play, burrows can be injected with an approved residual botanical insecticide dust or wettable powder or the nesting area can be worked with a soil/sand rake to a depth of 3 to 4 inches, then drenched with an insecticidal soap formulation and covered with a tarpaulin for a period of 7 to 14 days.
In most non-sensitive situations, burrows can be injected with an approved residual insecticide dust or microencapsulated, wettable powder or suspended concentrate formulation.
Likewise, paved garden walkways can be treated with an appropriately labeled residual liquid insecticide formulation in the spring to prevent wasps from burrowing into bare, sandy soil.
You can reach Wegner, technical director and staff entomologist for Varment Guard, at 614-794-8169 or e-mailgerry.wegner@varmentguard.com.
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