Wasp Wednesday: Beewolves

I was mistaken the first time I thought I saw a beewolf. It turned out that I was seeing a cuckoo bee in the genus Nomada. The real deal, the beewolves of the wasp genus Philanthus, are truly complex for solitary wasps. The most common species in North America is P. gibbosus, found from the Atlantic to the Pacific, and Canada to Mexico.

The European beewolf, Philanthus triangulum, which preys on honeybees, gained fame as one of the research subjects of Nikolaas Tinbergen, a Dutch animal behaviorist (ethologist) who shared the 1973 Nobel Prize in Physiology or Medicine. Tinbergen coined the name “beewolf,” and demonstrated that the female wasp was able to locate the buried entrance to her burrow by using subtle landmarks as cues. Our P. gibbosus does so as well. Meanwhile, we can’t remember where we parked our car.

A single female P. gibbosus usually excavates in bare, coarse sand, sandy-clay, or sandy loam soil, often removing small pebbles as she goes. A typical burrow extends for 15-24 centimeters, beginning as an oblique tunnel that eventually becomes horizontal. Some burrows may reach a length of a meter or more, but these may represent expansion of an existing nest by the succeeding generation. Individual cells at the end of very short tunnels radiate from the main burrow along its length.

Beewolves earn that name. The females target small bees, and even other wasps, as food for their larval offspring. Not all beewolves you see around flowers are there for nectar. The females will actively stalk bees that are busy gathering nectar and pollen themselves. A distracted bee could be a dead (well, paralyzed) bee.

Philanthus gibbosus is a well-studied insect, and has been recorded taking 35 species of bees and wasps as prey. The majority of victims are “sweat bees” in the family Halictidae, but also yellow-faced bees in the family Colletidae; and a couple members of the family Andrenidae. There are also records of aphid wasps in the family Crabronidae, genus Pemphredon falling prey.

A victim is stung immediately between its front legs, disabling a nerve center and rendering the bee paralyzed. The wasp then carries the bee beneath it, held in the wasp’s middle legs. It takes several bees to feed one larval beewolf wasp.

Male beewolves are highly territorial, perching on low twigs or leaves where they can intercept a female or chase off a competing male. Males also scent mark twigs and foliage, employing brushes of hairs on the underside of the abdomen to “paint” an odor that communicates their individual ownership of a small area. Their possession of a territory is usually short-lived, however, as competing males frequently displace resident individuals.

Interestingly, Philanthus gibbosus is known to engage in burrow sharing, whereby sibling females may occupy their birth nest for a short time before dispersing. There is also evidence that sibling females may expand their birth nest for at least one generation, depending on the condition of the burrow. This tendency toward sociality might be more of a population-level phenomenon than a behavior typical of the entire species.

One or more males may also spend nights, and periods of inclement weather, in the burrows of females. Males tend to be return to the same burrow each night.

Invincible as they might appear, beewolves are not immune to their own enemies. P. gibbosus is plagued by a host of adversaries, especially “satellite flies” in the genera Metopia, Senotainia, and Hilarella. The female flies follow female beewolves back to their nests, then deposit larvae at the lip of the entrance. The larvae crawl down the tunnel and become parasites of the larval wasps. The velvet ant Dasymutilla nigripes is likely a parasite as well; and the cuckoo wasp Hedychrydium dimidiatum is another suspect parasite. Adult beewolves can be killed by crab spiders lurking in flowers, and by robber flies that take advantage of the slow flight of prey-laden female wasps.

Beewolves are members of the family Crabronidae, and the subfamily Philanthinae. Worldwide there are about 136 species, about 30 of which occur in North America. The majority are rather diminutive insects, only ten millimeters or less in body length. They usually sport ornate patterns of black and yellow or white, the males with all-pale faces and no “tarsal rake,” the spines on the front legs that help the females dig.

Source: Evans, Howard E. and Kevin M. O’Neill. 1988. The Natural History and Behavior of North American Beewolves. Ithaca, NY: Comstock Publishing Associates (Cornell University Press). 278 pp.