Bee happy, they’re flower lovers

Bee wolves love nectar, prey on bees.

The wasp genus Philanthus is blessed with an appealing name: it means “flower lover” and refers to the fondness members of the genus have for feeding on flowers. They’re not unique in this, of course: Pollen and nectar from flowers are crucial food resources for an astonishingly wide range of insects. But flowers are indeed a likely place to find Philanthus.

“Flower lover.” It sounds so nice! Cue the pastoral music, and adorable video clips of happy pollinators puttering around on colorful blossoms.

Then suddenly, wham! The blissful scene vaporizes as a seemingly benign female Philanthus suddenly abandons her pollen eating, pounces on a bee, paralyzes it with sting to the underside of the thorax, wraps her victim up amid her spiny legs, and flies off for parts unknown.

The common name for the genus Philanthus, it turns out, is “bee wolf,” and it’s an apt sobriquet for these black-and-yellow wasps. Adults, it’s true, love flowers and feed on pollen. But larval Philanthus are carnivorous, developing in burrows dug by adult females and provisioned with paralyzed bees for the wasp larvae to eat.

A fair-size genus, Philanthus is represented by something like 140 species worldwide, perhaps a third of which occur in North America. So far, I’ve only found two species on the Vineyard, though ID of this genus is not always easy, and it is likely that I’ve overlooked a few. Wherever they occur, though, Philanthus species exhibit the split personality of being benign nectar lovers and, at the same time, aggressive predators of bees.

The nest-stocking habit of Philanthus, while it sounds bizarre to humans, is actually very common among wasps. The entire family this genus belongs to — Crabronidae — practices something similar, though the type of prey taken varies according to what genus or species of wasp. And the closely related family Sphecidea does the same. Another family, distantly related, captures spiders. From grasshoppers to beetles to leaf hoppers to cicadas, almost any kind of insect has a group of wasps that prey on it.

Philanthus gibbosus appears to be fairly common on the Vineyard, and is surely our most common and widespread Philanthus. I’ve found it on flowers at many locations (it’s common in my yard in Oak Bluffs), and seen nesting females at multiple spots on the Vineyard sandplain. P. gibbosus is mostly black, and sports yellow spots and abdominal banding that are pretty standard among wasps of many kinds. Perhaps the most helpful mark for identifying this wasp is a deeply dimpled exoskeleton, not always evident in the field but obvious in a decent photograph.

Like other members of the genus, the face of P. gibbosus shows a particular pattern of yellow spots. These are difficult to see accurately in the field, but even a poor head-on photograph often shows the pattern well enough to support an identification. Other useful ID cues include the pattern of yellow spotting on the thorax and the extent of banding on the abdomen. Like most wasps, members of this genus have little interest in humans, and don’t seem to mind being observed even at very close range.

Nest burrows of Philanthus can extend two or three feet underground, a depth that protects nests from both predation and extremes of temperature and wetness. It follows that these wasps are connoisseurs of soil composition, seeking out places that feature soil light enough and loose enough for easy excavation, but stable enough so that the lengthy burrows don’t collapse. Most of the nests I’ve found have been on lightly used dirt roadways in sandy areas; a little vehicular traffic may compact the soil just enough to stabilize it. Other observers have reported a preference in this genus for nesting in areas with deep-rooted herbaceous plants; again, the root systems may help stabilize the soil.

Female Philanthus wasps reportedly sleep inside their burrows, plugging the entrance from the inside with sand for protection as they snooze. I think they also may plug their nest burrows temporarily while they’re out hunting — anyway, some of the females that I’ve observed bringing prey back to their nests have had to relocate their burrows and do some digging to open the entrance. While digging, they resemble tiny terriers: they use their mandibles and front legs to loosen soil and kick out behind them, and once they get their momentum up, they throw an impressive jet of ejected soil out from between their legs.

Males also dig burrows to shelter in, but of course do not lay eggs and therefore do not collect prey. As is the case universally among bees and wasps, males also cannot sting. They are slightly smaller than females of their species, and reportedly use pheromones — specialized scent chemicals — to mark territories and attract females.

Given the (well-founded) concerns over declines in pollinator populations, the bee wolves may seem like undesirable insects. But recollect that they function as pollinators themselves, just like the bees they prey on. Philanthus is an important cog in the complex web of ecological relationships that surrounds us, often unnoticed and unobserved.