That Time of Year Again – Bees Beware

To every thing there is a season…


On several of my wanderings these past few days, I have heard noticeable buzzing sounds that indicate an acceleration and flyby of a large winged insect. I recognize these sounds to be from a fascinating group, the robber flies, family Asilidae. I remembered posting about them last year and when I looked it up, it was almost exactly the same week last summer when I started seeing these amazing aerial predators in the yard.

robber fly with honey bee looking from side

Robber fly with honey bee prey (click photos to enlarge)

And, once again, the first one I saw with a prey item last week had managed to capture a honey bee. Their preference for bees is one reason this particular species is also called the false bee-killer (although not really so false).

robber fly with honey bee

Close up of a killer

A closer look reveals some of the adaptations that make robber flies so efficient at catching their prey, which, by the way, they almost always do while on the wing. They have huge eyes for spotting flying insects; large wings powered by strong muscles in the humped thorax; and long spiky legs that help them maintain a grip on something once they have grabbed it in mid-air.

robber fly with honey bee looking from above

Face to face with a fierce predator

This one did what they all do after catching something – flew to a perch to start consuming its prey shortly after capture. Robber flies pierce and inject their victims with toxins that immobilize the prey and begin to liquefy them. They then fly to a nearby perch and begin to imbibe on the internal soup of their quarry.

This time, however, the meal was interrupted. What had drawn me to this particular perched fly was an intense buzzing sound, not made by this fly, but by a male robber fly hovering nearby. The male had the distinctive white patch at the tail tip I had seen last summer that allowed me to identify them as Promachus bastardii, which Bug Guide calls the Big Robber. Turns out, this loud, stationary buzzing is a prelude to mating. And, sure enough, the male waited for just the right moment and then jumped on the larger, feeding female.

mating robber flies

Mating robber flies

The act didn’t last long as I heard another buzzing sound and saw another male hovering nearby…the lady has two suitors. The first male buzzed off and the other male followed, so perhaps a territorial duel ensued elsewhere in the yard.

female resuming her meal

Female continued feeding

The female, meanwhile, continued feeding. That is, until I accidentally brushed her perch while trying to lean in closer for a better view. She immediately buzzed by my head, carrying her unfinished lunch to a less crowded perch. She will eventually lay eggs at the base of  plant or in soft soil or rotting wood. The larvae are rarely seen, but resemble odd worm-like creatures living in soil and soft wood where they consume organic matter and start their predatory career by capturing soft-bodied prey around them. Larvae pupate in the soil and emerge next year, or perhaps a year or more later. I guess I will need to be on the lookout for some egg-laying behavior and see if I can’t find a larva. In the meantime, I’ll listen for that buzzing sound and see what’s for dinner.


Pollinator Peril

In summer the empire of insects spreads.

~Adam Zagajewski

Last week I noticed a medium-sized robber fly noisily flitting about the garden. It landed on a cedar twig on the garden fence and I leaned in for a closer look.

Promachus bastardii

Robber Fly with distinctive terminal tufts (click photos to enlarge)

It was distinctive in the bright white patch near the tail tip, something I noticed from a few feet away. As I leaned in I could see the white was actually a pair of tufts of “hairs”. I figured this might be an easy one to identify so off I went for a look in Bug Guide. Sure enough, the white tufts are diagnostic of a couple of species. After reading more about the ranges and other characteristics, I decided this was a male Promachus bastardii.

Promachus bastardii tail

Promachus bastardii abdomen

Males of this species are characterized by white tufts of hairs along the edges of the abdominal segments, black on the dorsal surface of the abdomen, and the distinctive white tufts at the tip of the abdomen.

Promachus bastardii head

Promachus bastardii head and thorax

Robber flies have several features that set them apart from other flies…large size; eyes set wide apart for better depth perception; the hairy face or the mustache of bristles called the mystax (theorized to protect the eyes during struggles with large prey); the arching thorax containing superior flight muscles (an advantage for rapid lift and flight to snag fast-flying prey out of the air); a sharp beak, partially hidden by the mystax, which is used to stab their victims, injecting toxins and digestive fluids, which allow the predator to kill the prey and suck the juices out; and long, spiky legs with what look like talons at the tips for catching and holding their prey. This is one of the so-called Giant Robber Flies, due to the group’s large size. This species is actually one of the medium-large ones in the group, and this male (which is smaller than a female) was a little over an inch in total length. I found only one reference to a common name for this species, Bee Killer, for their habit of preying on various species of Hymenoptera. The scientific name immediately catches your attention, and translates to “the bastard’s champion”. Since the species was named in 1838, it is probably lost as to the reason, but I am curious. Perhaps it was named by a fan of bees.

Promachus bastardii 1

Bee Killer with prey, a bumblebee

A few days later while strolling in the yard, camera in hand, I heard a loud buzzing noise behind me and turned to find the same species of Robber Fly (perhaps the same individual male) subduing a prey item, a small species of bumblebee. These bees are common at my wildflowers, especially the Rosinweed (Silphium sp.) that is blooming now along the driveway. The fly had already stuck its beak into the bee, which soon quit struggling.

Promachus bastardii 2

Bee Killer is aptly named

Watching this drama unfold, I admired the strength and apparent agility that is needed to catch and subdue this type of prey. And I was thankful this guy is not two feet long…few creatures in these woods would be safe from such a aerial predator.


Tiger Swallowtail in spider web

Eastern Tiger Swallowtail in spider web

After writing this post, I found another peril for pollinators in this jungle of a yard. An orb weaver spider had strung a giant web across a main pathway through the yard and snagged one of the Eastern Tiger Swallowtail butterflies that have been feeding on the Joe-Pye Weed that is now blooming. A jungle indeed.