Robber Flies at Bruce Pit

Robber Fly (Promachus bastardii)

In 2014 and 2016 I found a small population of Acadian Hairstreaks at the base of the toboggan hill at Bruce Pit. It’s been a while since I’ve been there to look for them, or the Gray Treefrogs that I once found scattered on milkweed leaves on a hot August day in 2016, but with hairstreaks now flying I thought it would be a great time to go look for them – especially after finding my lifer Striped Hairstreak at South March Highlands earlier this morning (more on that to follow in a separate post). Hairstreaks are small butterflies with plain brown or gray wings that contain a multitude of colourful blue or coral spots when viewed up close. They always perch with their wings closed, often sit out in the open on leaves or branches between one and five feet above the ground, and are among my favourite local butterflies.

As soon as I arrived I headed to the bottom of the hill to look for the butterflies…and was disappointed to see very few wildflowers. When I had last visited this part of Bruce Pit in July 2016, the edge of the lawn was covered in a large swath of the pink, purple, and yellow flowers of Common Milkweed, Purple Cow Vetch, and Bird’s-foot Trefoil; now there were a few flowers scattered along the margins of the marsh, but it was a far cry from the rampant wildflower meadow I remembered from three years ago. Needless to say, I found no hairstreaks and no treefrogs.

However, I found something almost as interesting: a cooperative Robber Fly sitting on a leaf in the sunshine. These predators are often found in sunny forest openings and in open meadows where they wait for prey to fly by, then attack and feed on them. This was the first time I can recall seeing this particular species, which I would later learn is called Promachus bastardii, although it is common in the eastern half of the continent.

Robber Fly (Promachus bastardii)

A little later, on my way to look for the treefrogs in the milkweed meadow at the back of the off-leash area, I came across two more individuals mating.

Mating Robber Flies

I did not realize how abundant they were until I found even more in an open area at the back of the pond. There I found the males with their distinct white tufts of setae at the tip of the abdomen – females lack these tufts. They feed chiefly on flying insects, and while the majority of their prey consists of bees (which make up almost 60% of their diet), they may also eat beetles, other flies of the Order Diptera, and true bugs of the Order Hemiptera. Prey are captured in flight, and maneuvered into position by the Robber Fly’s legs until it finds a suitable place to insert its proboscis – usually behind the head.

Robber Fly (Promachus bastardii)

Although common names are not common among Robber Flies, this species is sometimes known as the False Bee-Killer.

Robber Fly (Promachus bastardii)

A second species was found nearby – Laphria flavicollis, closely related to the species I had found at the cottage in Prince Edward County earlier this month. However, while Laphria flavicollis has a yellow thorax much like Laphria thoracica, the mustache-like hairs below the eyes are yellow and the hairs on the abdomen are all black.

Robber Fly (Laphria flavicollis)

In addition to the robber flies, I found a couple of interesting moths while searching the milkweed field at the back as well- the first was this tiny One-lined Sparganothis Moth (Sparganothis unifasciana), a member of the family Tortricidae.

One-lined Sparganothis Moth

The second was a geometer moth that tucked itself beneath a leaf. It looks similar to many of the small whitish moths I see in the woods, so I was surprised when someone on iNaturalist provided an identification. Apparently this is a Small Engrailed (Ectropis crepuscularia), a common moth whose larvae feed on a number of different tree species.

Small Engrailed

The best lepidopteran of the day was this Gray Comma which flew in a couple of circles before landing on the gravel right next to me. The single black dot on the hindwings help to distinguish this from the Eastern Comma.

Gray Comma

It was great to see such a variety of insects at Bruce Pit, even if the critters that I really wanted to see were absent. This is why multiple visits throughout the summer are necessary – different species emerge at different times, and weather often plays a big part in both timing of emergence, and what might be around on any given visit. I’ll just have to visit again and hope that I’ll have better luck next time!


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