According to my friend Edna, black fly season has begun. Edna is one of the fine folks who participate in our local Bti Program, and it is through the efforts of her team, and other Bti teams around the Park, that our black fly populations are greatly reduced.
A couple weeks ago I stopped to talk to Edna as she was roping off the muddier parts of her driveway. I wanted to give her photographs of our “new” beaver pond because the Little Sucker Brook, which is now Little Sucker Pond, is one of her treatment sites. She told me she hadn’t been in there yet, but the test sites she had visited were still full of larvae in suspended animation – it was too soon to treat.
Last week I stopped by again while she was raking her front lawn. “Have you been in to the beaver pond yet?” I asked.
“No – I’ve been too busy treating other streams,” she replied.
“Really? I thought you said the larvae weren’t active yet.”
“They are now, and in huge numbers.”
It seems that the early spring-like conditions have warmed the streams enough to wake up the dormant larvae and before long we will be needing our bug shirts.
Black fly season usually doesn’t get into full swing until late May or early June around here, which is why all the locals are busily raking lawns and sweeping the winter’s sand into piles as soon as the last pile of snow vanishes, and sometimes before. They all want to get essential yard work completed before the black horde descends.
Birders have learned to tolerate the biting insects (adult females, who seek a blood meal before laying their eggs), donning bug shirts and bug dope, and even early spring wildflower enthusiasts have learned to tolerate the little buggers, within reason. But the insects are persistent, known to follow potential prey in large swarms. This can be a problem for tourism in the North Country, and in some extreme northern areas even the livestock suffers, leading to decreased meat or milk production, and sometimes even death.
With all this negative baggage, it seems that black flies need some good PR. A friend of mine commented that black flies had better have some sort of benefit, because otherwise they were just the Spawn of the Devil. So, I decided to find the silver lining of the black fly cloud.
Let’s begin with the larva (the little black squiggles on the leaf in the photo). Really and truly, it is a rather beautiful, and fascinating, creature. From its mouth it spins a silken thread which it forms into a sticky pad on some underwater surface. The only requirement for this surface is that it be in moving water (slow or fast, depending on the species), and that it be solid and stable. Rocks, woody debris, canoes wrapped around rocks…it’s all the same to the larva, as long as the surface isn’t slippery. Then the larva grasps this pad with a ring of hooks that are located on its nether regions. Thus established, the larva hangs on and thrusts its head into the flowing waters, extending two fan-like structures from its mouth that filter edibles from the current. Edibles for a black fly include very fine detritus, algae and bacteria. In other words, black fly larvae help keep our streams clean and healthy.
The days pass in a filtering oblivion. Perhaps the larva has relocated, moving slowly by alternately grasping the substrate with its ring of hooks and the fleshy proleg located on its thorax (near the head). Kind of like an inchworm. Should the current sweep it away, it spins out a silken life line and usually manages to get back to safety.
When it is time to pupate, the larva covers itself in a rigid cocoon, sort of like a sack, with the opening facing down stream. From this opening it sticks out its head and thorax. Attached to the thorax are two feathery tufts – gills, which ripple in the current and extract enough dissolved oxygen to keep the pupa alive.
More time goes by, and with the magic that only Mother Nature can wield, the pupa turns into an adult. The newly developed adult crawls out of the pupal case and, wrapped in a bubble, it floats to the surface and flies away. Some species of blackflies have been known to fly 80 km or more from their hatching grounds! And now the trouble starts, for the adults have only one thing on their minds: reproduction. And this means the female needs her blood meals.
We should count our blessings, really, because here in the North our species of black flies only produce one generation per year. Why, we could be living in the South, where several generations spring up each year, like rabbits. On the other hand, most of those species seem to prefer feeding on birds and non-human mammals. There’s always a trade-off.
And so, to save our tourism, to help out birders and botanists, to alleviate the nuisance that is The Black Fly Season, we now have The Bti Squads. Bti stands for Bacillus thuringiensis israelensis, a naturally occurring soil bacterium that was first discovered in Israel (hence the name). Studies showed that it was highly toxic to the larvae of mosquitoes and black flies, insects that in some parts of the world are vectors for many diseases.
Teams of trained professionals mix up a solution of Bti and pour it into streams when the black fly larvae are at a specific stage of their lives. The bacteria get into the guts the unsuspecting larvae and kill ‘em off within a couple minutes. Not all larvae are wiped out, though. Even though there is only one generation per year, they aren’t all hatched at the same time, so the Bti Squad has to treat the streams several times over the course of the season.
One is led to wonder, though, if there is any “collateral damage” wreaked on other species living in treated waters. Studies conducted by the EPA and various other organizations have shown that when used in proper amounts, beneficial insects (mayflies, stoneflies, caddisflies), other invertebrates (snails, bivalves, crustaceans), and fish (trout and bluegills) are not significantly impacted. Additionally, Bti has been proven to be non-toxic to birds and mammals (including people).
So, to recap, black flies are important filterers of our waterways. In fact, when water gets too full of organic matter or nutrients, the amount of dissolved oxygen decreases and very little can survive in it. Black fly larvae are indicators of good, clean water.
And let’s not forget that all-important food chain! Black flies are important food items for many critters further up the chain, like dragonflies and some species of birds. Fish eat them, too – at least the larvae. Maybe they eat adults – are there black fly flies used in fly fishing? And just for the record, those who claim bats eat black flies need to remember that these are diurnal insects, whereas bats feed at night.
So, while many of us would just as soon not have to deal with another black fly, we should all remember that every living thing on this planet has its rightful place. It’s only when the delicate balance is thrown out of whack that problems occur. So, greet the black flies this spring with a smile, for they will fill the bellies of the birds, dragonflies and fish that we love to encounter come summer.