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black fly

Black flies have a terrible beauty. From a distance, they are merely pests, which we must tolerate for a few weeks each year. Up close, they are carefully crafted machines with specialized sensory capabilities and sharp tools sculpted to pierce the skin of mammals and birds and extract the blood they require to nourish their eggs. You have to respect them from the standpoint of evolutionary design.

Yet I spend little time admiring these creatures. Indeed, I dread the two-week period in early May when adult female black flies (Simulium annulus) emerge in vast numbers and descend upon loons. Black flies render each day a trial for loons, which are trying to locate nest sites, mate, build nests and prepare themselves for four weeks of joint incubation. Hounded by flies throughout daylight hours when they are above water, loons drop their normal routine and begin to dive incessantly to minimize blood lost to the flies. Breeding pairs that have completed nests and laid eggs are often forced to abandon efforts to incubate them owing to the relentless insects.

In the past several days, fly populations have spiked, and loon pairs are now suspending their breeding efforts for the moment and simply ditching the flies. We hope that loons’ nesting efforts are not so closely synchronized with fly emergence this year as they were last, when black flies reduced loon breeding success dramatically. If this is a typical year, a small percentage of loon pairs (say, 10-20%) will abandon their eggs on account of the flies, but many more pairs will persevere in spite of the pests. So our hopes for a banner year of chick production are, as yet, intact.

Although their travails pale in comparison to what loons face, even loon researchers are impacted by black flies. Eric, Joel, and Chris are in the final stages of covering the study lakes for the first time. Since they are trying to observe leg bands of loons that are diving constantly to avoid flies, the LP team will confront a steep challenge in the next few weeks. I hope they are up to it!