Cool Weather Prolongs Black Fly Outbreaks

I have an almost-annual tradition. Each year at this time, I watch helplessly as black flies harass incubating loons, drive them off of their nests, and force them to dive to gain momentary respite. In years of mild infestation, I breathe a sigh of relief to see that the nest abandonment rate is low. In severe years, I sit down at my computer and try to determine what factors might cause the little demons to hammer loons so hard. Today, I found this. Rate of nest abandonment is strongly correlated with April temperatures. Specifically, cool Aprils seem to cause more nest abandonments. The pattern is strong.

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As usual, I am late to the party. Black flies are of considerable interest because of their negative impacts on humans and domestic animals, so we know a good deal about their biting habits. An hour ago, I excitedly e-mailed one of the world’s experts on black flies to report that cool weather seems to lengthen the period when these pests bother loons. “I am onto something”, I thought. “I must get the word out.” He politely informed me that cool water temperatures slow the emergence of the flies, and cool air temperatures stretch out their lives. He then pointed me to thirteen scientific papers from the past half-century that reached the same conclusion!

The momentary humiliation I experienced was a small price to pay for the knowledge gained. I am now poring over this literature to learn what I can. I have found that most studies of black fly biting patterns are quick-hitters — snapshots from black flies caught in traps over a year or two. Since we possess over two decades-worth of data, our nest abandonment finding expands the information pool considerably. In addition, we have not merely measured how many flies are around or biting, but their apparent impact on the breeding productivity of a well-loved bird. So my excitement about our result is only slightly diminished.

Setting aside the scientific significance of our finding, what does it mean for the Wisconsin loon population? We cannot control outside temperatures, so there seems little hope that the finding will help us mitigate the impact of black flies on loon nests. But if I am correct that cool Aprils are most damaging to loon nesting — and this is a big “if”, as I am still exploring the result — then we might have cause for guarded optimism. A warming climate, while harmful globally in many respects, might provide a slight lift to nesting loons.