Black Flies: another reason to reduce agricultural runoff

While summer has officially come to an end here in Michigan, some of us are still savoring the memories—and itching the bites—that we collected along the way. Black fly are a familiar pest to those who frequent woodsy or wet areas, and this summer the insects populations did especially well, much to the dismay of campers and swimmers everywhere.

You already know how mosquito populations can be influenced by storm water runoff; another pollutant discharge also has the potential to influence insect populations: agricultural runoff. Specifically, runoff from large-scale, commercial livestock farms, hereafter referred to by their government-designated moniker, CAFOs—Concentrated Agricultural Feeding Operations.

Feeding black fly. Photo: EPA

CAFOs earn their designation by meeting certain size requirements as outlined by the Environmental Protection Agency.  You can read more about how the EPA designates CAFOs here. The density at which animals are housed in a CAFO presents major problems in the storage and disposal of wastes. Just how major? The Union of Concerned Scientists estimated in a 2008 report (PDF) that CAFOs produce 300 million tons of waste per year, which is twice the amount of waste produced by humans in the U.S.

To deal with the excess of waste products, manure will be spread over fields adjacent to the facility or trucked, for a greater expense, to further locations. Unfortunately, this waste isn’t all-natural cow manure, either–waste collected from areas that house livestock often contains spoiled food or bedding, hormones and antibiotics, and pathogens like E. coli.

Now imagine what will happen to that freshly-spread manure during a rainstorm. You guessed it—a significant amount will be washed to the nearest river. That’s where the black fly connection comes in. Of the four genera of Black fly that occur in Michigan, one in particular will benefit from this influx of nutrients and dissolved organic matter; Simuliidae species include some of the most prolific summer black fly like S. venustum, S. decorum, S.vittatum and others.

CAFOs confine lots of animals in very little space. Photo: EPA

These insects begin their lives as eggs laid in a stream of moving water, then progress through several larval instars (stages between egg and pupae in metabolous species) leading up to formation of a cocoon, attached to a submerged object in the stream. Simuliidae black fly larvae tend to do very well below lake outlets because of the additional nutrients flowing out of the lake. If a runoff event coincided with larval development times, there could potentially be a huge positive effect on fly population, and a negative effect on their prey—us!

Of course Black flies aren’t the only reason to be concerned about agricultural runoff. This year Lake Erie experienced significant algae blooms believed to be due to runoff from farmlands in the surrounding drainage basin. Our drinking water supplies are also at stake; when rainwater soaks through the soil into groundwater, it can carry with it many of the pollutants it picked up on the surface. Recent studies have shown that conventional water treatment methods are not adequate to remove pharmaceutical compounds from drinking water. As caretakers of vast amounts of water resources here in Michigan, there are plenty of reasons for us to be concerned about the effects of CAFOs on the environment. A swarm of black flies may be simply an annoyance, but it also may be a signal of an environmental imbalance–one that we should pay attention to.

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