(Un) Natural Selection: A Toxic Tale of Flies and People

I confess, sickly people repulse me. The blame for some of this aversion lies with my upbringing. While the stomach flu and chicken pox were treated with maternal tenderness, I nevertheless inferred that falling ill was a sign of weakness. My mother was never bedridden and I can’t remember my father ever calling in sick, so I surmised that missing a day of school was a kind of failure, evidence of a delicate constitution hardly befitting a young man. But my internalized message does not compare to the one assimilated by my wife through her Midwestern, Protestant upbringing. Around her house—and presumably in the environs of Illinois farms—sickness was indicative of a potentially serious character flaw (on par with falling asleep on the couch while watching football). But familial messages aside, there are good evolutionary reasons for not associating with feeble folks. With respect to selecting mates, the infirm are bad genetic bets. And in terms of survival, there’s a good reason to avoid pallid people—they’re often infected with a communicable pathogen. All of this goes to explain, if not excuse, why I wasn’t all that keen on spending time with my university colleague Cheryl.* I’m neither a shallow cad who judges people based on their looks, nor an intolerant lout burdened with psychological baggage. I’m a well-adapted organism.

Horn flies are truly loathsome insects. They lay their eggs, and spend the maggoty portion of their lives, in fresh cow dung. The size of a small housefly, an adult feeds on blood up to twenty times per day, favoring the capillary-rich tissues at the base of a cow’s horns. Hence the insect’s name. This is not to say, however, that the flies are found only on their host’s head. When hundreds (in some cases more than ten thousand) of these insectan vampires are seeking a meal, the best spots are quickly occupied. So the back and sides of cattle become peppered with hungry—and horny—flies. Females lay up to two hundred eggs in their lifetimes, and the insect can crank through ten generations a year in warm climes, such as Louisiana. That’s where I met the horn fly, while studying for my doctoral degree in entomology at Louisiana State University. Although my dissertation research concerned a less noxious creature (the southern green stinkbug, which, despite its name, emits an odor reminiscent of cut grass), I was oddly attracted to these repulsive flies. They had a gritty, unapologetic, blue-collar authenticity to their lives. One had to respect a creature that had evolved the capacity to live its larval life in a pile of shit and spend its adulthood taking blood from an animal a couple of million times its own size.


I couldn’t figure out Cheryl’s behavior. Her peculiarities wouldn’t have bothered me had we not ended up on a university committee that required weekly gatherings of a dozen people from as many departments. The meeting frequency was annoying in its own right (some administrator urgently needed to create the appearance of faculty input on a policy, and after twenty-some years at the university I should have been clever enough to avoid such nonsense). But the ever-changing locations and erratic scheduling ensured aggravation. This difficulty was added to the normal challenge of convening a group of academics because the committee chair was allowing Cheryl to approve or veto all arrangements. We had to meet in particular rooms of select buildings—none of which were convenient to the majority of us. We could assemble only on specific days and at certain times which were subject to her unpredictable changes. And when one of the committee members was replaced after our first two meetings, I suspected it was Cheryl’s doing. The chair of the committee didn’t offer any reasons for accommodating her. As an ecologist trained to find patterns in the world and regularities in data, I was frustrated. After a month of bouncing from one room and time to another, I could discern no rationale for Cheryl’s avoiding particular buildings, times, and people.

Ron Byford couldn’t figure out what the horn flies were doing. During lunch beneath the Spanish moss–draped oaks on campus, he told me about finding a cattle herd on the outskirts of Baton Rouge that was infested with belly-dwelling flies. “It’s the strangest dang thing,” he said (as a good Baptist, he avoided cursing). “There aren’t any flies to speak of on the head, neck, or back.” “Maybe they’re avoiding the heat,” I proposed—only half jokingly. Having grown up in Albuquerque, I knew about triple-digit temperatures, but the sultry Louisiana summers had me sympathizing with the flies. “Nah, I was out there in the morning, before it gets hot.” Of course, hot is a relative term, and eighty degrees with 90 percent humidity isn’t hot by Louisiana standards. “Is there anything unusual about the cattle?” I asked. “Not really,” he replied. “They’re Brahmans, which aren’t all that common, but it’s hardly a rare breed. The farm manager has them ear-tagged, although by the looks of it he needs to double up.” The plastic ear tags held a slow release formulation of a pyrethroid insecticide, and there was increasing concern that horn flies were becoming resistant to the chemical. So all these flies somehow decided to invert the normal pattern of behavior and rest on the underside of cattle.


Cheryl turned out to be perceptive and eloquent, with a wickedly ironic sense of humor—traits that I find appealing. But her pale, gaunt appearance made her look chronically on the edge of collapse. I might never have figured out her mysterious pattern of behavior had we not ended up walking across the quad together. By chance, she’d emerged from the adjacent building just as I was heading to our meeting. As we chatted about the futility of our upcoming committee discussions, I started to head across the lawn at the center of campus. “No,” she said, “let’s go around.” “We won’t hurt the grass,” I replied, gesturing at the students racing across the green in pursuit of a Frisbee. “It’s the opposite that worries me,” she said. “Are you allergic to grass?” I asked. Cheryl adjusted her sunglasses and paused for a long moment. “I have MCS, multiple chemical sensitivity,” she said hesitantly, “and the grounds people spray for weeds.” As we headed down the sidewalk, Cheryl explained that she had terrible reactions to “synthetic chemicals”—herbicides on the university’s lawns, insecticides to control pine scales on the campus trees, paint fumes in buildings, perfumes on colleagues, cleaning products in restrooms, vapors from carpets, and just about anything else volatile. That explained why the committee was bouncing around among places and times: Cheryl was dodging chemicals (as well as the colleague who had been replaced after refusing to forgo cologne). My background in physiology made me dubious of the whole MCS thing; why did the organic chemicals we put on the lawn cause a reaction, but not the organic compounds released from cut grass? But at least I had an explanation for her pattern of behavior.

Horn flies aren’t the brightest insects (honeybees are considered the geniuses of the insect world), but they’re good at what they do: find cattle, drink blood, and reproduce. A heavy infestation of these pests can reduce a dairy cow’s milk production by 20 percent and cost beef cattle a half pound per day, which translates into serious economic losses to the producer. Livestock managers had long sought an ideal means for controlling the biting insects. The problems with spraying insecticides onto cows are that many chemicals are toxic to mammals, some compounds find their way into milk and meat, and other substances wear off in a matter of days and need to be reapplied. So the insecticide industry developed ear tags impregnated with various pyrethroids—synthetic versions of pyrethrum, a chemical found in chrysanthemums. In the course of both shooing away flies and grooming itself, the animal wipes the ear tag along its back and sides and the chemical diffuses into the hair, creating a no-fly zone. The pyrethroids are safe for cattle and aren’t absorbed into their tissues. Moreover, the slow-release formulation means that the cow treats itself on an ongoing basis. Think of this as a modified deodorant stick attached to an animal’s ear, providing all-day protection against pests. At least typical flies, those resting on the head and dorsal surface of the host, are killed. But a fly that prefers to spend its days on the cow’s ventral surface avoids being poisoned. Natural selection would rapidly favor such aberrant behavior, and we might hypothesize that belly dwelling would become the norm in a treated population. I proposed this explanation to Ron, who was intrigued but properly skeptical of whether behavioral resistance was the real explanation for this odd pattern.