Pesticide potency can depend on bug’s clock

Farmers and others plagued by insects often reach for an agricultural chemical to fight their marauders. But the timing of chemical bug whackers can be pivotal, a new study indicates. From mid-morning to evening, the potency of some pesticides can double or triple.

Timing pesticide applications for peak potency could offer farmers more bang for their increasingly cash-strapped budgets — and dramatically lower the quantity of poisons spewed into the environment. In other words, cheaper, greener bug control.

And although the new study focused on insecticides, plants exhibit similar daily oscillations in their metabolic activities. So I see no reason why a similar trend might not show up in the susceptibility to weed killers by dandelions, poison ivy or crab grass.

Most living things are governed by fairly regular daily oscillations in the activity of their genes. Materials cranked out by this genome-wide ebb and flow of production cycles over the course of roughly 24 hours orchestrate essential activities in the host; from when to eat or sleep, to the timing of work, peak attentiveness — even interest in mating. Louisa Hooven of Oregon State University and her colleagues now show that vulnerability to pesticides are among things that oscillate with a bug’s internal rhythms — its biological clock.

The OSU team didn’t pioneer work in this area. Others began reporting indications more than three dozen years ago of daily oscillations in bugs’ pesticide sensitivity. Some of the genes now known to underlie that vulnerability determine how quickly or effectively a poison will break down. Others have been implicated in the development of pesticide resistance.

Hooven’s group examined and compared published data on when dozens of various pesticide-vulnerability genes rev up or slow down. And they found that many of these oscillate throughout the day — often in concert with each other.

Production of some detoxifying enzymes, they note, tends to peak just after an insect’s normal meal time, precisely when dietary toxicants might be most abundant in the bug’s gut. But it takes energy to make those enzymes. So their production wanes in periods when a bug shouldn’t need them, such as at bedtime. Bottom line, that would seem to be when to target bugs with poisons.

To test the idea, the OSU scientists exposed various fruit flies over the course of a day to varying doses of any of four different classes of pesticides: a carbamate (propoxur), phenylpyrazole (fipronil), organophosphate (malathion) or pyrethroid (deltamethrin). The flies showed dramatic differences in susceptibility to the first two bug killers, but little difference is vulnerability to the last two. Hooven’s group reported its findings July 31 in PLoS ONE.

The pesticides exhibiting big time-of-day differences in potency indeed proved most toxic to the flies from dusk to dawn — or bedtime. Not, I’m sure, when any farmer would want to be out spraying toxic chemicals. Of course, the opposite timetable would probably hold for cockroaches, rats and other vermin that can be especially active at night.

But the new data provide useful hints for more than bug killing. For instance, humans might show a similar time-of-day variability in toxicity to pesticides and other toxic chemicals. If true, regulators might want to consider the likely timing of human exposures to such poisons when testing or establishing what doses might be safe.

Certain conditions can also reset or impair the functioning of a biological clock — such as unremitting daylight. In the new OSU experiments, for instance, flies raised on a strict schedule of 12 hours of light and darkness showed a clocklike rhythmicity in poison-detoxification activities. These daily rhythms largely vanished when the flies were raised in 24 hours of light. If what happens in fruit flies mirrors what happens in people, then artificial night-time illumination, shift work or jet lag may all alter — potentially deleteriously — the body’s ability to break down and eliminate toxic chemicals.

Moreover, Hooven’s group points out, “While here we focus on metabolism, [biological] clocks may also modulate absorption, distribution, excretion, and molecular targets of toxicity, and thus are likely to have broad influence on [vulnerability to chemicals].”

Bottom line: It’s not nice to fool Mother Nature — unless your goal is to vanquish vermin.

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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