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Spraying for Bugs? Check the Time

Scientists have found that the effectiveness of some insecticides depends on what time of day they're used. The results may also have implications for how people respond to medicines and to toxic chemicals.

When is a good time to kill a fly? We don't normally think about it, but researchers at Oregon State University say the time of day does matter. Jaga Giebultowicz and her colleagues tested four insecticides to see if their effect on fruit flies changed at different times of the day. Timing made a big difference for one bug spray.

"You could kill the same number of insects but with one-third of the pesticide applied at one time of day versus another," Giebultowicz says.

Killing time

Another insecticide was almost twice as effective depending on the time. Dusk appears to be the best time to spray, "but, of course, it's hard -- you don't want to spray your orchard when it's dark," she adds.

However, the later in the day you spray, Giebultowicz says, the less pesticide you'll need to use -- at least with some chemicals.

Fruit flies produce a number of enzymes that break down toxic chemicals and protect them from the poison. The researchers found the amount of several of these enzymes was controlled by the fruit fly's biological clock, rising and falling in sync with the sun.

Many plants naturally produce chemicals that are toxic to insects, and Giebultowicz suggests the fruit flies' biological rhythm may boost these enzymes at the best time to detoxify the chemicals without wasting the insect's resources.

Not all of the insects' detox enzymes are cyclical. Time didn't affect how the flies reacted to the other two chemicals Giebultowicz's lab tested. And they only tested fruit flies -- other insects may respond differently.

Beyond bugs

But Giebultowicz adds that the implications of her research extend far beyond the insect world.

"Biological clocks in insects and in humans are similar genetically," she says. So that means we may respond differently at different times of day when we're exposed to insecticides and other toxic chemicals; and it could also mean that medications may work better or worse depending on the time.

Giebultowicz says future studies should look into how both people and insects respond to chemicals over the course of the day. When it comes to flies, though, the fly swatter appears equally effective morning, noon, or night.

The research appears in the July issue of the journal PLoS One.