Corvallis, Oregon, USA
August 12, 2009
Researchers at
Oregon State University
(OSU) have discovered that the circadian rhythms or biological
“clocks” in some insects can make them far more susceptible to
pesticides at some times of the day instead of others.
With further research, the scientists said, it may be possible
to tap into this genetic characteristic, identify the times that
a target insect is most vulnerable to a specific pesticide, and
use that information to increase the effectiveness, reduce costs
and decrease the amounts of pesticide necessary for insect
control.
Approaches such as this may also be highly useful in programs of
“integrated pest management,” the researchers said, which aim to
minimize pesticide use, prevent development of resistance to
pesticides, and use a broad range of physical or chemical
control measures to enhance the long-term effectiveness of an
insect control program in crop agriculture.
The findings were just published in
PLoS ONE, a
professional journal, in work supported by the U.S. Department
of Agriculture, National Institutes of Health and National
Science Foundation.
 “We
found that it took triple the dose of one pesticide to have the
same lethal effect on fruit flies at the time of day their
defenses were strongest, compared to when they were weakest,”
said Louisa Hooven, a postdoctoral fellow in the OSU Department
of Zoology and lead author on the study. “A different pesticide
took twice the dose. This makes it pretty clear that the time of
day of an exposure to a pesticide can make a huge difference in
its effectiveness.”
In recent years, researchers have found that the genes which are
sensitive to the natural rhythms of day and night can have a
wide range of biological effects, on everything from fertility
to feeding patterns, sleep, hormone production, stress,
productivity, medication effectiveness and many other functions.
And they operate in multiple cells in many or most plant and
animal species, including humans.
In the newest work, circadian rhythms appear to coordinate
“xenobiotic metabolizing” genes, or the genes responsible for
breaking down and detoxifying various poisons, such as
pesticides. Besides that, it’s possible that circadian clocks
may also affect absorption, distribution, excretion, and
molecular targets of toxicity.
“This rhythmic defense mechanism may have evolved in order to
disarm the noxious compounds that plants produce to avoid being
eaten by an insect,” said Jadwiga Giebultowicz, a professor of
zoology at OSU. Other co-authors on this work included OSU
undergraduate students Katherine Sherman and Shawn Butcher.
The OSU study found that insect defenses against two commonly
used pesticides, propoxur and fipronil, were strongest during
mid-day, and weakest around dawn, dusk or the middle of the
night. The effectiveness of two other pesticides studied –
deltamethrin and malathion – did not seem to be so strongly
associated with time of day, at least with fruit flies.
“For this approach to be useful in agriculture or other places
pesticides are used, we will need to test specific insects
against specific pesticides, and we will probably find
differences in time of maximum effectiveness for various
pest-pesticide configurations,” Giebultowicz said. “In some
cases we may be able to greatly improve the effectiveness of
pesticides or allow the use of reduced doses.”
Although many pesticides have a residual effect, the researchers
said, the timing of the first exposure can be critical. Many
pesticides are repellent to insects, and if they are not killed
immediately they may simply avoid the residue, or in some cases
develop resistance to the pesticide – a critical and costly
problem in modern agriculture. Pesticide resistance has been a
driving force behind the evolution of the field of integrated
pest management, as growers realized that sustainable pest
control is not as simple as using the same pesticide, year after
year, which often becomes increasingly ineffective and more
expensive.
The new findings, the OSU researchers said, are also another
example of how circadian rhythms are important in other
detoxification systems in biology. In human medicine, a field
called “chronopharmacology” is already developing, based on the
observation that some medications are far more effective if
administered at one time of the day instead of another.
Research into the molecular mechanisms underlying circadian
rhythms was pioneered in fruit flies, but the OSU researchers
hope their future research will shed light on how the biological
clock influences responses to chemicals in humans.
“A fundamental understanding of the functional significance of
circadian rhythms in chemical exposures may facilitate
strategies to reduce adverse events in humans, promote control
of pest species and reduce pesticide use,” the researchers wrote
in their report.
“Our study strongly suggest that time of day should be included
in insect control strategies and human risk assessment of
chemical exposures, including pesticides,” they said. “In some
cases, the clock, together with the dose, may make the poison.”
As one of the largest academic units at OSU, the College of
Science has 14 departments and programs, 13 pre-professional
programs, and provides the basic science courses essential to
the education of every OSU student. Its faculty are
international leaders in scientific research.
Image credit: fruit fly used in this research, Drosophila
melanogaster - Wikipedia |
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