off the pesticide treadmill
By Steve Tally
professor Larry Murdock says that a pesticide resistance method
developed to prevent chemical resistance in insects can also
be applied to other organisms, such as weeds, bacteria and
fungal diseases. (Photo by Tom Campbell)
aren't known for their entertainment value, and even metaphorical
treadmills can be an aggravation.
farmers and agribusinesses have talked about being on the "pesticide
treadmill": A few years after a pesticide is introduced, insects
develop resistance to it. So another chemical is used--until the
bugs overwhelm that one.
chemical is used. Then another. And another.
Pittendrigh, assistant professor of entomology at Purdue, says it's
possible to stop the treadmill, or at least slow it to a barely
together with Larry Murdock, professor of entomology, and Patrick
Gaffney, of the University of Wisconsin-Madison, have developed
a method to use pesticides so that genetic resistance is severely
is called "negative cross-resistance," and it involves
using multiple pesticides in a precise way to stop the pests.
is 100 percent effective against its target, and that's where the
problem of chemical resistance arises.
If a pesticide
kills 98 out of 100 bugs, the only two left are both resistant to
the chemical. If those two mate, then all of their offspring also
will be resistant.
If the same
thing happens in field after field, soon entire populations of the
pest are immune to the effects of the pesticide.
insecticides are chemicals of some kind," Murdock says. "They
all act to kill insects by interrupting some physiological or biochemical
process. It may be the nervous system, it may be the digestive system
or it may be the hormonal system. That chemical interacts with a
specific membrane component or protein--we call those "target
sites"--to interrupt its function."
all of biology is variability, and even individual insects of the
same species can vary quite a bit.
for example, the lower southwestern part of Indiana. If there is
an insect infestation in that area, you could have tens of millions--or
even billions-- of that species of insect in that area at the same
time," Murdock says. "In this huge population of insects,
there are, by chance, a few individuals that are not vulnerable
to the particular insecticide that you use to stop the insect problem.
Their proteins don't bind it, or the site where the insecticide
attaches to the membrane or target enzyme is a little bit different
biochemically. So the chemical doesn't have the normal effect, and
that insect can survive."
just one in a million insects may be resistant to the insecticide,
that would mean that in a population of a billion insects, there
would be 999 other insects in the area that are also resistant.
they are lucky enough to meet up with another one that is resistant,
they'll mate and produce dozens or hundreds or even thousands of
offspring. All of these offspring will be resistant to the insecticide,"
Murdock says. "And these insects will have a huge advantage,
because the insecticide will have eliminated much of their competition
for food, so that population can grow quickly."
happens, the population of insects in an area rapidly comes to be
dominated by the new resistant strain of the insects. The result
is that the crop damage reappears because the insecticide is no
longer effective, and the insecticide is pulled from that market.
classic example was in Scandinavia in 1946," Murdock says.
"Right after World War II, DDT was used to control mosquitoes,
flies and lice. Soon people began noticing the number of houseflies
was increasing. Eventually, houseflies from that area could walk
on the DDT itself, and they were completely resistant to it."
were first used in the mid-1800s, there have been more than 500
reported cases of insects becoming resistant to the insecticide
meant to control them.