University Park, Pennsylvania
March 12, 2008
A cocktail of insecticides
containing a plant protein and a common insecticide may be more
lethal to crop pests than either ingredient used alone,
according to biologists. The one-two punch also inhibits the
insects' growth rate and reduces their chance of developing
resistance.
"We found a synergistic effect, where the two insecticides
together decreased the growth rate of caterpillars more than
either one did alone," said Dawn Luthe, professor of plant
stress biology at Penn State
University. "The insect mortality rate was also much greater
than the sum of mortality rates when only one insecticide was
used."
One of those insecticides, Bt, is commonly used around the
world. When insects feed on plants genetically modified to
produce Bt, the toxin binds to chemical receptors lining the
insects' midgut. This disrupts the receptor’s function,
eventually killing the insects. But researchers say some insects
always survive the ordeal and over time, subsequent populations
could develop resistance to the toxin.
Luthe and her colleagues Srinidhi Mohan and Peter W. K. Ma,
post-doctoral student and professor at Mississippi State
University, and W. Paul Williams, research geneticist at the U.S
Department of Agriculture, studied a unique plant-based
insecticide known as Mir1-CP.
Their goal is to see if Mir1-CP, when used in tandem with other
biological pesticides, such as the Bt toxin, can prevent pests
from developing resistance and make the toxin more effective.
"This protein, which we developed from certain strains of corn
from Antigua, breaks down other proteins and peptides in the
insects' gut," said Luthe, whose findings appear in the current
issue of PLoS ONE.
Unlike Bt, Mir1-CP breaks down proteins in a protective membrane
covering the midgut. This membrane acts as a barrier that
protects the caterpillar from toxins in the diet, and cycles
nutrients to the midgut.
"It is the caterpillar's first line of defense against toxins
and chemicals in its diet," said Luthe.
The researchers fed insects a sub-lethal dose of the two
insecticides to test the effectiveness of both insecticides on
the pests. They found that when used alone, a concentration of
Bt at five parts per billion killed 4 percent of all corn
earworms and 5 per cent of tobacco budworms. Mir1-CP, when used
at a concentration of 60 parts per billion, killed 8 percent of
the corn earworms and 3 percent of the tobacco budworms.
But when researchers added the two insecticides together, the
mixture killed 61 per cent of corn earworms and 57 percent of
tobacco budworms, which is more than 10 times better than either
by itself. Researchers saw similar results against the fall
armyworm and the southwest corn borer, when the insecticides
were used at slightly different strengths.
In addition to a high mortality rate among the insects, the
study indicates a significant decrease in the growth rate of the
survivors.
"We think that Mir1-CP is making holes in the membrane, which in
turn is making it easier for the Bt toxins to reach the insects'
midgut," explained Luthe, whose work is funded by the National
Science Foundation.
The findings have important implications for agriculture because
each year, insects cause major losses to farmers. Nearly 20 per
cent of major crops worldwide are lost to insects.
Genetically modified crops that produce the Bt toxin have
managed to check the insects to some extent but Luthe says
insects may be winning the fight.
"Researchers in the Mississippi delta have found resistance to
Bt among some insect populations in the region," Luthe noted.
"There is a chance that some time in the future Bt will not be
as effective against pests as it is now."
The Penn State researcher suggests strains of corn that
naturally produce Mir1-CP could be cross-bred with other strains
of corn that produce Bt to develop new varieties that are more
effective against pests.
"If you have two mechanisms of attack, it will take much longer
for an insect population to develop resistance to both modes of
attack, and help slow the development of resistance," Luthe
added. |
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