Testing Two Corn
Western corn rootworm
larvae (about one-quarter
Corn rootworms are the number-one
pest of corn, costing farmers hundreds of millions of dollars a year in
pesticides and lost crop yields. By being the first pest ever to evolve a way
of foiling crop rotations, the corn rootworm may have given genetically
modified (GM) crops their biggest boost ever. Now,
Agricultural Research Service
researchers are partnering with industry to test GM corn's ability to curb this
Crop rotations are the standard nonchemical way farmersboth
conventional and organickeep pest levels down. The idea is to plant a
crop, like corn, in a field or garden one year and then rotate to a different
crop, such as soybeans, the next year. Any corn rootworms that move in to breed
and feed on the corn will survive, but their offspring will starve to death the
next year, because they'll hatch in a field of soybeans.
Entomologist Wade French
examines corn roots from
plots that have been
artificially infested with
western corn rootworm. The
nontransgenic root on the
right shows considerable
damage from the pests, but
the transgenic root on the
left has very little damage.
But a few years ago, something
very new and strange happened. The western corn rootworms themselves began
rotating fields, to make sure future generations were always born in a
cornfield, not a soybean field. Adult beetles began flying out of cornfields
just long enough to lay their eggs in soybean fields that would become
cornfields the next spring, when their hungry larval offspring would hatch in
the soil. Northern corn rootworms took another tack: They laid eggs that took 2
years to hatch, so they'd hatch only when the fields were rotated back from
soybeans to corn.
Changing the Plant
Now, scientists are looking to control rootworm by using a GM corn that is
moving through regulatory channels for possible use in time for spring 2003
planting. YieldGard Rootworm corn has been genetically modified to produce a
protein toxin derived from Bacillus thuringiensis (Bt), a common
and naturally occurring soil bacterium. This bacterial insecticide kills
rootworms when they feed on corn plants.
Pitfall traps with timed
interval units are used
to study daily activity
cycles of beneficial ground
beetles in corn and soybean
cropping systems. Here
(left to right), biological
aides Ryan Hericks and Matt
Jones, entomologist Mike
Ellsbury, and graduate
student Chris Noble prepare
to collect beetles from a
One concept that will be tested
on a large scale is use of a mix of conventional and GM corn seed. The seed-mix
concept is based on cooperative research by ARS and Monsanto Company, St.
Louis, Missourithe developer of the new corn variety. In the field,
plants that grow from the conventional corn seeds in the mix serve as a refuge
for some of the beetles. This ensures there will be rootworm beetles not
exposed to Bt available to mate with any Bt-resistant survivors.
The offspring will likely not be resistant, which will slow the evolution of
beetles resistant to Bt.
Under a 5-year cooperative research and development agreement that is up for
renewal in 2003, the researchers tested various seed mixtures. They found that
those with 10 to 20 percent conventional corn seed provided the same rootworm
control, on average, as 100-percent GM seed. With rootworm control superior to
that offered by a conventional insecticide, the seed-mix approach may provide
an effective strategy to slow down the evolution of resistance to Bt.
While further research is required for seed-mix strategies, the U.S.
Environmental Protection Agency is reviewing Monsanto's application to sell
YieldGard Rootworm corn for 3 years only at first. This will allow Monsanto
time to conduct additional studies with ARS and university scientists.
Biological aides Dale Beckendorf
(right) and Heather Schilder
(center) help technician Eric
Bekendorf (left) prepare cages
used to catch corn rootworm
beetles for study.
The Bt corn offers a way
to reduce use of conventional insecticides, says Wade French, an ARS
entomologist at Brookings, South Dakota. French and colleagues are researching
the seed mix in cooperation with Monsanto and conducting other research on
rootworms. The wormlike larvae cause most of the damage to corn by devouring
plant rootstheir dietary mainstay. French and colleagues evaluate
feasibility of seed mixes by rating root damage and plant lodging (falling) and
monitoring numbers of adult corn rootworm beetles caught with emergence cages.
So far, all seed mixtures tested have worked as well as conventional
insecticide, and most of them did much better. But the mixtures with no more
than 10 to 20 percent conventional corn seeds worked best. "This makes us
think that a seed mix will control rootworms while also slowing down the
development of resistance," French says.
Biological technician Dave
Beck and biological aide
Sarita DeBoer examine
species of ground beetles
captured in fields rotated
with corn and soybeans.
But will the Bt corn also kill beneficial bugs such as ground
beetles, some of which are fierce predators of rootworms? The family Carabidae
includes more than 20,000 species of ground beetles, making them the second
largest group of beetles in the world. Ground beetles are ubiquitous and common
in both rural and urban areas, where the larger beetles are often mistaken for
In a study designed to investigate the potential impact of Bt corn on
ground beetle species composition, French's colleague Mike Ellsbury, also an
ARS entomologist, set up 105 pitfall traps, each one a single plastic cup, or
pit, for beetles to fall into. He placed the traps in experimental plots
planted with a mix of YieldGard Rootworm corn and conventional corn. The plots
were scattered over 40 acres of soybean fields on private farms. By keeping
track of the numbers and species of beetles in the test plots, the researchers
will have an indication of whether the the corn harms the insects.
Ellsbury collected 20,000 ground beetles this past growing seasonas
many as 5,000 in 48-hoursrepresenting 60 different species.
French also placed pitfall traps to monitor ground beetles on private farms,
where some farmers are growing another type of Bt corn to control corn
borer pests. These farms are part of a national areawide pest-management
project designed to reduce the corn rootworm population across the Corn Belt
with a variety of techniques, including nonchemical means.
Ellsbury is also
experimenting with three nighttime traps created by modifying a commercial
mosquito trap. He and colleagues converted the trap from one suspended on a
pole in the air to one buried just beneath the surface of the soil. The trap
nabs adult beetles as they race across the surface of cornfields at night,
devouring rootwormsand just about everything else they encounter. The
researchers attached the rotating trap mechanism to a circular panel of
recycled plastic so they could cover it with soil and crop residue, completely
disguising it, with only a single pitfall opening exposed at the ground
The trap is like a clock, set to rotate every 3 hours, so that beetles fall
into one of eight different collection bottles. Researchers can then see not
only how many and what kind of beetles are present, but alsowithin a
3-hour spanwhen they were caught. This tells researchers exactly when
different species of beetles are active, which offers clues as to who or what
With the decreasing effectiveness of the crop-rotation strategy in corn
farmers' arsenals, growers must increasingly turn to conventional pesticides or
beneficial insects to combat rootwormsor to a new weapon: corn plants
genetically modified to produce a pesticide that affects only corn
Comis, Agricultural Research Service Information Staff.
This research is part of Crop Protection and Quarantine, an ARS National
Program (#304) described on the World Wide Web at
Wade French is with the
USDA-ARS Crop and Entomology Research Laboratory, 2923 Medary Ave., Brookings,
SD 57006; phone (605) 693-5219, fax (605) 693-5240.
"Testing Two Corn Rootworm Controls" was published in the
issue of Agricultural Research magazine.