Corn Rootworm Damage To Bt Corn: Should We Expect More Reports Next Year?
URBANA, ILL.
By now, most corn producers throughout
Illinois and many areas of the Corn Belt
are familiar with the paper published by an
Iowa State entomologist who reported the development
of field resistance to the Cry3Bb1
protein expressed in some Bt hybrids that are
designed to prevent excessive damage by corn
rootworm larvae. Examples of Bt hybrids that
express this protein include YGRW, YieldGard
Plus, YieldGard VTRW, and YieldGard VT Triple.
In August and September I also reported on
fields in Henry, LaSalle, and Whiteside counties
that had been planted to Bt hybrids (expressing
the Cry3Bb1 protein) that had severe root pruning
and lodging. More recently, I have received
reports of similar damage in Carroll County.
Should we expect more reports of this type of
severe damage to Bt hybrids expressing this
specific protein in 2012?
Overall densities of western corn rootworm
adults were low to moderate throughout Illinois
in 2011. Densities were greatest in northwestern
Illinois, the area where most of our attention
has been focused on Bt failures this past
summer. However, even in this area of the state,
the population of western corn rootworms was
not alarmingly high. In fact, our statewide survey
of western corn rootworm adults in late July
and early August revealed averages of 0.64 and
0.81 beetle per plant for Lee and McLean counties,
the two highest averages among the 47
counties surveyed. Densities for most counties
were well below these averages.
Historically, an average of 0.75 to 1.0 beetle
per plant may lead to economic levels of larvae
the following season in non-Bt cornfields. Key
factors that will influence the potential for larval
damage next season include planting date (early
planting favors larval establishment) and soil
moisture during larval hatch (which occurs in
late May and early June). Heavy precipitation
that leads to saturated soils at the time of larval
hatch will suppress establishment.
If you experienced significant root pruning
(several nodes destroyed) and lodging in 2011
in a field planted to a Bt hybrid, consider the
following management recommendations:
Rotate to soybeans or another nonhost crop.
• Use a corn rootworm soil insecticide at
planting.
• Use a Bt hybrid that expresses a different
corn rootworm Cry protein than one that may
have performed poorly in your fields in 2011.
• Use a pyramided Bt hybrid that expresses
multiple Cry proteins targeted against corn
rootworms.
• Most importantly, consider a long-term integrated
management approach that includes
multiple tactics.
I believe there are some important points to
make about these recommendations: An integration
of these tactics is encouraged across
multiple growing seasons, not all at once. Using
multiple tactics should not imply that a Bt corn
rootworm hybrid plus a soil insecticide should
be a standard and routine practice in a single
growing season. Nor does it mean that using a
Bt corn rootworm hybrid in a field sprayed the
previous season for western corn rootworm
adults (beetle management program) should be
standard and routine. A corn rootworm soil insecticide
properly applied should provide acceptable
root protection in most producers'
fields. Similarly, a beetle management program
that is professionally conducted should provide
satisfactory root protection the following season.
What we need to avoid is throwing everything
including the kitchen sink at western corn rootworms
in a single growing season: a Bt hybrid
(also containing an insecticidal seed treatment),
a soil insecticide at planting, and a beetle suppression
program the previous summer. Not
only is this approach expensive, but ultimately
it may select for resistance more rapidly and
lead to unwanted environmental consequences.
In October, extension entomologists Chris Di-
Fonzo of Michigan State University and Eileen
Cullen of the University of Wisconsin published
a Bt trait table (Adobe PDF). It provides a good
reference to the types of Bt proteins expressed
by various hybrids, the insects targeted, the
type of herbicide tolerance offered, and the specific
refuge that can be used. With the introduction
of pyramided hybrids and seed blends
into the marketplace, this table should provide
a nice tool for growers as the use of Bt hybrids
increases-along with refuge compliance confusion.
I offer my thanks to these entomologists
for sharing this useful resource. Δ
DR. MIKE GRAY: Crop Sciences Extension Coordinator,
University of Illinois