THE SUDDEN AND
DRAMATIC SHIFT
OF THE WESTERN
CORN ROOTWORM
TO FIRST-YEAR CORN
C. Richard Edwards
Department of
Entomology
Purdue University
The primary means for man
aging the western com root
worm (WCR), Diabrotica vir
gifera LeConte, over the past 20
years, has been through crop
rotation.
More than 85 percent of the
crop acreage in Indiana is now
in a com/soybean rotational
system. Only about 12 percent
is continuous com.
Prior to the early 19705, most
of the acreage in the state was in
continuous com. Soil insecti
cides were applied as routine
insurance treatments to protect
com roots from attack by com
rootworm larvae. During this
time, more than two million
acres were treated per year at a
cost of well more than $lO mil
lion per year.
As Indiana producers
increased their acreage in rota
tion, many continued to treat
their first-year com (com after
soybeans), not understanding
fully that soil insecticides were
not needed for com rootworm
control.
By the late 19705, most pro
ducers had been convinced that
soil insecticides were not
needed in first-year com. Pro
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ducers were beginning to
accept and adopt the pest man
agement philosophy of manag
ing pests only whcn thcy are a
problem. This change resulted
in a reduction in the amount of
soil insecticides applied to
Indiana farmland by 750,000
pounds of active ingredient per
year. This not only resulted in
significant savings to the pro
ducer, but also to the
environment.
Approximately four years
ago we received a few reports
of rootworm larval feeding
damage in first-year com fields
following soybeans in north
western Indiana. Since WCR
eggs are laid in late summer
prior to hatch the next spring,
the adults had to have moved
into the soybeans to lay their
eggs.
There were questions con
cerning the possibility of vol
unteer com or some grassy
type weeds being present in
these fields, thus making the
fields attractive to the root
worm beetles. However, pro
ducers and agribusiness per
sonnel associated with these
fields saw little to no volunteer
com and/or weeds.
Each year since then, we
have seen a similar situation
develop, but only in a slightly
greater number of fields. The
trend continued and by 1995
hundreds of fields were
reported as having the problem
in northwestern Indiana, as well
as few other areas of the state.
It is difficult to say at this
point why this is happening.
There is speculation that the
beetle may be adapting to the
corn/soybcan rotational sys
tem. Perhaps the rootworm
population is simply greater
now and the adults are spread
ing out. They may be hedging
their bets and laying their eggs
in both com and beans.
Maybe we are dealing with
an insect that is genetically dif
ferent from WCR found in
areas where this has not been
identified as a problem. It could
be that the trend toward earlier
planting of com over the past
IS years and later rootworm
hatch over the past four years is
causing com and rootworm
development to get out of sync
with each other.
The early maturing com
fields may not be as attractive
to late emerging beetles, thus
their movement to succulent,
flowering soybeans.
The problem could also be
associated with the trend to
reduced tillage systems and
narrow rows. Crop residue is
not turned under in these fields
and this may be providing a bet
ter environment for beetle sur
vival and egg laying. In these
fields, the com residue remains
on top of the ground for several
years before it completely
breaks down.
Perhaps volatiles given off
by the decaying com plants in
soybean fields are attracting the
beetles into these fields. There
are any number of possibilities
as to why this could be happen
ing. Although extended dia
pause, a problem associated
with northern com rootworm,
has been mentioned as another
possible reason for WCR larvae
being found in first-year com,
data to date from Illinois show
that this is unlikely the cause.
Obviously producers, agri
business personnel, and univer
sity staff are concerned about
this development. This situa
tion could put producers back
on the treadmill of applying soil
insecticides prophylacticly in
all com fields. If this happens.
/ ' Iff
Corn Talk, Lancaster Farming, Saturday, February 10,1996-Pafl# 7
we will have lost much of what
we have gained in managing
this insect with alternatives to
pesticides.
The chemical industry will
undoubtedly target this new
market. However, we know
that for all first-year com fields
will require treatment. Perhaps
a small percentage of fields will
be affected, yet unless we
obtain some answers and deve
lop some guidelines, most
fields could be treated.
The Clinton Administration
has set a goal of 75 percent of
all production acreage under
pest management by the year
2000. If not properly addressed,
this problem could make it
more difficult for Indiana to
reach this goal and could be a
step backward for Indiana’s
pest management program.
The following are studies
under way at Purdue to help us
determine what is happening
with this insect:
• Field study. Three sets of
test fields have been identified
for each of four counties in
northwestern Indiana. Each set
consists of a soybean and com-
Year-Round Business
(Continued from Pag* 3)
narrow-row and grid com
planting becomes more widely
used. Wolf figures equipment
will evolve to suit producers’
needs.
Like most farmers, Dan
Wolf is quick to trade ideas on
what has and has not worked
and gleans tips from other pro
ducers whose successes he
observes and admires. Along
with keeping up on the latest
trends from reading, Wolf says
he gathers ideas visiting with
others at supplier and produc
tion meetings, and, of course,
while waiting in line to unload
grain at the mill.
Getting a “heads up” on the
field work through pre
planting applications is one
practice that helps the Wolfs
stay on schedule during plant-
panion com field. For the three
com fields in each county, one
represents early pollination,
another mid pollination, and the
third late pollination.
The three pollination periods
are important to determine if
the beetles stay in any of the
fields for a longer time period
due to the presence of pollen,
their preferred food. Samples to
determine rootworra beetle
numbers have been taken in
both com and soybean fields to
see how populations fluctuate
over time. Whole plant obser
vations have been made in com,
while sweep net samples and
yellow sticky card-counts have
been taken in soybeans.
In 1996, cooperating produc
ers will treat approximately
’/< of their 1995 soybean test
fields, which will be rotated to
com in 1996, with a soil insecti
cide for WCR control. In July
1996, the root systems in both
the treated and untreated areas
will be evaluated for rootwoim
larval damage. At harvest,
yields will be taken to deter
mine if differences occur.
(Turn to Pag* 9)
ing season. A small amount of
chemical is applied ahead of
planting, spreading the work
load and the cost, along with
getting some effective, early
weed control.
“We started that several
years ago and we liked the
results,” said Wolf. “We also
put some fertilizer out early,
figuring we can generally
depend on some April rains,
while the cool weather holds
down the nutrient volatility.”
With field work season only
a few weeks away, the Wolfs
are finishing up equipment
maintenance, finalizing plant
ing plans and continually
watching the markets. And as
soon as the last piles of ice and
snow disappear and the ground
dries and warms, they’ll roll
out the planter to greet a new
season of challenges and
changes in the com growing
business.
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