Page 30—Com Talk, Lancaster Farming, Saturday, April 17,1993
Corn Rootworm
Elson Shields
Department of
Entomology
New York
State College
Of Agriculture
And Life
Sciences, Cornell
During the past 10 years,
western com rootworm (WCR)
has established itself as the
major insect pest of com grown
for silage and grain in New
York.
Since the first detection of
WCR iji western N.Y. in 1979,
populations of this insect have
continued to increase and have
moved east to the eastern sea
board. Results from field scout
ing during 1992 have indicated
that 60-70 percent of the conti
nuous com fields in western
and central New York are at
high risk for economic loss
from WCR feeding damage
during the 1993 growing
season.
It these continuous
com fields are planted
to com in 1993, grow
ers should consider
the use of a soil
insecticide.
Corn Rootworm
Biology and Manage
ment; Two species of
corn rootworm
(CRW) can cause
economic loss in our
commercial corn
fields. Northern com
rootworm adult bee
tles are bright lime
green and have been
inhabitants of our
com fields for many
years. The western
corn rootworm,
which is yellowish
with dark stripes, has
invaded New York
only in the past few
years and currently is
most numerous in
western and central
New York.
Adult CRW beetles
are found in com
fields from pollina
tion until the first kill
ing frost (late July
September). During
this time, the adult
females are laying
eggs in the soil cracks
and around the bases
of com plants which
will overwinter and
hatch in late May the
following year. New
ly hatched larvae
locate the young com
plants and begin feed
ing on the developing
roots. Larval develop
ment and root feeding
damage is completed
by mid July; larvae
pupate and emerge as
adult beetles during
late July and early
August to begin lay
ing eggs to complete
their life cycle.
CRW larvae dam
age com by feeding
on the root system and
if present in sufficient
numbers, will reduce
com yields by inhibit
ing the ability of the
com plant to uptake water
and nutrients.
Recent field studies
(1991-1992 growing seasons)
using field plots artificially
infested with WCR eggs by
Paula Davis from the Entomol
ogy Department at Cornell Uni
versity has shown that com
grown for silage is much more
sensitive to WCR larval feed
ing damage than com grown for
grain. In general, root damage
ratings of 3.0-3.5 have been
considered the range of damage
ratings where economic yield
losses begin to occur.
Results from the 1991-92
field studies indicated that sil
age losses both years were eco
nomically significant with a
root rating of approximately 3.0
(100 eggs/row foot) and the
value of the crop loss approxi
mately equaled the cost of the
soil insecticide used to control
WCR larvae in com. By con-
eerless
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trast, com grown for grain
could tolerate a root rating of at
least 4.0 (300 eggs/row foot)
before the yield loss from WCR
larval feeding approximately
equaled the cost of the soil
insecticide needed to control
WCR larvae.
Yield losses from WCR lar
val feeding demonstrated in
this study were physiological
yield loss (the com plant did not
produce the yield) rather than
harvest loss from lodging.
Lodging of com plants usually
occurs only after the root rating
damage is 4.0 or greater.
Fields at Risk: Fields planted
to continuous com are at grea
ter risk of economic CRW
infestations than first-year com
because CRW eggs arc laid the
previous fall in existing com
fields. Fields in continuous
com production are more likely
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to develop economic CRW
infestations the longer com is
continuously planted to the
field. Continuous com planted
after late-planted com the pre
vious year is at high risk
because the late-pollinating
com is attractive to the adult
CRW, resulting in heavier than
normal egg laying in the field.
Management: Potentially
damaging CRW populations
arc managed by rotating the
field to a non-host crop or by
using soil insecticides incor
porated in the seed bed at plant
ing or cultivation. The need to
rotate the field or use a soil
insecticide next year to manage
CRW can be determined by
counting the number of adult
CRW beetles per 55 com plants
(5 plants in 11 different field
locations within a field) in each
com field during and shortly
after pollination (see Cornell
Extension Fact Sheet 501.00).
If these beetle counts exceed 1
beetle per plant as a field aver
age, the field is at high risk for
CRW damage, providing the
field is planted to com in 1993.
If the field cannot be rotated
and must be planted to com in
1993, then a registered soil
insecticide is recommended at
planting or during cultivation
next spring.