Page 22—Corn Talley Lancaster
Corn Rootworms Cause Havoc
UNIVERSITY PARK
(Centre Co.) A Penn State
entomologist said this year’s
growing season is proving to
be one of the worst in recent
memory for stalk lodging and
yield reductions caused by
com rootworm.
“Com growers in the state
have seen drought-stressed
com plants fall over after re
ceiving much-needed moisture
from recent rains,” said Dennis
Calvin professor of ento
mology in the College of Agri
cultural Sciences. “These
plants had few roots left after
the ravenous feeding of com
rootworm larvae. Rains soft
ened the soil and winds blew
the plants over because the
weakened root systems could
not hold them up.”
Calvin said the rootworm ef
fect on com yield depends on
the percentage of the field that
is “lodged,” a term used by en
tomologists to describe plants
that have fallen over due to
damage by the com pest. An
other factor affecting yield is
the moisture conditions for the
remainder of the growing sea
son.
“Com plants have a remark
able ability to recover from in
sect injury, but it’s really de
pendent on moisture condi
tions,” Calvin said. “Good
moisture conditions will allow
many plants to regenerate a
root system, but these plants
still will sustain a 10 to 20 per
cent reduction in grain yield if
they have been lodged.”
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Calvin estimates grain yield
reductions may reach SO per
cent in some fields if moisture
conditions remain poor during
the growing season. However,
if com plants remain erect and
adequate moisture is present,
they will sustain very little re
duction in yield.
Calvin said favorable weath
er conditions and other factors
allowed com rootworms to
thrive this year. “A mild winter
provided an excellent climate
for egg survival,” he explained.
“A warm and dry April was
followed by an extended cool
period in May that delayed
com emergence and com root
worm development. This delay
in emergence led to com plants
with root systems that were
smaller and more vulnerable to
injury from rootworm larvae."
Because of cool spring
weather, the com rootworm
egg hatch also was delayed.
The rootworm larvae usually
appear in May, but this year’s
weather conditions put off the
pests’ emergence until mid- to
late June.
“Soil insecticides are de
signed to protect the corn’s
root system against com root
worm larvae for only six to
eight weeks,” Calvin said. “An
insecticide applied to a com
field that was planted in late
April would have degraded to a
level too low to protect vulner
able com roots by the time egg
hatch began. Even fields
planted in mid-May had lost
much of the soil insecticide’s
Features include:
ability to protect the root sys
tem by late in the insect’s egg
hatch period.
“To make matters worse,
drought conditions developed
during the period when com
rootworm larvae were actively
feeding, preventing com plants
from regenerating new roots to
compensate for those lost,”
Calvin said. “These factors in
combination have led to ex
tremely high numbers of adult
beetles feeding on newly
emerging silks. In a number of
fields, silk feeding becomes so
intense that it interferes with
com pollination, causing fur
ther yield reductions.”
Calvin said farmers can use
insecticides in mid- to late
August to control adult com
root worm beetles under certain
conditions. “If five or more
adults are counted per plant
feeding on green silks and the
silks are being cut back into the
silk channel of the husk, then
pesticide control is justified to
prevent interference with pol
lination,” Calvin said. “Keep in
mind that high numbers of bee
tles on whorl-stage com should
not be used to make a treatment
decision. Adults in these fields
will tend to move to fields in
green silk when availble. The
females are drawn to freshly
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silking fields because they of
fer pollen that is needed to de
velop their eggs. An insecticide
applied to whorl-stage com
will kill the majority of com
rootworm beetles, but will not
last long enough to protect the
plants from silk clipping.
“Once green silks begin to
emerge, new beetles will be
drawn into the field from sur
rounding fields,” he said. “If
the insecticide has broken
down, it cannot protect the
field over the 10- to 14-day
period required for pollination,
potentially making a second
application necessary. An in
secticide application for adult
control should be timed to the
beginning of silk emergence.”
Calvin said farmers should
be prepared to harvest early to
minimize harvesting problems
due to root system degenera
tion. “Once roots have de
graded, plants will pull out of
the ground as the com picker
moves through the field, caus
ing problems with plugging,”
Calvin says. “In severely
lodged fields, harvesting will
be considerably slower be
cause of the difficulty in find
ing and staying in the row.”
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