Don’t Let Look-Alikes Fool You
URBANA, ILL.
In the mad rush to finish up what has been a
challenging corn planting season, speed often
takes priority over attention to detail, said
Robert Bellm, University of Illinois Extension
crop systems educator.
“This is especially true when it comes to crop
scouting,” Bellm said. “When thousands of
acres need to be covered in a very short time
frame and the late planting date leaves little opportunity
to replant a damaged, less than optimum
stand, many growers tend to err on the
side of caution and apply pest control measures
at the earliest sign of a problem.”
High commodity prices make this reaction understandable,
and even hard to argue against,
he said. Still, a pest management practice needs
to be economical and return more dollars to the
grower than what it costs to apply.
“This is where correct pest identification becomes
a critical factor,” Bellm said. “There are
several pests of corn that can cause very similar
damage symptoms, and are easily mistaken
for one another if one hurries and does not observe
closely.”
Only one of these pests, the black cutworm, is
actually manageable once the corn seed has
been placed in the ground. Misidentification can
lead to a pesticide application that is less than
worthless. So, what are these “look-alike” problems
and how are they differentiated in the
field?
One type of damage currently being observed
in newly emerged corn is cutting of plants at the
groundline, Bellm said.
“The quick reaction
is to call in the
sprayers to control a
black cutworm infestation,”
he said.
“However, redwing
blackbirds and grackles
also cut off corn
seedlings as they tug
and peck in an attempt
to access
the seed that is
buried in the
ground.”
Often this tugging
and pecking will result
in a small funnel-
shaped hole in
the ground at the
base of the plant, a
symptom not observed
when the
damage is caused by
black cutworm. When
damage is caused by
birds, the discarded
seed coat will often be
observed close by. Bird damage is more commonly
found in fields where the habitat is conducive
to blackbird nesting: adjacent to wheat
fields, pastures, wetlands, and tree lines.
“An overhead power line makes an excellent
staging area for birds to rest on,” he said. “An
observant crop scout can actually hear and observe
birds entering and leaving the field in
these types of areas.”
A second type of look-alike damage can occur
when corn reaches the V-3 to V-5 growth stage.
“Dead heart” or “dead whorl” can occur when a
corn plant is too large, or a black cutworm is too
small to actually cut it off below ground. In this
instance, the cutworm will often wrap itself
around the corn plant and create a feeding hole
until it consumes the underground growing
point of the plant, he said.
The plant remains upright but the whorl
quickly wilts and deteriorates. The look-alike
pest in this instance is often wireworm, which
will also tunnel into the base of the plant and
destroy the growing point. Commonly, both
pests can be found in the same field at the same
time. Unfortunately, there is no rescue treatment
for wireworm infestations, so if wireworm
and not black cutworm is the culprit, a pesticide
application is useless, Bellm said.
“The most obvious method of differentiating
between these two pests is to dig up the plant
and find the offending larvae,” he said. “They
aren’t always polite enough to stick around at
the scene of the crime, so a back-up diagnostic
key is to observe the size of the hole bored into
the base of the plant. Wireworm tunnels are
normally no more than a couple of millimeters
in diameter, while a cutworm tunnel will be up
to five millimeters in diameter.” Δ
Top Photo:
Black cutworm feeding
injury. Note large
diameter hole.
2nd Photo From Top:
Black cutworm larva
can often be found at
base of damaged plant.
Note the lack of funnelshaped
hole,
differentiating this
from blackbird damage.
3rd Photo From Top: “Deadheart”
caused by insect
tunneling into growing
point below ground.
Bottom Photo: Wireworm
feeding injury. Note small
diameter hole in base
of plant.