C2—Lancaster Farming, Saturday, June 23,1984
NEWARK, Del. As field corn
approaches the whorl stage,
growers can expect to see a variety
of insects feeding on the whorl or
inner leaves of plants.
First generation European com
borers, true armyworms, and
common stalk borers should all be
active now, says University of
Delaware extension pest
management specialist Joanne
Whalen. Each insect produces a
characteristic type and amount of
feeding injury, and before making
a control decision it’s important to
know which one is causing the
damage.
Whalen advises farmers to look
for com borer damage when plants
reach 18-24 inches in height.
Generally, shorter com is not
attractive to ECB moths unless
egg laying is extended over a long
period of time or taller com is not
available. Even when eggs are laid
on small plants, there is little com
borer feeding on com less than 18-
inches tall because younger plants
contain high levels of a chemical
called DIMBOA which is toxic to
larvae.
When checking for corn borer
whorl feeding, the specialist says
to look for characteristic “window
box” or “shot-hole” injury on the
inner leaves of plants. Peak
feeding usually occurs in mid-
June, but since spring weather
delayed egg laying this year,
larval activity will probably peak
later. Growers should scout fields
weekly unitl com reaches the pre
tassel stage.
Base control decisions on a
random sample of 20 consecutive
plants in five locations throughout
a field. “In general” Whalen says,
“we recommend an in
secticide treatment when 50
percent or more of the plants
checked have fresh borer feeding.
No treatment will be effective once
larvae begin to bore into the
stalks.”
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Insects feeding in whorl stage corn
If entire leaves are destroyed or
if large, ragged holes appear along
leaf margins, look for armywork
larvae in the whorls. This insect
does infest small plants and may
cause severe damage. But
regrowth usually occurs unless the
growing point is killed.
Armyworm outbreaks are most
likely to occur under the following
conditions: (1) if spring weather
has been cool and wet; (2) if fields
had pre-plant weed growth; (3) if
no-till com was planted in a cover
crop; or (4) if com was planted
near wheat fields infested with
armyworms.
Start scouting for armyworm
injury during the early whorl stage
and continue until the late whorl
stage. Be sure to check for the
presence and size of larvae.
Whalen recommends treating if
more than 25 percent of the plants
are damaged and larvae are less
than IV4 inches long. Larvae larger
than that are almost through
feeding so there’s no point treating
for them, she added.
While scouting fields for com
borers and armyworms, producers
may also find stalk borers feeding
in the whorls. Stalk borer moths
lay their eggs early in the fall on
dead grasses and weeds. Over
wintering eggs hatch from mid-
May to mid-June and larvae
initially feed on nearby grasses.
The insect is easy to identify
because of the purple saddle on its
midsection and the white
longitudinal stripes on either end.
In the past, infestations occurred
along field borders next to grassy
areas, but with the increase in no
till com acreage in Delaware,
infestations have been occurring
throughout no-till fields with pre-
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Crooked Acres Farm in New Hope,
Pennsylvania is the high herd in Bucks
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and his wife Elizabeth have five other
children; Jim, Betsy, Dan, Jeffrey, and
Becky. David is also married and he
and his wife Christine have three
children: Andrew, Peter, and Matthew.
In addition to outstanding milk
production (19,337 milk and 753 fat
(3.9%) this herd was one of only about
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plant grass weed groth and/or a
cover crop.
Whalen says stalk borer larvae
damage com in two ways. They
either burrow into the stalk at
ground level and move up through
the plant, causing wilted leaves
and buckled plants; or they feed in
the whorls, producing ragged holes
in the leaves before bonng into the
stalk. Base treatment decisions on
a sample of 20 consecutive plants
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The managing partners of Crooked Acres
Farm, New Hope, PA are pictured with
their homebred excellent cow, Crooked
Acres Million Aggie (Ex-90) who produced
21,021 milk and 831 fat (4.0%) at 5 years,
4 months of age. From left to right,
Christine (Dave's wife) holding 2 year old
son Peter, David, Leonard, Andrew, and
Elizabeth (Leonard’s wife) holding her 1
year old grandson Matthew.
The cropping program on this
Eastern Pennsylvania farm consists of
70 acres of hay crop, 50 acres of corn
for silage, and 45 acres of small grains.
All the corn is raised for silage. High
moisture shelled corn is purchased at
harvest, as Leonard does not feel it has
been cost effective in past years to
grow his own.
In addition to high moisture shelled
corn, corn silage, hay crop silage, and
baled hay are fed to this high
producing, high testing herd. Farm
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a field. If larvae are still feeding m
the whorls and high numers are
moving from grassy areas into the
corn, treat with an insecticide.
Treatment will have no effect,
once larvae have bored into the
stalks.
Delaware residents who want
further information on in-season
crop pest activity can call the
University of Delaware’s crop pest
hotline, 1-800-345-7544.