Alabama Agricultural Experiment Station
Auburn University
Auburn University, Alabama
Lowell T Frobish, Director
Leaflet 103
June 1987
Identification of
Lesser Cornstalk Borer
Damage to Soybeans
D.A. HERBERT and T.P MACK
INTRODUCTION
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Cover Fig. Adult male (bottom) and adult fe-
male (top) lesser cornstalk borer.
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D)ESCRIPTION OF THE STUDY
lage with an in-row subsoiler. Lesser corn-
stalk borer populations were monitored
weekly by sampling 200 plants and by exam-
ining each for insects and damage. Presence
of lesser cornstalk borer was verified by lo-
cating larvae actively living within stems or
within silken tubes attached to the plants.
Larvae also were found to be associated with
plant roots near the soil line. Stages of insect
development and characteristic symptoms of
plant damage by lesser cornstalk borer were
documented.
RESULTS AND OBSERVATIONS
Identification of the Insect
Adult lesser cornstalk borers are small
moths, approximately 12 mm (1 inch = 25
mm) in length, that are linear-shaped when
resting on vegetation. These moths are active
primarily at night and in the twilight hours
and can be easily flushed from daytime
under-canopy resting spots. A number of
moths seen flying by a grower walking crop
rows can alert him to possible infestations.
Adult males, cover figure (bottom), are tan
with a dark grey stripe down the center of the
back, while females, cover figure (top), are
more uniformly dark grey The first genera-
tion in soybeans can occur at seedling emer-
gence; thereaftei, infestations can occur any-
time during the season.
Adult female lesser cornstalk borers de-
posit small (0.6 mm diameter), greenish-
yellow eggs which turn pinkish-white on
plant stems near the soil line. Newly hatched
larvae are pink and approximately 2 mm
long. Mature larvae, figure 1, are bluish-
green with dark transverse bands and reach a
length of approximately 16 mm. As larvae de-
velop they construct a series of silken tubes,
figure 2, covered with soil particles which
they attach to feeding sites at the bases of
plant stems. Tubes are used as a refuge dur-
ing periods-when larvae are not actively feed-
ing. Larval development can vary from 16
days during hot and dry periods to 40 days
when temperatures are cooler. When ma-
ture, larvae transform into small greenish-
yellow pupae (usually in the silken tube) in
the soil near the base of infested plants. Pu-
pae darken to brown as they age.
Damage
It is the larval stage that damages both soy-
bean seedlings and mature plants. Larvae
usually bore into the main stem at or just be-
low the soil line. Several seedlings can be
damaged by a single larva. Seedlings may be
cut off at the soil line or extensive main stein
tunneling may occur causing damaged seed-
lings to wilt in place. Plants in the later veg-
etative stages are damaged when larvae bore
into the stems and tunnel upward, figure 3.
In older plants, larvae make successive bor-
ings into the stein near the soil line, thereby
riddling important nutrient conducting tis-
sues, figure 4. This damage is significant be-
cause it reduces a plants ability to withstand
other stresses, such as drought and disease.
Slight damage may cause varying degrees of
stunting. Heavy damage causes plants to turn
yellow, wilt, and die; dead plants may either
remain standing in a characteristic brown
teepee-shaped appearance, figure 5, or
lodge, figure 6, due to pressure from wind or
cultivation equipment.
There are several indicators of the potential
for or presence of lesser cornstalk borer in-
festations in soybeans. First, most infesta-
tions are associated with extensive periods of
hot and dry weather. Soybeans that are late-
planted in June (i.e., double-cropped with
wheat), when the weather is normally hot and
dry, are more likely to be attacked. Plants in
soils that dry out quickly, such as sandy soils,
are also more susceptible to attack. Easily
noticed symptoms of attack are wilted seed-
lings and yellowed, stunted, or lodged older
plants. Lesser cornstalk borer infestations
most often begin in several locations within a
field (i.e., where areas of sandy soil occur).
Other insects and certain diseases can cause
similar symptoms in soybeans, but a survey
can confirm the presence of lesser cornstalk
borers. Examinations of several damaged liv-
ing plants can be made by carefully uprooting
plants and looking for characteristic bore
holes and/or silken tubes near the base of
plant stems. Also, splitting of several stems
allows one to determine if tunneling by larvae
has occurred.
Prevention of Damage
Adequate soil moisture is probably the
best defense against lesser cornstalk borer.
Management practices which optimize or
conserve moisture (i.e., early planting, no-
till cultivation, and irrigation) are helpful in
areas prone to infestations. For example, sea-
son-long irrigation is cost prohibitive for most
growers, but one or two waterings when
plants are young and most susceptible to
lesser cornstalk borer attack could reduce
populations and be cost effective.
The use of insecticides is another control
method. Chlorpyrifos (Lorsban?) at 0.5 to
2.0 pounds technical per acre is the only cur-
rently recommended granular insecticide for
control of lesser cornstalk borer in sdybeans.
Best control results when this material is
lightly incorporated at planting in narrow
bands in the row. Cost can be minimized by
limiting treatments to fields with previous in-
festation histories or those with predomi-
nantly sandy soils. If outbreaks occur in fields
not treated at planting, early season applica-
tions can be considered. Currently there are
no economic injury levels for lesser cornstalk
borer in soybeans. However, agronomic stud-
ies indicate that a minimum of 4 to 6 undam-
aged plants per row-foot are needed for op-
timal yields. If existing larval populations
threaten to reduce stands below that density,
chlorpyrifos granules can be applied at rec-
ommended rates in narrow bands or broad-
cast. Lesser cornstalk borer larvae do crawl
on the soil surface in and between crop rows
and some efficacy can be obtained with sur-
face applications. Costs could be minimized
if treatments were applied to 'hot spots'
rather than over entire fields. It is important
to apply post-plant treatments before exten-
sive stem damage has occurred. Stem dam-
age is permanent and although applications
may reduce larval numbers, they may not en-
hance yields if many plants have already been
damaged.
SUMMARY
The lesser cornstalk borer, an important
insect pest of soybeans in Alabama, has pop-
ulation outbreaks primarily during hot and
dry years. The larvae of this insect (2-16 mm
in length) can be identified by their bluish-
green, banded color pattern or by the pres-
ence of their silken tubes attached to plant
stems. Larvae are most often found in stems,
roots, or soil near the base of infested plants.
Larvae make successive borings into the
stems near the soil line, riddling important
nutrient-conducting tissues. Heavily dam-
aged plants turn yellow, wilt, and die; dead
plants may remain standing in a teepee-
shaped appearance, or lodge.
Agronomic studies indicate that 4 to 6 un-
damaged plants per row-foot are needed for
optimal yields. If existing lesser cornstalk
borer larval populations threaten to reduce
stand density below this level, then control
measures would be profitable. However. a
knowledge of conditions favorable to the de-
velopment of infestations can alert growers to
potential problem situations and allow for
more effective preseason management strat-
egies.
FIG. 1. Lesser cornstalk borer larva inside a
soybean stem.
FIG. 2. Characteristic silken tube made by a
lesser cornstalk borer larva.
FIG. 3. Young soybean stem that has been
tunnelled into by a lesser cornstalk borer
larva.
FIG. 4. Damage to an older soybean stem
caused by lesser cornstalk borer larvae. Note
the number of bore holes, and the enlarged
stem diameter.
FIG. 5. Brown and teepee-shaped dead
plants caused by lesser cornstalk borer at-
tack.
FIG. 6. Soybeans that have lodged because
of lesser cornstalk borer damage to the main
stem.
Information contained herein is available to all persons without regard to race, color, sex, or
national origin.
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