Indianapolis, Indiana
April 21, 2005Source:
Mycogen Agronomy Update
Producing a successful crop is
challenging enough without the threat of yield-robbing pests.
Whether it's an insect pest that has been encountered for years
or the threat of something new, growers should scout their
fields early and often this year to ensure their crop is
protected. Though pests observed will vary according to
geography, the following tips from Jeff Housman, District
Agronomist, Mycogen Seeds, will help growers make the most out
of their insect scouting trips this season.
Black cutworm
"Scouting for black cutworms should be done at least once a week
for a three- to four-week period after the crop emerges," says
Housman. "Black cutworm feeding may be present above and below
the soil surface, so growers and their crop consultants should
look for areas where plants have not emerged, plants that have
been clipped off and are lying beside their stump, or plants
that have died within the row. Sometimes they may have been
clipped below the soil surface."
Black cutworms migrate in
spring, laying eggs on winter annuals. Once eggs are laid,
cutworms will start cutting approximately 300 heat units later.
The worm has four dark tubercles of unequal size on top of each
abdominal segment, and mature larvae can be up to 1½ inches
long. Total feeding time for the larvae ranges from 20 to 31
days, and those in the 4- to 5-instar stage cause the most
damage.
If cutworms average less than ¾
inch in length, an insecticide should be considered if 2 to 3
percent of the plants are wilted or cut. If cutworms are longer
than 1 inch, treatment should be applied if 5 percent of the
plants are cut. If the field has a plant population of 20,000 or
less, these treatment thresholds should be lowered.
Before deciding on a control
treatment, Housman advises growers to dig around the cut plants
and assess the size of the cutworm. Treatment is not an
economically viable option if the cutworms are about to finish
their larval feeding cycle. Even if a seed-applied insecticide
is used, growers should still be aware of the level of cutworm
feeding in their fields. These products have activity on
cutworms, but are not fail-safe. The Herculex® I
Insect Protection trait will also control black cutworms,
although minimal feeding may appear before the cutworm is
controlled.
Corn rootworm
"Corn rootworm larvae can cause significant damage by feeding on
plant roots, leaving plants vulnerable to lodging during storms
with heavy rains and high winds," says Housman. "To avoid this,
growers should be diligent in checking for root feeding and
identifying common signs of corn rootworm, including brown scars
on the root surface, tunnels within the roots and various
degrees of root pruning."
The corn rootworm lays eggs
over winter in the soil. The larvae hatch during late spring and
summer, feeding on corn roots for nearly three weeks. From June
until the first week of September, they pupate from larvae into
adults. When the corn is silking during mid-to-late July until
September, crop scouts and consultants should scout the fields
to look for pregnant or gravid female beetles, and identify
fields in need of treatment.
Though today's rootworm Bt
varieties are an economically viable option to help control
heavy infestations of larvae, growers should still monitor for
corn rootworm presence. One way to do this is to place pheromone
sticky traps in fields and observe how many corn rootworm
beetles are captured in the trap over time. "August is the prime
time to monitor the flights of adult corn rootworm beetles,"
says Housman. "The usual treatment threshold is five adult
beetles in the trap per day. At this point, aerial or ground
spray-on insecticides can provide very consistent control, but
for greatest effect they should be sprayed before the beetles
lay their eggs."
European corn borer
When scouting fields for corn borer, growers should look for
masses of 15 to 35 eggs on the underside of leaves. Egg masses
are usually found near midrib, and are laid in an overlapping
pattern. Larvae are cream-colored and range in length from .04
to 1.18 inches long, depending on maturity.
More mature corn plants
(usually taller than 18 inches) are susceptible to corn borer
injury, and early planted corn is most susceptible to larval
feeding by the first generation of borers. The larvae feed on
all parts of the corn plant except the roots.
Typical early feeding signs of
European corn borer include shotholes in the leaves as they
emerge from the whorls. Damage from this pest is usually caused
by the early larval stage where pests chew on the leaves. In
later larval stages, or second generation, the European corn
borer tunnels into stalks, ears and ear shanks. Late season
signs of severe corn borer feeding include broken stalks and
tassels, poor ear development and dropped ears. "Though in-plant
Bt traits have minimized the impact of the corn borer,
it is still important for growers to scout susceptible fields
early and often," advises Housman.
Western bean cutworm
Originally found in Colorado and other Western states, western
bean cutworm (WBCW) now has been found feeding in Nebraska,
South Dakota, Kansas, Iowa, Minnesota, Missouri, Wisconsin and
the western edges of Illinois. This pest can cause substantial
yield loss and reduced grain quality if left unchecked.
"Growers should scout for WBCW
larvae mid- to late season," says Housman. "Look for larvae that
are dark brown in color, with a faint diamond shape marking on
their back. As they mature, they become a light, gray-pinkish
brown color." Mature larvae may be up to 1½ inches long, and
have three dark stripes on the segment immediately behind the
head.
WBCW larvae feed on pollen in
the developing tassel, or on silks. As the ear develops, the
larvae feed primarily on developing kernels. The treatment
threshold is reached when 8 percent of corn plants have egg
masses or small larvae. Once larvae move into the ears, they are
protected by the corn husks, making insecticide treatments
virtually ineffective. Housman suggests that growers consider
planting hybrids with the Herculex I trait, the only in-plant
trait on the market that offers protection against WBCW.
Resources
Growers who need assistance in identifying a pest or in
determining a treatment threshold should consult their local
agronomist or university. These local advisers are familiar with
insects of particular concern in a local area, and will be able
to help devise an effective control plan.
For further information, or
to speak with an agronomist in your area, please contact Stephen
Smith, Agronomy Services Manager with Mycogen Seeds, at (317)
337-4662 or via e-mail at
sesmith@dow.com.
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