Des Moines, Iowa
July 19, 2007
Source:
Pioneer Hi-Bred Internationals,
Inc.
Entomologists confirm early
captures of western bean cutworm (WBC) moths in Iowa, Indiana,
Illinois, Nebraska, Wisconsin, Minnesota and Missouri. Corn
producers should begin scouting now for egg masses and young
larvae. Timing an insecticide application can be critical, and
applications should be made before larvae enter the silks.
Marlin Rice, professor of entomology at Iowa State University,
reports that as of July 10, 76 of Iowa’s 99 counties have
reported WBC moth captures. Captures at this point are fairly
widespread with high trap captures in Clay, Cherokee, Guthrie,
Hardin, Humboldt, Monona and Woodbury counties.
According to John Obermeyer, IPM specialist at Purdue
University, northwestern Indiana pheromone trap cooperators are
reporting higher early trap counts in comparison to last year.
“It is too early to speculate on this year’s flight intensity,
but numbers so far are higher than last year,” says Obermeyer.
“Now is the time to scout for egg masses and young larvae.”
The life of a WBC
Adult WBC moths emerge in late June through July, mate and begin
laying eggs immediately. WBC moths lay eggs in masses from five
to 200. Eggs will turn purple by the fifth day of development
and hatch as larvae one or two days later.
Young WBC larvae feed on tassels and silks, but eventually
tunnel through the silk channel to reach the developing kernels.
Direct yield loss occurs as larvae consume all or parts of
developing kernels. Partially consumed kernels may be attacked
further by ear molds or secondary insect feeders that enter the
ear through the WBC feeding channel.
Because of the labor intensive nature of scouting, the critical
timing needed for insecticide applications and the possibility
that multiple treatments may be necessary, insecticides may not
be an economical or effective solution to the WBC problem.
Fields planted with in-plant control of WBC with the Herculex® I
and Herculex® XTRA insect protection traits have shown to be
effective in protecting corn against WBC. Comparing fields with
Herculex I or Herculex XTRA to fields without this technology -
a conventional hybrid or a hybrid with another Bt trait - is the
best way to evaluate the trait’s effectiveness against WBC.
The Herculex I gene protects the corn plant against European and
southwestern corn borer, western bean cutworm, black cutworm,
fall armyworm, corn earworm, while the Herculex® RW trait
protects against western and northern corn rootworms. The
Herculex XTRA insect protection trait contains both Herculex I
and Herculex RW traits.
Scouting recommendations
Obermeyer suggests that egg scouting begin once moths become
active.
“In five different areas of a field, inspect 20 consecutive
plants for egg masses which are laid on the upper surface of the
top leaves of corn,” says Obermeyer. “Also look for larvae that
may have hatched and crawled to the whorl and begun to feed. For
hybrids lacking resistance to WBC, a treatment threshold of 8
percent of the plants with an egg mass and/or larvae in the
tassel is suggested.”
Obermeyer adds that timing is critical. As soon as pollination
begins, larvae will make their way into the ear via the silks
and become impervious to insecticides.
The 8 percent nominal threshold was developed at the University
of Nebraska, notes Rice.
“A nominal threshold is one that is based upon a person’s
understanding of the pest’s biology tied together with field
experience, and it is rarely based on rigorous research,” says
Rice. “The University of Nebraska threshold was developed during
the years when field corn was worth about $2 per bushel, but
with corn in the $3.50-per-bushel range now, it would make sense
to cut the threshold in half, to 4 percent, of the plants
infested with an egg mass.”
The University of Nebraska recommends the following nominal
economic threshold:
- Eight percent of plants
with egg masses or small larvae
- If eggs have hatched,
spray at 95 percent tassel emergence
- If tassels already are
emerged, spray when most of eggs are expected to hatch
“It sometimes doesn’t hurt to get
a second opinion,” says Rice. “Last winter, I spoke to Dr. Earle
Raun, a private crop consultant from Nebraska. Dr. Raun’s
threshold is about half of the University of Nebraska threshold,
or right in the ballpark, if we halved the Nebraska threshold
based on a current higher cash value for the crop. The
perspective I appreciate with this threshold is that it
incorporates a ‘field experience’ component and increases as the
crop matures. This suggests that it would require more insects
to cause economic damage in the later plant stages.”
Nominal threshold recommended by Dr. Earle Raun:
- Five percent of plants
with egg masses or small larvae on silking/blister/early
milk-stage corn (R1 to early R3)
- Twenty percent of
plants with egg masses on mid-milk-stage corn (R3)
- No threshold on
late-milk/dough/early-dent-stage corn (late R3 to early
R5) unless seed production field
Pioneer Hi-Bred, a DuPont
business, is the world’s leading source of customized solutions
for farmers, livestock producers and grain and oilseed
processors. With headquarters in Des Moines, Iowa, Pioneer
provides access to advanced plant genetics in nearly 70
countries. DuPont is a science-based products and services
company. Founded in 1802, DuPont puts science to work by
creating sustainable solutions essential to a better, safer,
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and construction; communications; and transportation.
® Registered trademark of Pioneer Hi-Bred.
Herculex insect protection technology by Dow AgroSciences and
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