AgWeb
07-19-07
Time to Scout for Western Bean Cutworms
AgWeb.com Editors
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.”
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.
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,” he says. “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