December 2020 - 16

Whipping
WORMS
Pest control strategies for
organic sweet corn
European corn borer (Ostrinia nubilalis) Photo: Mariusz Sobieski/Bugwood.org
By Dean Peterson
VGN Correspondent
European corn borer, fall armyworm
and corn earworm are problem pests
for all sweet corn producers, but are
especially troublesome for organic
growers.
The wide adoption of Bt field corn
dramatically changed the European corn
borer populations organic sweet corn
growers face.
" European corn borer numbers
have collapsed in the eastern U.S.
because of the widespread adoption
of field corn expressing Bt toxins, "
said Kris Holmstrom, vegetable IPM
program associate at Rutgers University.
Holmstrom recently spoke on pest control
in organic sweet corn production at the
New Jersey Agricultural Convention and
Trade Show in Atlantic City.
Despite steep declines in European
corn borer numbers, sweet corn growers
must still be vigilant because there can be
occasional outbreaks.
Start scouting for European corn borer
at the whorl stage when the plants reach
about 16 inches tall. Check five consecutive
plants each in 10 random locations
throughout the field. The number of plants
showing signs of injury multiplied by two
is the total injury percentage.
The traditional threshold in New Jersey
is to treat when 12% of the plants are
injured. " Some states may have different
thresholds, " Holmstrom said.
Continue scouting weekly as tassels
emerge. Surviving larvae will emerge
with the tassels and can damage the
plants again. Once pollination begins,
the larvae will bore back into the
plants and the developing ears, and are
impossible to control.
Several types of Bt products are
available for organic sweet corn growers.
DiPel contains the Bt toxin (Cry 1Ab)
and previous trials in New Jersey have
shown it works best applied as granular
formulations dropped into the whorl.
Xentari contains the toxins (Cry 1A(a,b),
C, D) and also has activity against
armyworm.
" Products like DiPel and Xentari are all
acceptable as foliar sprays, " Holmstrom
said. These toxins are unacceptable
for organic growers when they are
genetically-engineered into the plant. Bt
products are best used in the whorl stage
because larvae must ingest the Bt toxin.
Spinosyns are a relatively new group
of products that are extremely fast-acting
and effective on larvae. " Spinosyns are
another bacterial-derived product, "
Holmstrom said. " Bt needs to be digested
and causes gut perforations. Spinosyns
affect the nervous system. " Entrust
(spinosad) is the only spinosyn approved
for organic use.
The parasitic wasp Trichogramma
ostriniae is specific for European corn
borer and releases of the wasp have
provided effective control. However, with
European corn borer populations down
dramatically, the economics of buying
and releasing the wasp are questionable.
" If you're at treatment threshold, look
for a way to put DiPel granules into the
whorl, " Holmstrom said. " Entrust is also
highly effective, but there are regulatory
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ftil
16 | VegetableGrowersNews.com
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December 2020

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