Agricultural Innovations
Organic Insect Management in Sweet Corn
Scouting, thresholds and management methods for key caterpillar pests
By Ruth Hazzard and Pam Westgate
Department of Plant, Soil and Insect Sciences, UMass Extension Vegetable Program, University of Massachusetts Amherst.
Introduction
W
hen customers flock to markets in search of sweet
corn, they want it to be of the highest quality --
sweet, fresh and worm-free. Yet, in ecological or organic
production of sweet corn, achiev ing worm-free corn is one of
the most difficult challenges. In the Northeast, three major
caterpillar pests -- corn earworm, European corn borer, and
fall armyworm -- invade ears and cause ugly feeding dam-
age. Without effective controls, it is impossible to produce
high quality corn throughout the season.
This fact sheet discusses an in tegrated strategy for control-
ling these three caterpillar spec ies using methods that meet
current organic certification standards. Any grower inter-
ested in methods that are safe for the applicator and the envi-
ronment may be interested in this approach. The compo-
nents of this strategy are 1) monitoring to determine pest
pressure and need for treatment a nd, if necessary, 2) a direct
treatment of each ear with a microbial or botanical insecti-
cide carried in vegetable oil to control corn earworm, 3)
Trichogramma releases and/or foliar applications of Bacillus
thuringiensis (Bt) or spinosad to control European corn
borer and/or 4) foliar applications of Bt or spinosad for fall
armyworm control.
Inside this fact sheet:
#
Introduction
#
Corn Earworm
#
European Corn Borer
#
Fall Armyworm
#
Putting it all Together
#
Conservation of Natural Enemies
#
SARE Research Synopsis
#
References
SARE Agricultural Innovations are based on
knowledge gained from SARE-funded projects.
Written for farmers and agricultural educators,
these peer reviewed fact sheets provide practical,
hands-on information to integrate well-researched
sustainable strategies into farming and ranching
systems. The articles are written by project
coordinators and published by SARE.
Northeastern U.S. The methods presented in this fact
sheet were developed in the Northeastern U.S.; how-
ever, they can be used anywhere in North America
where these pests are a problem. The mix of caterpillar
pests and the timing of their infestation in corn depend
upon geographic location, requiring the use of localized
IPM scouting for each pest. This method has not been
extensively tested in southern regions where corn ear-
worm over-winters and pressure may be greater than in
the Northeast.
G EOGRAPHIC R ANGE :
Fact Sheet Practical applications for
sustainable agriculture
For the online version go to www.sare.org/publications/factsheet/0105.htm
Sustainable Agriculture Research & Education
Organic Insect Management in Sweet Corn SARE 2
Corn Earworm (Helicoverpa zea)
The corn earworm is a widespread pest, which is also
known as tomato fruitworm, and cotton bollworm. In
southern areas of the U.S., corn earworm over-winters and
infests sweet corn throughout the season. Corn earworm
moths reach northern states through annual migrations,
invading late-season corn from mid-July through Septem-
ber. In New England, large numbers of moths can arrive
suddenly on storm fronts that move up the coastline and
river valleys. The
heaviest numbers
are found in
coastal areas, but
corn earworm
can be a devastat-
ing pest in late-
season corn any-
where in the
Northeast.
Adult moths have
light tan scales
and are about 1 ¼
inches in length.
Distinctive fea-
tures are a dark
spot on the fore-
wing, a dark band
near the margin of
the hind wing and,
in live moths, bright green eyes. Female moths lay single
cream-colored, globe-shaped eggs on the silk as well as
other parts of the plant. They are attracted to the odor of
corn silk; dried silks are less attractive than fresh silk as
egg-laying sites. They lay an average of five eggs per day
over their seven- to14-day life span. Eggs hatch in three to
seven days, depending on temperature, and newly hatched
larvae move directly down the silk and into the ears to feed
at the tip of the ear. Unlike European corn borer and fall
armyworm, earworm larvae do not tunnel through the
husk to reach the ear. Corn earworm caterpillars reach 1 ½
to 2 inches when full grown and have small bumps and
hairs that give the body a rough texture. They can be
brown, tan, green, or pink, with light and dark longitudinal
stripes (photo A). Their head capsule is always golden
brown.
Monitoring and thresholds
Trapping moths is a critical IPM tool for monitoring corn
earworm flight activity. Either blacklight or pheromone traps baited with corn earworm lures can be used. Black-
light traps can be placed near corn fields, but not necessar-
ily in them, and give a reasonable estimate of populations
up to one mile away from fields. Traps should be checked
daily, and capture of any corn earworm moths should trig-
ger treatment [1]. The pheromone trap should be placed in
freshly silking corn with the lure at ear height (photo B).
Lures are suspended in an opening at the base of the trap
and replaced every two weeks. Two traps per field, at least
50 feet apart, are
recommended.
When the silk
dries, move the
traps to a new
block of corn in
fresh silk. Count
the moths captured
in each trap twice
weekly. Trap cap-
tures totaling two
moths per week
per trap indicate
that a damaging
population of corn
earworm is pre-
sent. Damage will
increase as trap
captures rise [1, 9].
Monitoring on your
own farm provides
the most accurate
and timely informa-
tion on corn ear-
worm flights; how-
ever, regional data
also can be used. Cooperative Extension systems in many
states maintain Helicoverpa zea trapping networks and
report captures regularly throughout the season. Contact
your county or state Extension system to determine what
information is available in your area.
Control: foliar spray applications
Foliar sprays of Bacillus thuringiensis (Bt ) will not control
corn earwor
m. Recent research suggests that spinosad,
which became available in an organic formulation
(Entrust™) in 2003, can suppress corn earworm if used on
a three- to four-day spray schedule, as in the same manner
that broad-spectrum insecticides are used in conventional
production. Further work is needed to evaluate the effec-
Photo A. Caterpillar pests in sweet
corn: corn earworm Photo B. Heliothis net trap with corn
earworm (CEW) lure in silking sweet
corn for monitoring arrival and popula-
tion of CEW. Note that the base of the
trap, where the lure hangs, is at the
level of the corn silks. Because of the
very heavy moth pressure at this site
(>50 moths per week), counting was
made easier by covering top with plastic
bag and inserting a vapor strip.
