Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
Hot Topics!
First-generation European corn borers have run their
course in the whorls throughout the state. In most areas of
Illinois, larvae have developed to at least the fourth instar,
which then bores into the stalks. Larvae in stalks cannot be
controlled with insecticides. Begin making plans to scout for
second-generation corn borers in a couple of weeks, especially
in areas that got hit hard by the first generation. This page.
FOR IMMEDIATE RELEASE
No. 16 / July 11, 1997
Corn rootworm adults should begin emerging in cornfields
throughout the state very soon. Full-grown larvae have been
observed in many fields, and after a few days as pupae, the
adults will emerge from the soil—male western corn rootworms first, followed by northern corn rootworms and western
corn rootworm females. Stay alert in east-central Illinois,
where there is concern about rootworm damage to corn planted
after soybeans. Page 139.
Hot
Topic
INSECTS
Executive editor: Kevin Steffey,
Extension Entomologist
Current Status of European Corn Borers
Web subscriptions available:
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For subscription information, phone
217.244.5166, or e-mail
[email protected]
In This Issue
❏ Southwestern Corn Borers in
Southern Illinois, 138
❏ Flights of Corn Earworm Adults
Have Been Heavy in Some
Areas, 139
❏ Time to Begin Watching for Corn
Rootworm Adults, 139
❏ Two Girls for Every Boy: Sex
Ratios of Western Corn Rootworms in Soybean Fields, 141
❏ Potato Leafhopper–Resistant
Alfalfa, 141
❏ Conditions Favorable for Gray
Leaf Spot, 142
Reports during the first full week of July and the early part of the second
week confirmed our assessment of European corn borer infestations in Illinois. In some areas of the state, infestations were quite heavy; and, in other
areas, corn borer activity was minimal. The outbreaks were localized, not
widespread as they were in 1996. However, some consultants, dealers, and
growers dealt with some severe infestations. Bill Craig, a private consultant
in Macoupin County, indicated that infestations of first-generation European
corn borers in Macoupin County and parts of Montgomery County were the
heaviest he had observed in 20 years. John Thieme, with Zeneca Ag Products, observed a field in Henry County in which 83 percent of the plants
were infested with an average of 3.3 larvae per plant.
Most people now are finding that larvae (fourth instars) have already bored
into the stalks or are just about ready to do so (third instars). Obviously, the
time for treatments to control first-generation European corn borers has
passed. Although you might still find some small larvae in the whorls, if
most of them have tunneled into the stalks, it’s too late to treat.
Now it’s time to wait for the second generation to begin. We have no idea
what we will face; but in areas where the first generation was impressive,
keep your eyes open. If mortality factors (weather, natural enemies) don’t
reduce the numbers of borers that complete the first generation, the second
generation likely will be larger. Also stay alert in areas that escaped the first
generation problems. Again, good survival of corn borer adults, eggs, and
early instar larvae will increase the potential for economic infestations of
second-generation borers.
137
Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
Ron Hines, a senior research specialist
at the University of Illinois Dixon
Springs Agricultural Center in “deep”
southern Illinois, was splitting stalks
to evaluate a Bt-corn trial during the
first week of July; and he observed
newly hatched European corn borer
larvae. And they are catching European corn borer moths in their light
traps. Apparently the second generation of these corn borers is under way,
so we will have little time to relax
between the two generations throughout the state. Table 1 shows some
information regarding accumulated
degree-days (developmental threshold
of 50oF) from initial capture of moths
in the spring to first occurrence of life
stage or activity of first- and secondgeneration European corn borers. If
you are charting degree-day accumulations, this information should provide a “biofix” for your area and a
way to keep track of the speed of
development of the second generation
of European corn borers
Kevin Steffey, Extension Entomology,
(217)333-6652
Southwestern Corn Borers in
Southern Illinois
Ron Hines also reported some information about southwestern corn borers, a pest of corn only in the southern
counties of the state. During his stalksplitting evaluations at the Dixon
Springs Agricultural Center, he found
nearly full-grown larvae in the stalks.
However, the numbers present this
year are not nearly as large as they
were last year, when several fields in
southern Illinois had economic infestations. Ron anticipates the flight of
adults to begin in a couple of weeks,
marking the beginning of the second
generation of this pest.
Because southwestern corn borer
larvae can be confused with European
corn borer larvae, a reminder of distinguishing characteristics and differences in biology is in order. This is
most important for the second generation because southwestern corn borers
cause distinctly different damage to
stalks than European corn borers.
Southwestern corn borer females may
deposit eggs singly or in small masses
of up to five eggs on either the upper
or lower surface of a corn leaf. The
eggs are oval, flattened, and creamcolored when first deposited, and, if
deposited in masses, overlap like the
scales of a fish, much like the eggs of
European corn borers. However,
within 48 hours after the eggs are
deposited, three pink to red transverse
bars appear on the eggs.
Small southwestern corn borer larvae
might be confused with European corn
borer larvae. However, a southwestern
corn borer larva appears to have indistinct bands across its body, from
which hairlike setae arise. The
bandlike appearance becomes more
pronounced as the larvae grow, until
the spots formed by the tubercles
(small, knotlike bumps) become visibly distinct in the fourth instar. The
last (fifth) instar has very distinct,
large, dark tubercles. Like European
corn borers, the last two instars tunnel
in the stalk. If you have a good magni-
Table 1. Accumulated degree-days (developmental threshold of 50°F) from initial capture of moths in the
spring to first occurrence of life stages and general activity of European corn borers (from European
Corn Borer: Ecology and Management, NCR Publication no. 327, Iowa State University, Ames).
Accumulated
degree-days
First occurrence
of stage or event
Days to first
occurrence a
General activity
First generation
0
212
318
435
567
792
1,002
First spring moth
Egg hatch (first instar)
Second instar
Third instar
Fourth instar
Fifth instar
Pupa
16.3
6.6
6.5
6.6
10.2
7.6
Egg hatch (first instar)
Second instar
Third instar
Fourth instar
Fifth instar
8.2
4.1
4.3
5.1
9.0
Pinhole leaf feeding
Shot-hole leaf feeding
Midrib and stalk boring
Stalk boring
Stalk boring
Changing to adult
Second generation
1,404
1,510
1,627
1,759
1,984
a
Pollen and leaf axil feeding
Leaf axil feeding
Sheath and collar feeding, midrib boring
Stalk boring
Stalk boring
Average number of days of development to reach the first occurrence of the stage or event since initiation of the
previous stage listed. For example, it takes approximately 16 days from first moth capture to egg hatch; first instars
require approximately 6.6 days to develop to second instars; etc. The number of days varies if temperatures are
cooler or warmer than average.
138
Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
fying glass or microscope and a little
patience, another characteristic also
distinguishes the two species. On the
bottom of the prolegs (the peglike
false legs on a caterpillar’s abdomen)
of the southwestern corn borer, the
tiny hooks (called crochets) form a
complete circle. On a European corn
borer larvae, the crochets on the bottom of the prolegs do not form a complete circle.
Whorl-feeding injury caused by southwestern corn borer larvae can be confused with injury caused by both European corn borers and fall armyworms. Early instar southwestern corn
borer larvae “windowpane” the leaf
tissue, much like small fall armyworms; however, the windowpaning
usually is more extensive. Injury
caused by larger southwestern corn
borer larvae resembles whorl-feeding
injury caused by European corn borers, except more leaf tissue is removed. The injury is less ragged looking than injury caused by fall armyworms, and southwestern corn borer
larvae do not produce the dark, coarse
frass produced by fall armyworms.
Southwestern corn borers have two
generations per year in southern Illinois. The second generation of southwestern corn borers causes damage
that is much more severe than damage
caused by European corn borers. The
larvae that will overwinter tunnel very
low in the plant, girdling the stalk
from within. Girdling greatly weakens
the stalks, inevitably resulting in
lodged corn.
For the second generation of southwestern corn borers, begin scouting
about 2 weeks before corn tassels and
continue for 2 weeks after pollination
is complete. Look for egg masses and
larvae on the leaves, or for larvae
behind the leaf sheaths. A treatment
may be justified when 25 percent of
the plants have eggs, or larvae in the
whorls or behind leaf sheaths.
Results from the western Corn Belt
and from the Dixon Springs Agricultural Center indicate that Bt-corn provides effective control of southwestern
corn borers. However, just as you
should be observing the absence (or
presence) of European corn borers in
Bt-corn, keep your eyes peeled for
signs of southwestern corn borers, too.
We need to monitor whether this new
technology continues to perform in the
field as it has in numerous research
trials.
Kevin Steffey, Extension Entomology,
(217)333-6652
Flights of Corn Earworm Adults
Have Been Heavy in Some Areas
Rick Weinzierl, Extension entomologist on campus, reported a couple of
weeks ago that the numbers of corn
earworm adults captured in traps were
extremely large in southwestern Illinois (100 to 400 per night). Flights
elsewhere were very light (0 to 1 or 2
per night in east-central Illinois), light
(3 to 6 per night at Dixon Springs),
low to moderate (6 to 10 per night
near LaSalle-Peru, and 3 to 5 per night
near Belvidere), to heavy (46 per night
in one trap in central Illinois, and 20 to
100 per night in western Illinois).
Although this news is a bit old, I
thought you might want to keep corn
earworms in the back of your mind.
Field corn rarely is economically
damaged by corn earworm larvae, but
their presence can increase the incidence of ear rots. It should also be
interesting to watch for the presence
of corn earworm larvae in Bt-cornfields. Bt-corn does not provide outright control of corn earworms, but
research indicates that it “messes up”
their development. Other research
indicates that the incidence of aflatoxin associated with corn earworm
injury is reduced in Bt-corn hybrids.
Producers of seed corn and sweet corn
always are concerned about corn earworms. To place the previously mentioned moth counts in perspective, the
guideline for protecting seed corn is to
treat if 10 or more moths are captured
in a pheromone trap for several consecutive nights. However, only fields
silking and/or pollinating during this
time would be candidates for treat139
ment. This information was taken
from Seed Corn Pest Management
Manual for the Midwest, published by
Purdue University. Rick Weinzierl
offers the following treatment guideline for corn earworms in sweet corn:
Earworm sprays should begin within
1 or 2 days after first silk whenever
moth counts consistently are more
than 2 to 3 per night. Spray intervals
should be based upon moth counts and
temperatures—as short as every 2
days when moth counts exceed 50 to
100 per night and temperatures are in
the 90s; as long as every 4 to 5 days
when moth counts are under 10 and
daily highs are in the 70s. Just one
more caterpillar to think about!
Kevin Steffey, Extension Entomology,
(217)333-6652
Hot
Topic
Time to Begin Watching for Corn
Rootworm Adults
In last week’s issue of the Bulletin (no.
15, July 3), Mike Gray brought you
up-to-date on the progress of corn
rootworm larvae feeding below
ground. Reports about rootworm larval damage to corn roots have begun
to trickle in. We always anticipate
some reports about rootworm damage
in fields of corn planted after corn if
an insecticide was not applied at planting, or if the insecticide failed to control the larvae adequately. However, in
recent years, concern about rootworm
larval injury to corn planted after
soybeans has increased dramatically in
east-central Illinois, where western
corn rootworm females are laying
eggs in soybeans (refer to next article).
Consequently, people in the affected
area usually are on alert to determine
if the problem has reoccurred or
spread. Howard Brown with Pioneer
Hi-Bred International already has
observed numerous corn rootworm
larvae in part of a field in Champaign
County that had not been treated with
a soil insecticide. He observed as
many as seven larvae per plant. A
grower in Iroquois County discovered
Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
some lodged corn in spots in one of
his fields and determined that rootworm larvae had severely pruned the
root systems. So it’s time to make an
extra effort to determine whether corn
rootworms have injured roots in cornfields.
It’s been stated more than once, but
egg hatch of corn rootworms this year
was delayed. Although we generally
would have found corn rootworm
adults in cornfields by now, their appearance is delayed, just as it was last
year. However, during the next couple
of weeks, both western and northern
corn rootworm adults should begin
emerging in fields throughout the
state, so we should be vigilant. When
emergence of large numbers of rootworm adults coincides with silking
and pollination, the threat of the
beetles’ clipping silks becomes a reality.
Western corn rootworm males emerge
first, followed by western corn rootworm females. After emergence and
mating, about 14 days elapse before
the females begin laying eggs. Western corn rootworm beetles are about
1/4 inch long. The background color
for both males and females is yellow,
but the two sexes differ somewhat in
their markings (Figure 1). On males,
nearly the entire front half of each
wing cover is black; only the tips of
the wing covers are yellow. Females
usually are slightly larger and have
three distinct black stripes on the wing
covers, one on the outside of each
wing cover and one in the middle.
Gravid (pregnant) females have distended abdomens.
Northern corn rootworm beetles
emerge soon after emergence of western corn rootworms begins. Northern
corn rootworms also are about 1/4
inch long, but they have no distinct
markings (Figure 1). Newly emerged
northern corn rootworms are cream
colored or tan, but they become green
as they age. The females are slightly
larger, and pregnant females have
distended abdomens.
Figure 1. Western (top), northern (bottom left), and southern (bottom right) corn
rootworm adults (illustration from Seed Corn Pest Management Manual for the Midwest, Purdue University).
140
You also may encounter 12-spotted
cucumber beetles (also called southern
corn rootworms, although technically
only the larvae are called southern
corn rootworms). These insects do not
overwinter in Illinois. They arrive
from southern states early enough in
the growing season that they frequently are found alongside the western and northern species. The adult
southern corn rootworm is slightly
larger (3/8 inch long) than the western
and northern corn rootworms, is yellow-green, and has 11 conspicuous
black spots on the wing covers (Figure
1). They rarely cause much silk-clipping injury.
If cornfields are not silking or pollinating when corn rootworm beetles
begin seeking food, both western and
northern corn rootworms will feed on
the epidermal layer of corn leaves and
partially or totally strip the leaves of
green tissue. This type of injury seldom is economically important, but it
looks fairly ugly. The northern species
also will feed on the blossoms and
pollen of many species of weeds.
Start making plans now to scout for
rootworm beetles. The first objective
of scouting for rootworm adults is to
determine their potential to interfere
with pollination. In commercial field
corn, treatment may be justified if you
find five or more beetles per plant,
pollination is not complete, and silk
clipping is observed. In seed corn,
treatment is justified if the silks on 20
percent of the plants have been
clipped to a length of 3/4 inch or less,
pollination is still taking place, and
rootworm beetles are present. A second objective of scouting for rootworm beetles is to determine the potential for rootworm problems next
year, either in corn after corn or corn
after soybeans. Guidelines for corn
after corn have been determined and
will be offered in a future issue of this
Bulletin. However, research on scouting soybeans to determine whether or
not rootworm larvae will be a problem
in next year’s corn is still under way
(refer to next article). We’ll give you
Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
as much information as we can as our
summer research projects progress.
havior of western corn rootworms in
cornfields.
A third objective of scouting for rootworm beetles is to determine whether
you should prevent them from laying
eggs this year to prevent rootworm
larval injury next year. This practice
has specific guidelines for corn after
corn (to be printed in a future issue of
this Bulletin), but not for corn after
soybeans. Again, we’ll offer any suggestions we can ascertain during this
year’s research activities, which now
include a large-scale areawide study in
which the primary objective is to
prevent corn rootworm adults from
laying eggs. Keep watching for new
information.
During the past few months, we have
been examining about 2,000 yellow
sticky traps from our study last year
and recording the numbers of western
corn rootworm males and females.
Male western corn rootworms emerge
from the soil before females. Traps
placed in both continuous and rotated
cornfields capture males first. As
adults continue to emerge, the ratio of
females to males increases. Studies by
Purdue entomologists Larry Godfrey
and Tom Turpin reported in 1983
showed that the percent of females
was greater in first-year corn than in
continuous cornfields (slightly less
than 60 percent in first-year corn). The
authors of this study stated that “the
presence of more female beetles in FC
(first-year corn) fields could result in
higher WCR [western corn rootworm]
egg density in the soil in the fall and a
higher larval damage potential the
following year.”
Kevin Steffey, Extension Entomology,
(217)333-6652
Two Girls for Every Boy: Sex
Ratios of Western Corn Rootworms in Soybean Fields
As the corn growing season enters
July, you might be concerned about
this year’s population of corn rootworms. Last year was the first season
of a three-year project to develop an
economic threshold for western corn
rootworms captured with Pherocon
AM yellow sticky traps in soybeans.
This threshold will help us predict the
potential level of damage growers
might expect in the next year’s corn
crop. As we have reported previously
for 1996, the number of adults captured in corn was lower than the number captured in soybeans, with the
peak occurring near the second week
of August.
We have suggested that western corn
rootworms in east-central Illinois have
developed a new behavior of laying
eggs in soybean fields, which would
explain the recent increase in the incidences of damage to corn planted after
soybeans. We do not have an explanation for this new behavior, but our
trapping efforts last year will allow us
to compare this new behavior (egg
laying and movement into soybean
fields) with what we know about be-
As you might have expected, we have
learned that the percent of females in
soybeans is similar to the percent of
females in first-year corn. Data gathered from a soybean field in Vermilion
County in 1996 revealed that females
in the western corn rootworm population were 57.6 percent during the last
week of July, increased to 71.0 percent
the next week, and decreased to 61.8
percent during the last week of trapping (ending on August 19). Growers
who continued to put out traps after
August 19 captured 60 to 75 percent
females.
At the end of 1997, we should be able
to report a preliminary threshold for
western corn rootworms in soybeans.
Information to date suggests that we
should expect the trends of increased
populations of western corn rootworms in soybeans and increased
incidences of reports of larval injury
to corn planted after soybeans to continue.
The similarity in sex ratios of western
corn rootworms in soybean fields and
in first-year corn after soybeans sug141
gests that the pest may be selecting
soybeans as another egg-laying site.
We will use yellow sticky traps again
this year in both cornfields and adjacent soybean fields to monitor the
behavior of the insects. This information will provide some insight into the
factors that are influencing the change
in egg-laying behavior of western corn
rootworms. Stay tuned as we continue
to study this dynamic pest.
Matt O’Neal, Entomology Graduate
Research Assistant, (217)244-2637
Potato Leafhopper–Resistant
Alfalfa
A lot of folks are talking about the
varieties of alfalfa that are resistant to
potato leafhoppers. Several seed companies are selling this alfalfa, and
many of us are curious about how it
holds up in the fields this year, particularly if densities of potato leafhoppers are large, as they are in some
areas of the state right now.
To help us stay abreast with the performance of potato leafhopper–
resistant alfalfa, Dave Feltes and Jim
Morrison, Extension educators in IPM
and crop systems at the Quad Cities
and Freeport Extension centers, respectively, are monitoring leafhopper
densities in a strip trial in northwestern Illinois. They will be counting
adults and nymphs and assigning an
injury rating to at least six different
plots throughout the summer. At least
one of the varieties is susceptible. The
comparisons should be interesting. I
would imagine that counts of adults
may not be different among the plots
(nothing in the resistant varieties repels leafhopper adults). However,
counts of nymphs and injury ratings
should be quite low in the leafhopperresistant plots if the varieties hold up.
We’ll provide information as it is
gathered to keep you informed.
Kevin Steffey, Extension Entomology,
(217)333-6652
Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
PLANT DISEASES
Conditions Favorable for Gray
Leaf Spot
With the high temperatures, frequent
rainfall, and high humidity, conditions
are favorable for gray leaf spot (GLS)
to appear on corn leaves. Howard
Brown, agronomist for Pioneer HiBred, reported finding lesions on
lower leaves in fields in west-central
Illinois. The disease is present but
does not represent any real threat to
crop development at this time; however, it is increasing in severity across
the Midwest, due in part to the adoption of reduced and no-till practices.
This pathogen survives readily in corn
debris and sporulates profusely in the
early spring if weather conditions are
favorable. The disease is most severe
in continuous no-till corn and can
cause extensive damage in reduced
tillage fields if crop rotation is not
practiced.
Lesions are identified by their rectangular or blocky appearance on susceptible corn plants. Lesions are pale
brown or gray to tan in color and are
1/4 to 2 inches in length. They are
restricted by the veins and usually
have blunt or squared-off ends. In
susceptible hybrids, lesions may coalesce, causing extensive tissue necrosis.
GLS typically appears on the lower
leaves because the spores are being
either windblown or rain-splashed
from previous crop debris. Extensive
blighting may result, followed by
death of the plant and stalk breakage
or lodging. If favorable environmental
conditions occur, GLS may kill entire
fields before they mature.
GLS is favored by warm, humid conditions and frequent rainfall. Fungal
spores can survive at humidities as
low as 60 percent, but infection and
colonization of the host does not occur
unless relative humidities climb above
85%. This pathogen has a long latent
period when no symptoms are visible.
This period may last from 2 to 4
weeks. Thus, once initial symptoms
are evident, the disease severity may
already have reached the epidemic
point.
Control of gray leaf spot should begin
with identification of potential problem fields and the selection of resistant or tolerant hybrids for these fields.
Because GLS is favored by high humidities, only tolerant or resistant
hybrids should be planted in these
fields. River bottom fields, for example, are typically humid and offer
the most favorable environment for
GLS infection. Crop rotation is also
important for GLS control. This pathogen cannot survive for extended periods without a host plant. Thus, rotating soybeans or another nonhost crop
helps to reduce the inoculum level.
Plowing heavily infected fields also
reduces carry-over inoculum levels.
Once buried, the fungus cannot produce spores and infect the corn crop.
However, care should be taken with
plowing, especially with regard to
slope of fields and erosion considerations. Separation of fields can be a
minor, although important, method of
reducing infections by GLS. Because
this pathogen is primarily wind dispersed, corn crops should not be
planted adjacent to a field with high
corn residues if GLS was a problem in
that field the past season. If winds
blow across the residues, spores may
be transported to the new corn crop
and early infections begun.
Field scouting for GLS should begin
at least 2 weeks before tasseling and
continue until at least 2 to 3 weeks
past tasseling. If lesions are at or
above the ear leaf, a fungicide application of Tilt may significantly improve
performance. It is difficult to determine an exact threshold for leaf damage from GLS because hybrids vary in
their response to infection. Some hybrids can tolerant rather extensive
damage before yield losses appear
while others will react to lesser
amounts of damage. Various estimates
place the leaf area damage in the range
of 10 to 25 percent on the bottom four
leaves before economic losses can be
expected to occur. This allows for a bit
of risk adversity on the part of the
producer when determining the need
for fungicide applications. Decisions
to apply fungicides should be made
with the performance of the hybrid in
mind, as well as the 30-day weather
forecast. If past experience indicates
that this hybrid is sensitive and the
forecast is for continued favorable
weather (warm, high humidity, rainfall), then a yield benefit can be expected from a Tilt application. Expected response to Tilt applications
can range from 10 to 30 bushels per
acre, depending on the factors mentioned. However, the key to economical benefit for any fungicide is scouting and an application made based
upon the presence of the pathogen.
The critical period for protecting the
plant is usually 2 weeks before to
2 weeks after tasseling. However, if
leaf blights are present earlier and
appear to be moving upward on the
plant, earlier fungicide applications
may be needed. This practice is not
recommended because it usually
means that you will need other applications made later, when they would
be far more effective.
H. Walker Kirby, Extension Plant Pathology (217)333-8414
The information provided in this publication is for educational purposes only. References to commercial products or trade names do not constitute an endorsement by the University of Illinois and do not imply discrimination against other similar products.
142
Pest Management & Crop Development Bulletin • No. 16 / July 11, 1997
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