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UNIVERSITY OF
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NATURAL HIST. SURVEY

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ORN ROOTWORM

PEST MANAGEMENT

IN CANNING SWEET CORN

W. H. Luckmann, J. T. Shaw,
D. E. Kuhlman, R. Randell, C. D. LeSar

ILLINOIS NATURAL HISTORY SURVEY

STATE OF ILLINOIS
DEPARTMENT OF REGISTRATION AND EDUCATION

NATURAL. HISTORY SURVEY DIVISTORN

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URBANA, ILLINOIS
MARCH, 1975

Research contributing to this publication is supported in part by USEPA
Grant SC 802547-5 and CSRS Grant 316-15-97 involving research in coop-
eration with agricultural experiment stations in Missouri, lowa, Indiana,
and Ohio, and the Illinois Agricultural Experiment Station.

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COVER PHOTO: Northern corn rootworm adult (upper left) and western corn rootworm
adult (lower right). Photo by Larry Farlow.

Corn Rootworm Pest Management
in Canning Sweet Corn

W.H. Luckmann, J.T. Shaw, D. E. Kuhlman,
R. Randell, and C. D. LeSar

THREE SPECIES of rootworms — the northern, southern, and west-
ern corn rootworms — attack sweet corn in Illinois.

The southern corn rootworm, Diabrotica undecimpunctata how-
ardi, also called the spotted cucumber beetle, is widely distributed and
some individuals will be found in every cornfield in Illinois. It over-
winters in the adult stage, oviposits in the spring and summer, and has
several generations per year. Where southern corn rootworms are abun-
dant, severe damage can occur in late-planted sweet corn (sweet corn
coming into silk about September 1). However, this insect usually is
not a serious problem and populations rarely reach damaging numbers
in the canning areas of central and northern Illinois (latitude 40—42°
N). Thus, these guidelines on pest management concern only the north-
ern and the western corn rootworms.

The northern corn rootworm, Diabrotica longicornis, is a very com-
mon insect in Illinois cornfields. Although it is distributed throughout
Illinois, damaging populations rarely occur south of U.S. Route 50
(latitude 39° N). The western corn rootworm, Diabrotica virgifera,
invaded the northwestern corner of Illinois in 1964 and is now present
throughout the northern two-thirds of the state. The western corn
rootworm is a product of modern agriculture. It became a serious
problem in irrigated corn in western Nebraska about 30—40 years ago.
It developed resistance to the cyclodiene soil insecticides in the 1950's
and the resistant strain began to migrate and disperse. Up to 1955,
the western corn rootworm was found only in Nebraska and portions
of Colorado, Kansas, South Dakota, and Iowa (Chiang 1973). Since
then it has moved eastward, southward, and northward into all of the
corn belt states. In time, the western corn rootworm will probably dis-
perse to occupy the same range as the northern corn rootworm.
AUTHORS: W. H. Luckmann, Professor and Head, Section of Economic Ento-
mology, Illinois Natural History Survey, and Agricultural Entomology, Illinois Agri-
cultural Experiment Station; J. T. Shaw, Technical Assistant, I.N.H.S.; D. E.
Kuhlman and R. Randell, Extension Specialists in Entomology, I.N.H.S. and

University of Illinois College of Agriculture; C. D. LeSar, Graduate Assistant in
Entomology.

o ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 54

BIOLOGY AND BEHAVIOR

In biological characteristics the northern corn rootworm (NCR)
and the western corn rootworm (WCR) are quite similar. In the
agroecosystem, corn is the only host, though it has been shown experi-
mentally that the immatures of both species can develop on several
species of native grasses (Branson & Ortman 1966, 1967a, 1967b).
Adults feed on pollen, silks, and kernels of corn and on the fruiting
parts of many other plants, including weeds.

The NCR and WCR have only one generation per year. Egg lay-
ing begins in late July and continues into September in the soil in fields
of corn, with intensive oviposition occurring in August and early Sep-
tember. In general, the eggs are concentrated in the rows at the base
of the corn plants in nonirrigated fields and between rows in fields
with irrigation. The NCR prefers to oviposit at the base of the corn
plants in the plant row and most eggs are deposited in the upper 4
inches of soil. Ovipositional behavior of the WCR is slightly different
as females oviposit many of their eggs between the plant rows and
eggs are often deposited at a greater depth in the soil. Following ovi-
position, eggs develop slightly and diapause through fall and winter.
The eggs do not begin to develop again until spring. In Illinois the
eggs begin to hatch in June and first adults appear about mid-July.
A few eggs may not have hatched by the time first adults appear, but
the egg stage normally lasts about 10 months.

The larvae feed on corn roots and concentrate near the base of
the plant. The larvae consume the roots, thus decreasing nutrition
supplied to the ears, and in strong wind and rain storms plants with
severe root damage may become lodged. The larval plus pupal stages
last about 1 month. In cages in the laboratory, adult rootworm females
will live up to 2 months and oviposit for 3-4 weeks, during which time
they will deposit about 400 eggs (range 300—1,000 eggs). In the field,
adult life probably does not exceed 5—6 weeks. There is a preoviposi-
tion period of 2-24 weeks. Fig. 1 illustrates the life cycle of the NCR
and WCR and the period of time each growth stage can be found in
Illinois. The ovipositional curve shows when intensive egg laying is
occurring in cornfields.

The adults may remain for a considerable time in the field where
they emerge. Later they disperse, seeking pollen in late-maturing corn,
alfalfa, weeds, and ornamental plants. NCR adults congregate on silks
and in tips of corn ears, whereas WCR adults have less tendency to
congregate and they will be found on other parts of the plant as well

CORN ROOTWORM PEST MANAGEMENT IN CANNING SWEET CORN 5

Eggs in soil

Adults feeding on
pollen, silks, ears

Pupae

Overwintering eggs in soil

JAN.-APRIL MAY JUNE JULY AUG. SEPT. OCT-DEC.

Fig. 1.— Life cycle of the northern and western corn rootworms in Illinois.

as the ears. WCR adults readily feed on corn leaves. ‘Thus, counts
made in ear tips will show a predominance of NCR adults, whereas
visual counts on the entire plant may show a predominance of WCR
adults. As the NCR and the WCR live together and have similar
habits, there is some interaction between the two species. Several re-
searchers have reported a displacement of NCR by the WCR (Hill
1967, Ortman & Fitzgerald 1964). Interspecific matings have been
observed in the field and F: progeny of crosses were produced in the
laboratory (Hintz & George 1965, 1967). The F: adults have the
phenotype of the WCR.

Larvae of the NCR and the WCR are resistant to the cyclodiene
insecticides over much of their range. Organophosphate and carbamate
soil insecticides applied at planting time are used for control. Rotating
corn with other crops is a means of control. Usually, only the larvae
cause economic damage, but adults congregating and feeding on fresh
silking ears can interfere with pollination, and in these instances insecti-
cides are applied for adult control.

FIELD RECORDS

Detailed field observations were made in blocks of canning corn in
major sweet corn production areas in Illinois in 1973 and 1974
(Boone, Ogle, and Vermilion counties). In the field study, select fields
were divided into three groups, namely early-planted fields that were

6 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 54

harvested before August 8, midseason fields that were harvested by
August 20, and late plantings that were harvested after August 20.
Each of the fields in each group was examined numerous times during
July, August, and September. Counts were made of numbers of adult
rootworms in sweet corn fields during the 2—3 weeks before harvest.
Further, soil samples were taken and the numbers of rootworm eggs
were recorded at the time the fields were harvested, and again about
October 1. Results of the postharvest examinations did not differ sig-
nificantly from the observations made at the time of harvest, indicating
that while adult rootworms inhabit fields of standing corn they do not
readily invade or oviposit in areas where the standing corn plants have
been removed. The results are summarized in Table 1. Interpreting
these field observations along with published data already available
allows us to draw certain conclusions:

1. Fields of sweet corn harvested before August 8 will attract few
ovipositing corn rootworm adults, since the fields are harvested before
initiation of intensive oviposition.

2. Fields of sweet corn are not attractive for rootworm oviposition
after they have been mechanically harvested (no standing stalks in the
field following harvest).

3. Aerial or ground sprays of carbaryl insecticide applied for con-

Tasite 1.—Summary of observations on the density of northern and western
corn rootworm adults and eggs in early, midseason, and late plantings of can-
ning sweet corn in Vermilion, Ogle, and Boone counties, Illinois, in 1973 and

1974.

Density of Adult Rootworm Eggs
Crop Rootworms Per Pint of Soil
Characteristics Prior to in the Fall,
Harvest Sept. 15—Oct. 1
Early planted, silking an occasional 0-1
in July, mechanically adult
harvested by August 8
Midseason planted, 0.1—1.0 0-5
silking in late July and per plant
early August, mechanically
harvested by August 20
Late planted, silking in an occasional 0-1
August, European corm adult

borer and corn earworm
control program, mechanically
harvested late August or
September

CORN ROOTWORM PEST MANAGEMENT IN CANNING SWEET CORN 7

trol of the European corn borer (ECB) and the corn earworm (CEW )
are very toxic to adult rootworms, and ovipositing adults are eliminated
from sprayed fields once the spray program is begun.

4. Sweet corn fields that are harvested about mid-August (August
8-20) may contain some eggs since intensive rootworm oviposition is
occurring, and these fields are usually not protected by sprays or dusts
applied for ECB or CEW attacking the stalk and ear.

5. Since corn rootworm eggs are oviposited in one season and re-
main in the field until the next season, it is possible to plant sweet corn
in fields that are virtually devoid of rootworm eggs due to early har-
vesting or spraying of sweet corn the previous year. Thus there is
little or no need for soil insecticides for rootworm control on sweet corn
when pest-management guidelines are followed.

THE MANAGEMENT PROGRAM

Annual planting of sweet corn in fields that were planted to sweet
corn the previous year is the key to the rootworm pest-management
program for canning corn. You can avoid corn rootworm problems by
carefully designing your planting schedule in relation to harvesting
dates of the previous season. However, the planting guidelines that
follow apply only to sweet corn following sweet corn where the crop
was mechanically harvested the previous year, or, if the crop was hand
harvested, the standing stalks were mechanically destroyed immedi-
ately following harvest. The guidelines presented in this circular do
not apply when sweet corn is planted in fields that were in field corn
the previous year.

Adult rootworm counts of less than one per plant are below the
damage threshold. It will not be necessary to count adult rootworms
in fields in Categories 1 and 4 of the following section. Normally there
would be no Category 3 fields, since sweet corn producers regularly
treat to control ECB and CEW. You may wish to count adults in
Category 2 fields, and the color photo on the cover of this circular will
aid in easy recognition of the adults. Count adults on 25-50 plants to
provide a representative sampling of the field regardless of size. The
count should be made in early morning or early evening anytime dur-
ing the 10 days preceding harvest. Some adults will be visible on the
leaves and behind leaf sheaths but many will be in the silks and ear
tips. A good method of counting is to grasp the ear tip and silk in one
hand, cut off 1 inch of the ear tip with a knife and place it and silks
into a plastic bag. Keep the bag tightly closed to prevent escape of the

8 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 54

adults. Put the bag containing the 25—50 ear tips and silks into a
freezer to kill the adults. Then the contents can be easily examined and
a record made of numbers of adult rootworms per plant.

Occasionally adult rootworms will become so abundant that they
interfere with pollination of the ear. When this occurs, the adults can
be suppressed with a single insecticide treatment applied to the foliage
and ears with aerial or ground equipment.

The planting guidelines are given below and the potential for
damage from corn rootworms is illustrated in Fig. 2. In some cropping
seasons these calendar dates must be shifted slightly (2—4 days) to
adjust for early or late crop and pest development.

CaTEecory 1. — Current year’s planting is scheduled for sweet corn
fields that were mechanically harvested before August 8 the previous
year. Little to no rootworm oviposition occurred in these fields. Poten-
tial for larval damage during the current season is negligible. No root-
worm soil insecticide is needed.

CatTecory 2.— Current year’s planting is scheduled for fields
where the sweet corn crop was harvested after August 8 the previous
year, and there was no spray program for ECB and CEW the previous
year. Rootworm females oviposited eggs in these fields with potential
for low to moderate damage. Use a soil insecticide at planting time to
prevent damage by rootworm larvae if the adult rootworm count in the
field was one adult or more per plant the previous year. Normally only
a small acreage of sweet corn will be in Category 2, since a field enters
Category 4 as soon as the first application of insecticide in the multiple
treatment program has begun for ECB and CEW control.

CatTEecory 3.— Current year’s planting is scheduled for fields
where the sweet corn crop was harvested after August 20 the previous
year and no ECB or CEW spray program was in effect. These fields
were very attractive for oviposition by female rootworms in the pre-
vious year. Potential for damage by rootworm larvae is moderate to
high. Use a rootworm soil insecticide at planting.

Catrcory 4.— Current year’s planting is scheduled for fields that
received sprays for ECB and CEW the previous year. Carbaryl or a
spray mixture containing carbaryl insecticide used for controlling these
stalk and ear pests is very toxic to adult corn rootworms, and rootworm
oviposition was suppressed or virtually eliminated. Potential for dam-
age from rootworm larvae is negligible. No rootworm soil insecticide
is needed.

CORN ROOTWORM PEST MANAGEMENT IN CANNING SWEET CORN

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REFERENCES

Branson, T. F., and E. E. Orrman. 1966. Host range of larvae of the western
corn rootworm. North Central Branch, Entomological Society of America
Proceedings 21:40—-41.

Branson, T. F., and E. E. Orrman. 1967a. Host range of larvae of the western
com rootworm. Journal of Economic Entomology 60 (1) :201—203.

Branson, T. F., and E. E. Orrman. 19676. Host range of larvae of the northern
com rootworm (Coleoptera:Chrysomelidae). Journal of the Kansas Ento-
mological Society 40 (3) :412-414.

Curanc, H. C. 1973. Bionomics of the northern and western corn rootworms.
Annual Review of Entomology 18:47-72.

Hi1, R. E. 1967. Changes in distribution and dominance of rootworm species
in Nebraska, 1948-1966. North Central Branch, Entomological Society of
America Proceedings 22: 163—164.

Hintz, A. M., and B. W. Grorce. 1965. Hybridization of northern and western
corn rootworms. North Central Branch, Entomological Society of America
Proceedings 20:61.

Hintz, A. M., and B. W. Georce. 1967, Laboratory experiments on hybridiza-
tion of Diabrotica species. North Central Branch, Entomological Society of
America Proceedings 22:96.

Ortman, E. E., and P. J. Frrzceratp. 1964. Developments in corn rootworm
research. 19th Annual Hybrid Corn Industry-Research Conference Proceed-
ings 19:38-45.

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