Western Corn Rootworm, Diabrotica virgifera virgifera LeConte
Western corn rootworm (WCR) adults are approximately ¼ inch long
and yellow with three black stripes on forewings (elytra). The stripes
on female wing covers are distinct, with one on the outside of each wing
cover and the third one along the middle, where the wing covers meet.
The stripes on male wing covers tend to merge across the wing covers.
The white, football-shaped eggs are less than 0.004 inch long. Mature
larvae are approximately ½ inch long, slender, and white with brown
head capsules and a dark plate on the top side of the last segment. Pupae
are white, and their appearance is similar to the adult beetle.
corn rootworm have one generation per year with larvae present from
May through July. Adults are abundant from July through September.
A female may lay an average of 500 eggs over several weeks in clutches
of about 80 eggs. Most eggs are laid in the upper 6 inches of soil
during late summer. During dry years, females will enter cracks
in the soil to lay eggs as deep as 10 to 12 inches. Eggs remain
dormant until the following spring. Eggs are concentrated near the
base of corn and soybean plants, and are spread, both horizontally
and vertically, by tillage.
Western Corn Rootworm Adult
Corn Rootworm Larvae
larvae hatch in late May or early June and, if corn is present,
begin feeding on roots. Larvae complete three developmental stages
(instars) as they feed and tunnel in the corn roots. In mid-to late
July, pupation occurs and adults emerge a short time later.
in fields begin to senesce, adults will disperse to neighboring, later
planted, cornfields or feed on pollen from soybeans, alfalfa, and several
species of weeds. The life cycle is completed when adults mate and females
lay eggs during August and September. A strain of Western corn rootworm
found in east-central Illinois will lay eggs in both corn and soybean
fields. As a result of this behavioral shift of the WCR, extension entomologists
recommend monitoring soybean fields according to the procedures outlined
in: Western Corn Rootworm Insect Information Sheet (No. 1) Because WCR
still inhabit and lay eggs in corn, growers also should follow recommendations
for monitoring corn fields where corn will be planted again the following
Corn Rootworm, Diabrotica barberi Smith and Lawerence
Northern corn rootworm (NCR) adults are approximately ¼ inch
long, with females usually larger than males. Their color varies
with age and among individuals. Both sexes of newly emerged adults
are cream to light brown and become yellowish to pale green over
time. Eggs, larvae, and pupae of the WCR are similar in appearance
to the same stages of Western corn rootworm. The life cycle of the
Northern corn rootworm is similar to the Western corn rootworm except
this species has not been shown to lay eggs in soybean fields.
Northern Corn Rootworm Adult
Southern Corn Rootworm Adult
Corn Rootworm, Diabrotica undecimpunctata howardi Barber
Southern corn rootworm (SCR) adults are approximately 3/8 inch long
and yellow to green with eleven black spots on the forewings (elytra).
Eggs, larvae, and pupae of the SCR are similar in appearance to
the same stages of NCR and WCR. This species overwinters as an adult,
instead of in the egg stage, like WCR and NCR. However, adult SCR
do not overwinter successfully in Illinois. Populations of SCR migrate
from southern states beginning in April. Root and foliage feeding
by the SCR is seldom a problem in Illinois
Larval feeding on corn roots may reduce yields. Injured root tips feature
brown lesions. In some fields, entire nodes of roots may be pruned severely.
Pruned roots are less capable of supplying water and nutrients to the
growing ears and moderate to severe root pruning may result in lodging
and significant losses at harvest. Larval injury also may make roots more
susceptiable to root and stalk rot fungi. High adult densities may clip
silks resulting in poor pollination and reduced kernel set.
Corn Fields for WCR and NCR Beetles
Growers are encouraged to monitor cornfields for WCR and NCR adults to
determine the need for a soil insecticide the following year if corn will
be planted again in the field. Extension entomologists recommend the following
procedure for monitoring WCR and NCR beetles.
- Scout every week from mid-July through early September in cornfields
that may be planted in corn again next year.
- Count all WCR and NCR beetles on two plants in 25 widely separated
locations in each field (total of 50 plants). The distance between each
plant sampled must be great enough to prevent disturbing beetles on
other plants that will be examined. Avoid border rows. Scouting a 40-acre
field will take about 45 minutes.
- Approach plants quietly to prevent disturbing beetles to ensure more
accurate counts. All beetles found on a plant should be counted, including
any located on the ear tip, tassel, leaf surfaces, and behind the leaf
axils. Record the number of beetles per plant.
- Divide the total number of beetles per field by 50 to calculate beetles
per plant. If the average number of beetles for any week during the
monitoring period meets or exceeds recommended thresholds (see table
below), growers should consider applying a soil insecticide or rotating
to a non-corn crop in that field.
1. Thresholds for Corn Rootworm Beetles for Different Plant
Density and Cropping Sequences
plants per acre
number of beetles per plant - continuous corn
number of beetles
per plant - first-year corn
Susan T. Ratcliffe (email@example.com)
Michael E. Gray (firstname.lastname@example.org)
Kevin L. Steffey (email@example.com)