E-49
DEPARTMENT OF ENTOMOLOGY
field crop insects

MANAGING CORN ROOTWORMS - 1987
Marlin K. Bergman and C. Richard Edwards, Extension Entomologists

Three species of corn rootworm are commonly found in Indiana-the western, northern, and southern. Pictures of the beetles, larvae, and damage to corn roots and ears are shown in Extension Publications E-83 and E-84. The potential for most problems can be determined by timely scouting and sampling.

BEETLE SAMPLING
Adult Counts to Determine Silk Damage. Begin checking corn fields for the presence of beetles as soon as silks protrude from the tip of the developing ear. If, after examining plants in at least 10 representative areas of the field, you see no rootworm beetles or silk damage, check the field again within 3 days. Even if no beetles are found, recheck fields twice a week until silks have turned brown.
Most Indiana corn does not require a spray for silk protection. However, when tassels are shedding pollen and green silks are continuously clipped back to 1/2 inch or less with beetles present, a yield loss may result. A yield loss will seldom occur when less than five beetles are present per plant. Refer to Table 1 for insecticides that control the adult beetle.

Adult Counts to Determine Potential for Larval Problem Next Season. Cornfields should be surveyed weekly for beetles from late July through early September. Make 2 counts in each of at least 20 equally separated locations in a field and at least 25 rows from field margins. Each count is made by randomly selecting a plant and counting the beetles on the leaves, stalk, tassel, around the ear, and on or in the silks.
To determine the number of beetles on or in the silks, grab the silks in your hand, slowly open your hand, count the number of beetles in the silks, and record the number. Repeat this procedure for a second plant that is within 10 feet of the first plant. Then move to the next area of the field and sample two more plants. Determine the average number per plant. Remember, when beetles are present, samples to determine the average number per plant must be made weekly.

Table 1. Insecticides for beetle control.
Insecticide	Formulation	Rate/acre
carbaryl	80% Sprayable	1.25 lb
(Sevin)	4 Flowable	1 qt
(Savit)	XLR	1 qt
chlorpyrifos	4 lb/gal EC	1.5 pt
diazinon (AG500)	4 lb/gal EC	1 pt
fenvalerate (Pydrin)	2.4 lb/gal EC	5.33 oz
malathion	57% EC	0.2 gal
methyl parathion (Penncap-M)	2 lb/gal FM	2 pt
permethrin (Pounce)	3.2 lb/gal EC	4 oz
phosmet (Imidan)	50 WP	1 lb

COOPERATIVE EXTENSION SERVICE • PURDUE UNIVERSITY • WEST LAFAYETTE, INDIANA 47907