Corn

Cooperative Extension

Pest Management

Sampling Guidelines

Growth Stage: Pre-Planting
Pest	Sampling Method
White grub, Wireworm	Sample one square foot of soil 12 inches deep; one sample should be taken for each 10 acres with a minimum of 5 sites per field; field should not be tilled before samples are taken and soil temperature at 6 inches should be 45-50 degrees F.
Growth Stage: Emergence to Early Whorl
Cutworms	Examine 10 plants in 10 locations for the presence of leaf feeding (small irregular holes) and cut plants; also look for live cutworms and estimate the average size of the larvae
Slugs	Examine 10 plants in 10 locations for the presence of feeding damage and slime trails; you will need to observe plants at night or during cloudy conditions to actually observe slugs feeding in plants; be sure to also check for slugs under surface trash and in open seed slots
Flea beetles, Cereal leaf beetle, and European corn borer	Examine 10 plants in 10 locations, count the number of beetles per plant and percent plants infested for the two beetles; count the percent infested plants for European corn borer, pull the whorls our of 5-10 plants per field to determine the size of ECB larvae as well as the average number per plant
Growth Stage: Mid-Whorl – Tassel Emergence
Stalk borer, True Armyworm, European corn borer, Corn Earworm, Fall Armyworm	Examine 10 plants in 10 locations and count the percent infested plants
Growth Stage: Silking
Corn rootworms	Count the number of beetles on 5 plants in 10 locations

Insect Thresholds

	Growth Stage
Pest	Preplant	Emergence to 4 leaf stage	5 leaf stage to silking
White Grubs	1 per sq. ft.
Wireworms	1 per sq. ft.
Slugs	3 per sq. ft.	3-5 per plant
Cutworms		1-2 leaf – 3% cut or 10% feeding damage 2-4 leaf – 5% cut and larvae present
Corn flea beetle		5-6 per plant and 50% plants damaged
Common stalk borer		2 leaf – 4% infested plants 3 leaf – 6% infested plants 4 leaf – 10% infested plants
European corn borer		50% plants infested (irrigated)80% plants infested (non-irrigated)	50% plants infested (irrigated)80% plants infested (non-irrigated)
True armyworm		25% plants infested and larvae < 1″	25% plants infested and larvae < 1″
Cereal leaf beetle			10 per plant and 50% plants damaged
Fall armyworm			75% plants infested with 1 worm or 50% plants with 2 or more worms
Northern corn rootworm *			2 adult beetles per plant
Western corn rootworm *			1 adult beetle per plant

* if both present – divide number of northern corn rootworms by two and add to number of western corn rootworms

Corn | Commercial Crops | Sustainable Production Systems | Cooperative Extension | University of Delaware

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