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Western
Bean Cutworm females lay eggs on corn or
dry edible field bean leaves. Cornfields in the late-whorl stage are most
attractive to the females for egg laying. Eggs are laid in masses of 5 to 200,
usually on the upper surface of the top leaves. The eggs are about the size of a
pinhead. When first laid, the eggs are white. As the eggs develop they turn tan
and then purple just before the larvae hatch. Newly hatched larvae are
approximately 1/4 inch in length and are dark brown. Young larvae are tan with a
darker, faint diamond-shaped pattern on their backs. As the larvae mature, they
become a pinkish tan or pale brown and reach a body length of 1 1/2 inches. When
the larvae hatch, they first feed on pollen and then move to the corn ears. The
larvae feed there for several weeks before they drop to the soil to form a
subterranean overwintering chamber. By the end of the five instars, considerable
feeding damage can occur. In corn, one larva per plant usually does not cause
severe damage but the ears may contain up to 10 larvae, which can substantially
reduce yield, because western bean cutworms are not cannibalistic, compared with
corn earworms.
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