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Corn Earworm (Helicoverpa zea Boddie)
Description and Biology: Moths of this insect are 19 mm long, with a wing span of 38 mm. They vary in color from dusty yellow to reddish brown. Females are active in the evening and live about 12 days. Each female may deposit 350 to 3,000 eggs. Eggs are flattened spheres, prominently ribbed and 1.2 mm in diameter. When deposited, they are white, but soon darken and hatch in three to five days. White, newly hatched larvae grow rapidly and become variously colored, ranging from pink, green, or yellow, to almost black. Many are conspicuously striped. Down the side is a pale stripe edged above with a dark stripe. Down the middle of the back of larger larvae is a dark stripe divided by a narrow white line that makes the dark line appear doubled. Fully grown larvae are robust and 38 to 50 mm long. Pupation occurs in the soil and is the overwintering stage.
Symptoms and Damage: Corn earworms infest both whorls
and panicles of sorghum. Infestation of panicles is considered more serious
than infestation of whorls.
Monitoring: Sampling for corn earworms in sorghum whorls
and panicles requires different procedures. Holes in leaves as they unroll
from the whorl are evidence of whorl infestation by corn earworm. These
leaves are ragged or have a row of holes across them. To locate corn earworm
larvae in the whorl, the whorl leaf must be pulled from the plant and
unfolded. The whorl of an infested plant contains frass produced by a
larva as it feeds. Because of their cannibalistic habits, there usually
is only one corn earworm larva per plant whorl.
Management: Natural mortality suppresses abundance
of corn earworms in sorghum whorls and panicles, as do predators, parasites,
pathogens, and cannibalism among larvae. Infestations in sorghum whorls
and panicles usually are less in early-planted than in later-planted sorghum.
Whorl infestation by corn earworm usually is not severe enough to justify
insecticide application. Also, corn earworms within the whorl of sorghum
are protected from insecticide. An important management tactic against
corn earworm infesting sorghum panicles is to use hybrids with loose (open)
panicles. Natural mortality in early instar larvae can be high. Insecticide
application usually is justified when there are two corn earworms about
13 mm long or one larva longer than 13 mm per panicle. Although larger
corn earworms are more difficult to control with insecticide, corn earworms
in sorghum usually are controlled more easily than in cotton.