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Corn Earworm (Helicoverpa zea Boddie)

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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.
Larvae, which hatch from eggs laid on sorghum leaves before panicles are available, migrate to and feed on tender, folded leaves in the whorl. When damaged leaves unfold, they are ragged with "shot holes." Although this may look dramatic, damage to leaves usually has little effect on yield, and control of larvae during the whorl stage of sorghum growth usually is not justified economically. Chemical control may be justified if larval feeding is reducing leaf area by about 30% or is damaging developing panicles or growing points within the whorls.
Corn earworm infestation of sorghum panicles is of greatest concern because larvae feed on developing grain. Small larvae at first feed on flowering parts of panicle spikelets, then hollow out kernels. Later instar larvae completely destroy maturing grain. About eighty percent of the damage is caused by larvae in the last two instars. Larval excrement or frass is common in infested panicles and may be seen on the tops of upper leaves and on the ground under plants. Under certain conditions, molds also may be common in infested panicles. An insecticide application would be justified at one to two larvae per panicle.

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.
Inspections of sorghum panicles should begin soon after flowering and continue at five-day intervals until hard dough. To examine sorghum panicles for corn earworm larvae, shake randomly selected panicles vigorously into a 19-liter bucket. Larvae then can be seen and easily counted in the bucket. This "beat-bucket" technique permits detection of even small larvae (less than 6 mm long) commonly overlooked. At least thirty panicles from a field should be inspected to ensure reasonable reliability of sample results. Fields to be sampled that are larger than 32 hectares should be divided into sampling areas no larger than 32 hectares.

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.