Stalk Borer
Papaipema nebris (Guenee), Noctuidae, LEPIDOPTERA


DESCRIPTION

Adult - The forewings of this moth are basically reddish or grayish-brown marked with distinct white spots or obscure smoky areas. The outer third is paler and bordered by a thin white line. The hind wings are grayish-brown on the upper surface and fawn-gray below. The wingspan ranges from 25 to 40 mm.

Egg - The longitudinally ribbed egg may be spherical or slightly flattened and measures 0.4 to 0.6 mm in diameter. White when first deposited, it gradually turns brownish-gray or amber before hatching.

Larva - Early larval instars are brown and have a dark brown band around the middle and brown or purple longitudinal stripes on all but the first four segments. The mature larva is solid white or light purple and may reach a length of 31.8 mm.

Pupa - The light brown pupa gradually darkens as it matures and is about 16 to 22 mm in length.


BIOLOGY

Distribution - The stalk borer occurs in all areas east of the Rocky Mountains from southern Canada to the Gulf of Mexico. Highest populations are associated with fields and fence rows with large-stemmed weeds. In North Carolina, economic infestations are most common in the Piedmont, particularly in no-till plantings.

Host Plants - Stalk borers tunnel in almost any large-stemmed plant. Their host range encompasses at least 44 families and 176 species of plants. Some cultivated crops subject to infestation include corn, cotton, potato, tomato, alfalfa, rye, barley, pepper, spinach, beet, and sugarbeet. Although many weedy plants are infested, giant ragweed is preferred.

Damage - Stalk borers migrating from another host infest corn seedlings 6 to 60 cm (2 to 24 in) high causing 2 types of injury. Larvae that enter the plant through the lower stalk tunnel upwards severing the leaves from below - in this case, infested stalks are hollow, and apparently healthy green leaves wilt and die. Other larvae climb plants, enter from the top, and feed on buds and rolled leaves. As they unfurl, new leaves display ragged holes which increase in size as leaves develop. Both forms of injury result in destruction of tassels, production of suckers, and deformation of the upper plant. Soon after borers enter the seedlings, the stems often break. Frass is usually evident around the base of more mature infested plants. Once past the "whorl" stage, corn is somewhat resistant to stalk borer and recovers more readily from damage. Damage is sporadic but most commonly associated with border rows of conventionally planted corn and with no-till plantings.

Life History - Stalk borers overwinter as eggs on weedy plants. In May, newly emerged larvae feed as leafminers on broadleaf plants or as stem borers on grasses. On all hosts, larvae eventually bore into stems and feed until they kill or outgrow their host. When this occurs, they emerge at night and tunnel into new plants, including seedling corn. Developing through 7 to 16 instars, stalk borers mature in their second host. Late in July, borers emerge, construct individual cells in the soil, and begin a 4-week pupal period. Stalk borer moths emerge in late summer and deposit eggs singly or in masses between the leaf sheath and stems where they remain until the following spring. One generation occurs each year.


CONTROL

Stalk borers cannot be controlled once they have entered the plant; therefore, control measures are strictly preventative. Destruction of weeds in fields and along fence rows result in the elimination of many primary hosts from which the borers infest corn.

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