Potato Tuberworm
Phthorimaea operculella (Zeller), Gelechiidae, LEPIDOPTERA


DESCRIPTION

Adult - The small, slender moth has narrow gray forewings with dark brown spots. The hind wings are yellowish-brown. Both sets of wings are fringed - the forewings near the base and the hind wings along the lower margin. The moth's body is about 8 mm long and its wingspan about 13 mm. The female is slightly larger than the male.

Egg - The oval egg is about 0.5 mm long. Pearly white when first deposited, it gradually turns yellow and finally brown.

Larva - The larva, upon hatching, is creamy white with a dark brown head and prothorax (area immediately behind the head). The larva varies from green to pink as it matures and just before pupation takes on a purplish cast. The larva is 13 to 19 mm long when fully grown.

Pupa - White with green blotches at first, the spindle-shaped pupa soon turns brown. It is about 8 mm long and enclosed in a flimsy, white silken cocoon.


BIOLOGY

Distribution - The potato tuberworm is a cosmopolitan pest, occurring in most areas where potatoes or other solanaceous plants are grown. It occurs in at least 25 states from the Atlantic to the Pacific coasts.

Host Plants - The tuberworm generally attacks potato foliage and tubers, but will also feed on tobacco, tomato, eggplant, pepper, jimsonweed, nightshade, and horsenettle.

Damage - Potato tuberworm larvae act as miners of leaves, petioles, and stems of the above crops and as borers in potato tubers and, occasionally, tomato fruits. Tuberworms feed and tunnel between upper and lower surfaces of leaves causing grayish papery blotches which become brownish and very brittle. Such injury is usually concentrated on older, lower leaves.

The caterpillars enter tubers near the surface of the ground by moving through cracks in the soil. Tubers covered with at least 5 cm of soil are not subject to infestation. Tubers exposed at harvest often are infested soon afterwards by larvae moving from the foliage. Therefore, infestations are likely to increase in areas where cull potatoes are allowed to remain in the field following the harvesting of the spring crop.

Larvae tunnel through potatoes, filling tunnels with excrement and webbing on which disease-causing fungi grow. Such potatoes are unsightly and of little food value. Larvae usually enter tubers near eyes, covering the small entrance holes with webs and excrement. Infestations are more evident a few days later when a pink coloration develops in the flesh around entrance holes. More excrement also is present. Larvae cut galleries 2.5 to 7.5 cm (1 to 3 in) long in the tubers, either just beneath the skin or deep in the flesh.

Life History - Potato tuberworms may overwinter as larvae or pupae in the soil or in potatoes that are not subjected to freezing temperatures. Cull dumps, storage houses, and cellars are all suitable hibernation sites. The weak-flying moths emerge in spring and dart from plant to plant when disturbed. They are most active at dusk and dawn. Each female deposits, singly, 60 to 200 eggs in 4 days or less. Eggs are usually deposited on rough surfaces such as potato tuber eyes or the hairy leaf undersurfaces. Hatch occurs 3 to 6 days later, depending on temperature. Larvae feed and mature in 7 to 10 days under ideal summer conditions, but take longer at cooler temperatures. When fully grown, larvae leave their host and pupate in the soil near the bases of plants, in leaf remains, or in some other suitably sheltered site. A new generation of moths emerges in 6 to 9 days. Five or 6 generations occur each year.


CONTROL

Preventive measures usually are effective in controlling the potato tuberworm. Some important control practices include: 1) keeping potatoes well hilled so there are always at least 5 cm of soil over the tubers, 2) cultivating or irrigating to prevent deep cracks in soil, 3) planting fall potatoes as far as possible from the location of the spring crop, 4) destroying cull potatoes, perhaps by feeding them to livestock, 5) eradicating volunteer plants early in spring, 6) harvesting potatoes soon after maturity and removing them from the field immediately after digging, 7) storing tubers, when possible, at temperatures below 10 degrees C(50 F), 8) screening storage area from egg-laying moths, and 9) fumigating or steam cleaning sacks before they are reused. Practices to avoid include planting infested seed pieces, covering newly dug potatoes with vines, and leaving piles of potatoes out overnight when egg-laying moths are active.

Tuberworms are rarely a problem in fields where rigid pesticide schedules are followed. Once an infestation develops in a field, chemical treatment should begin at once and be repeated until the pest is controlled. For recommended pesticides and rates, consult the current North Carolina Agricultural Chemicals Manual.

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