Limabean Vine Borer
Monoptilota pergratialis (Hulst), Pyralidae, LEPIDOPTERA


DESCRIPTION

Adult - This brownish-gray moth has whitish scales on the edge and across the end of its forewings. The forewings are marked with small black streaks and are black along the veins. Hind wings and the outer edges of the forewings have a brownish-gray fringe-like border. Male's hind wings are white, while the female's are more grayish. The wingspan is 21 to 23 mm.

Egg - The dull gray oblong-oval egg is 0.7 mm long and 0.4 mm wide.

Larva - Gray when very small, the larva gradually becomes bluish-green and sparsely covered with long yellow hairs. Behind the black or brown head capsule is a yellowish-brown prothoracic shield. When fully grown, this caterpillar is 18 to 25 mm long.

Pupa - The yellowish to reddish-brown pupa is 12 to 14 mm long and is enclosed in a 16 mm long cocoon.


BIOLOGY

Distribution - Although this pest occurs throughout much of the east central and southeastern U.S., it is primarily a problem along the Coastal Plain from Delaware and Maryland south to Florida and west to Alabama. The limabean vine borer also occurs in some southwestern states like Arizona.

Host Plants - This pest heavily infests pole and lima beans. Though its occurrence on other hosts is rare, this caterpillar has been reared on snap bean, cowpea, and dahlia. Large stemmed bean varieties are preferred.

Damage - Young larvae feed on leaves, slightly skinning the lower epidermis and leaving tell-tale frass, webbing, or excrement behind. As larvae mature, they bore into stems, typically just above or below nodes, and hollow out cavities. As a result, infested stems gradually swell and form galls up to 70 mm long and 20 mm in circumference. The galls eventually turn brown and develop a woody texture. Short, loose, silky frass tubes are attached to the entrance holes on the galls. Usually, infested plants are weakened and have lower yields. When galls are located near the tips of small stalks, the tops of plants often wilt and full-sized pods cannot be produced. The extent of damage varies with the position of the gall and the vigor of the host plant.

Life History - Limabean vine borers overwinter as prepupae on or near the soil surface. Moths emerge from late April to mid-May in eastern North Carolina and deposit eggs on leaves or in naturally occurring depressions in host plant stems. Two to 6 days later, eggs hatch and larvae begin feeding on leaves. After feeding for 4 to 7 days, larvae bore into stems where they continue feeding and complete their development. When fully grown, larvae bore exit holes in the galls, drop to the ground and enter the soil where they spin cocoons and pupate. A new generation of moths emerges about 15 days later. Larvae usually can be found from May through October. Later in the season, larvae may take over galls formed by previous generations instead of creating their own. In North Carolina, approximately three generations occur each year.


CONTROL

Hand removal of galls does more harm than good to infested plants. Fall plowing or winter cultivation helps reduce populations by destroying overwintering prepupae. Where practical, crop rotation is recommended. Foliar sprays to control this pest should begin when pods start to form. For up-to-date chemical recommendations, consult the current North Carolina Agricultural Chemicals Manual.

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