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.
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.
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