Montana Field Guide

The Treble-bar moth (Aplocera plagiata) varies considerably in size (wingspan 30 - 38 mm). The forewing is brown to gray with triple lines containing faint cross-bars. The cross-bars give the appearance of sheet music (perhaps the origin of the common name "treble-bar"). The forewing has a dark apical dash with yellowish or orangish shading. The hindwing is pale gray with a tiny black discal spot (Skinner 1984 and Chinery 1986).

The “inch worm-like” larvae develop through various stages and change from their initial light brown/tan and green color to darker brown with deep shading along their upper back and with dark stripes along their body. When they are fully mature, the larvae will measure 22 mm long (Kelleher and Hulme 1984, Powell et al. 1994, Rees et al. 1996, and Harris 2003).

The Treble-bar moth has two generations per year. The first generation adults emerge in May and June and do so in fewer numbers than the summer generation. The second generation adults appear when the host plants (St. Johnswort, Hypericum perforatum) are flowering, usually from August to September. Mating and oviposition begins shortly after the adults emerge. The females lay up to 300 eggs onto upper and lower leaf surfaces or on stems. The eggs are oval-shaped and pearl white. They will incubate for 5 - 7 days. The adults make frequent erratic flight patterns when disturbed and land a short distance away before taking flight again. They are strong fliers which enable them to easily seek scattered patches of host plants (Harris 2003).

When the larvae are disturbed they will mimic dead twigs. The first generation appear in early August. Newly emerged larvae feed on St. Johnswort leaves, avoiding toxic leaf glands until they are old enough to have developed their immunity. The mature larvae burrow into the soil to pupate which takes 12 - 17 days to complete. The pupae are slender and light greenish-golden brown. The immature larvae overwinter in the soil and resume feeding the following spring. The larvae can withstand high and low temperatures, but harsh winters, high elevations, and northern locations may effect the overwintering generation if they do not develop sufficiently before winter. Moist soils can cause fungal infection (Harris 2003).

The Treble-bar Moth is commonly associated with well drained soils in warm continental climate areas. Dry sites with rocky or sand-based soils and some limestone are suitable. Wet climates or moist soils are unsuitable (Harris 2003).

The larvae primarily feed on the leaves and flowers various species of St. Johnswort (Hypericum). The spring generation feeds on leaves and is capable of substantially defoliating plants. The summer generation feeds on flowers. Foliar feeding is usually observed before the larvae are located (Harris 2003).