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St. John's Wort Inchworm - Aplocera plagiata
Other Names:  Treble-bar Moth

Non-native Species

Global Rank: GNR
State Rank: SNA

Agency Status


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General Description
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).

Range Comments
The Treble-bar moth is found throughout the Palearctic region and the Near East. It is native to Europe, and was introduced to British Columbia in 1976, and the Ottawa area of Ontario in the 1980's in an attempt to control St. Johnswort. The Treble-bar Moth has since spread to Quebec (Harris 2003). The first US release was in Oregon in 1989 (Oregon Department of Agriculture 2007). It is reported to be established in Idaho, Montana, Oregon, and Washington (Rees et al. 1996).

Observations in Montana Natural Heritage Program Database
Number of Observations: 16

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

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).

Food Habits
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).

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Citation for data on this website:
St. John's Wort Inchworm — Aplocera plagiata.  Montana Field Guide.  .  Retrieved on , from