Treble-bar moth (Aplocera plagiata)
This week’s crawly is a moth of many names. Meet the treble-bar moth (Aplocera plagiata), AKA the St. John’s wort inchworm, AKA the defoliating moth.
Don’t panic! It’s a moth specifically imported for the sole purpose of defoliating, so it’s not a bad thing.
By any name and for whatever purpose, the treble-bar is a subtle beauty.
Their main moniker comes from, you guessed it, the three darker bands – or “bars” – on the outer edge of their wings. It’s probable the “treble-bar” common name refers to the bars’ resemblance to sheet music.
The St. John’s and defoliating moth common names stem from the reason the treble-bar was imported to Canada from their native region in Europe. They were brought over in hopes they would be a bio-control agent for St. John’s wort plants (Hypericum).
In some areas, including Oregon, common St. John’s wort (Hypericum perforatum) is considered a noxious weed as it is toxic to livestock.
That said, they are also medicinal and, in some cultures, thought to ward off evil spirits. Moreover, there is a large-flowered species, the “sunburst” St. John’s (Hypericum frondosum) which is native to the Southeastern United States and is often used for landscaping.
Finally, we have two other St. John’s species; the western St. John’s wort and tinker’s penny (H. scouleri ssp. nortoniae and H. anagalloides respectively) which pose no problems.
The treble-bar moth’s caterpillars will happily dine on any St. John’s wort species; they aren’t picky about which are weeds, native wildflowers or imported sunburst beauties.
Still, don’t panic. While they were imported to defoliate St. John’s, that didn’t pan out. They do dine on them but didn’t make the hoped-for dent in Canada’s problem population.
Because treble-bars have only been in North America since the 1980s, we have few of them in Oregon. Here they can be found from the coast to the Cascades. Treble-bar ‘pillars dine exclusively on St. John’s so they aren’t in competition with native moths or butterflies for resources. If North America had a built-in “defoliator of St. John’s” we wouldn’t have imported treble-bars.
Now, we know how treble-bars got their many names, that they are pretty and that they originally hail from Europe – but wait! There’s more!
Let’s talk about their super savvy ‘pillars. St. John’s wort isn’t just toxic to livestock, it’s also toxic to treble-bar ‘pillars. So how can it be their only food source?
Young treble-bar ‘pillars avoid eating the glands in St. John’s wort leaves that produce toxins (livestock are not so discerning). As the ‘pillars progress through their instars (larval growth stages), they build immunity to the poison and older ‘pillars earn that “defoliator” common name by chowing down on the full foliage.
Like monarch caterpillars, the treble-bar ‘pillars’ deadly diet makes them unappetizing to potential predators. While many moth and butterfly caterpillars eat only at night for maximum stealth, treble-bars dine all day. Because they are day-feeding, they evolved natural sunscreen; their cuticle (soft outer exoskeleton) filters out 80% of ultraviolet rays.
Adult treble-bars prefer the nightlife. Your best bet for seeing these beneficial beauties is in the early morning around porch lights, or any time of day blending in with tree bark. They have two broods a year, so look for adults in May and again in August.