Like the mourning cloak butterfly (Nymphalis antiopa), Milbert’s tortoise shell (Aglais milberti) is another butterfly that hibernates over the winter as an adult and therefore is seen very early in the spring.
Also called a fire-rimmed tortoise shell, this beautiful butterfly is black with a wide orange submarginal (just inside the outer edge of the wings) band that grades toward yellow at the inner edge of the band. Along the margin or outer edge of the wings is a narrow black border that may contain some blue spots. Each forewing displays two orange patches in the costal area (front edge).
Widespread throughout the Northern Hemisphere, Milbert’s tortoise shells can be found in wet areas near woodlands, marshes and moist pastures. This Milbert’s tortoise shell was along the Tule River (Shasta County CA).
The adult female lays eggs in large batches of up to 900 eggs on the undersides of nettles, the food eaten by the caterpillar (larva). Young caterpillars often feed together on the nettles, but older caterpillars (later instar stages) will feed alone. The pupa occurs in a folded leaf secured with silk. Adults usually feed on flowers such as goldenrod, lilacs and thistles and will also eat sap and rotting fruit. One reference noted that Milbert’s tortoise shell butterflies will also eat dung. Two, occasionally three, broods occur each year.
The genus name for Milbert’s tortoise shell was previously Nymphalis and this butterfly is often still referred to by that name. Another common name is brush foot butterfly.
Milbert’s tortoise shells are a most welcome bit of color as they flit about in a landscape that is only beginning to show green.