In spring it feels like we live in paradise. We are surrounded by glowing green meadows and hillsides. Wildflowers are blooming, weeds decorate the landscape everywhere, and our gardens are waking up with new blooms each week.
Spring rains have led to lush growth in our wildlands and gardens, and feeding on this abundant spring foliage are a variety of native butterfly caterpillars, some of which may be sadly mistaken for pests.
When we think of butterfly gardens, what comes to mind are the flower nectar plants that attract butterflies. But we shouldn’t overlook the vital importance of the larval host plants caterpillars feed on. Each species of butterfly caterpillar eats plants in specific families. They cannot feed on just any plant. Butterflies have evolved with specific plant families and have developed the capability to process and tolerate the various chemicals — some toxic — that these plants contain. A spring flying butterfly like the Mourning Cloak feeds primarily on willows, which are not toxic but have bristly hairs on their backs to deter predators from eating them.
A spring butterfly that feeds on toxic plants is the Pipevine swallowtail butterfly (Battus philenor). Pipevine caterpillars are colored a striking black and red to warn predators they are toxic. These caterpillars are so toxic, birds actually vomit when they eat them. When caterpillars consume toxic plants, they become toxic for life, even when they transform into butterflies, from the chemicals contained in the toxic plants. Many are colored with red or orange colors to deter predators like birds from eating them. The Monarch butterfly is another example.
Mourning Cloaks (Nymphalis antiopa) are the earliest butterflies to be active in the year. Here in the North Bay they are active only until early summer to midsummer, when they enter a hibernation phase. They become active again in the fall for a short time before they find a safe place to overwinter in tree crevices or under the overhang of buildings. They are a mid-sized to large butterfly with charcoal black wings with a distinctive pale gold band on the bottom of each wing. Above this band are iridescent blue dots. Wing margins are irregular. They are primarily seen in riparian areas, but also in city parks or urban areas where they can find their larval host trees like willows, birch or elm. The first Mourning Cloak butterflies you see in spring are adults that have overwintered from an April or May hatching the year before. They fly during the first warm winter days when temperatures reach above 60 degrees and bask in the sun with their wings outstretched. Adults that have overwintered look faded and worn compared to the first deep black and gold offspring of the early summer season.
Mourning Cloaks are the longest-lived butterfly, surviving about ten months. The adults nectar on blooming willows, but feed also on tree sap, or gain minerals from mud. The females lay eggs in clusters on willows, poplars, American elm and birch. Caterpillars feed together communally in large webbed aggregations. Because of feeding in numbers, these caterpillars may be mistaken for pest insects and sprayed with pesticides, so be aware. Caterpillars of butterflies and moths form an important food for bird young, and spraying pesticides will deprive young birds of much needed food. The caterpillars often pupate away from the tree they fed on and pupae can often be seen on nearby building walls. Hosting these spring caterpillars is a wonderful event and should be enjoyed. Willows have a proliferation of leaves — and feeding caterpillars won’t harm them.