Life Histories of Cascadia Butterflies

Life Strategies

18 life histories of cascadia butterflies and this book offers an introduction to the lesser known parts of a butterfly’s life. We have uncovered many details and secrets of the immature lives of Cascadia butterflies, but many more remain to be discovered. Life Strategies Despite their fragile appearance and sometimes ephemeral existence, butterflies are tough, seasoned survivors. Every delicate butterfly flitting over a flowery meadow is the survivor of millions of generations of resilient ancestors that endured, outsmarted, or avoided drought, disease, predators, parasitoids, competitors, storms, volcanoes, and even ice ages. Survival in the butterfly world requires adaptation, flexibility, and strategy, and Cascadia butterflies have become experts in all these endeavors. Host Plants: Butterflies are inextricably linked to plants, not only as nectar sources for adults but also for larval food. Most adult butterflies will use whatever nectar source is available, provided the timing is right and they are physically able to access the nectar (proboscis length vs. flower depth); however, larvae of every butterfly species have adapted to certain host plants, some depending entirely on a single kind of plant for survival. Many other butterflies have adapted to use several closely related plant species, and a few are able to utilize a very wide range of plants. A butterfly species that overwinters as an egg needs fresh succulent food as soon as it hatches, in most cases early in the spring, so only plants that grow early in the spring are candidates as hosts. Butterflies that overwinter as pupae will eclose to adults in spring, immediately needing nectar but not requiring a larval host until sometime later, when the plant selection may be different from that available to species overwintering as eggs. Life cycle timing, or phenology, is an important determinant in host-plant selection. Why have some butterfly species adapted by depending on only one or a few plants while others use many? There is no doubt that the ability of plant generalists, such as the Painted Lady, Gray Hairstreak, and Variegated Fritillary, to use many kinds of host plants has rendered them highly successful, allowing them to colonize vast areas, worldwide in the caseofthePaintedLady.Specialization on only one or a few plants can also be a highly successful strategy if those plants are widespread and common (e.g., the case of the Cabbage White). California Tortoiseshell larvae feed almost exclusively on Ceanothus spp., but they are highly successful as the plants are widespread and common Aglais milberti (Milbert’s Tortoiseshell) eggs on Stinging Nettle. Vanessa cardui (Painted Lady) larva on Canada Thistle surrounded by sharp spines. 19 life strategies over vast areas of the mountainous West. The Mountain Parnassian uses only stonecrops, greater fritillaries only violets, and Milbert’s Tortoiseshells only Stinging Nettle, but these plants are widespread and the butterflies are very successful. Niche exploitation occurs throughout the natural world, and Lepidoptera are no exception in a natural process of testing the boundaries and seeking a new advantage against competitors; however, it can also be a risky strategy, with some butterfly species becoming restricted to a single plant species that may be very limited in distribution and/or abundance. The Johnson’s Hairstreak specializes on a species of Arceuthobium (dwarf mistletoe) that grows on mature hemlock trees, but with the human deforestation of old growth forests, this habitat has largely disappeared, and the Johnson’s Hairstreak is now at risk. In Washington State the Golden Hairstreak feeds only on chinquapin, a broadleaf tree that barely reaches into this area, so this population is tentative and at the mercy of commercial logging operations, although it is secure farther south in Oregon. Some butterflies, such as Euphydryas editha (Edith’s Checkerspot), have adapted to using two different host plants, one for young larvae in the summer (Castilleja), and a different host for the spring larvae (Collinsia or Plantago) after passing the winter halfgrown . Such alternate hosts can be very different, although they are usually from the same or a closely related plant family. Chemical cues undoubtedly play a strong role in selection of alternate hosts; some unrelated or introduced plants may contain chemicals similar to the preferred host, thus inducing larvae to experiment. The part of a plant preferred by larvae can be very important, and each species of butterfly tends to specialize on only one or a few parts. Some species feed only on leaf buds, others on young leaves, others on mature leaves. Other species require flower buds, flowers, seeds, or even stems. The different...