Oregon Silverspot Butterfly

The Oregon silverspot butterfly (Speyeria zerene hippolyta) is one of two Oregon butterflies listed as threatened or endangered under the U.S. Endangered Species Act. Once found in coastal headlands from northern California to southern Washington, it has disappeared from all but a handful of sites along the Oregon coast. In addition to habitat losses, one possible factor in its decline has been fire suppression, which allows grass to overshadow its obligate larval host plant, the western blue violet (Viola adunca).

The Nature Conservancy's (TNC's) 280-acre Cascade Head Reserve in southern Tillamook County is home to one of the surviving populations. During the last ten years, butterfly monitoring at Cascade Head has shown a dramatic decline in the number of butterflies seen flying. From an average of over 1,000 adults seen most years prior to 1992, only 57 butterflies were recorded in 1998. In response, TNC has initiated a habitat management plan that will use controlled burning to increase host plant populations. TNC, the Oregon Zoo, and the Woodland Park Zoo have embarked on a butterfly-rearing program designed to maintain genetic variability in the population and increase the likelihood of its natural recovery.

During the flight season, female butterflies are brought from Mt. Hebo and induced to lay eggs in laboratories at the Oregon Zoo and Woodland Park Zoo. The population at Mt. Hebo is stable, so U.S. Fish and Wildlife scientists determined that removing some females would not hurt the whole population. Every day, the butterflies are fed nectar, a solution of sugar water and egg white, by placing them on saturated cotton balls.

Their eggs are collected daily, and put into petri dishes. After about 10 days, many of the eggs hatch out into tiny larvae. Each larva is about one millimeter long, and under a microscope, can be seen to be a perfect miniature caterpillar! At this stage, the larvae do not eat, but they are given drinks of water by placing them on moist filter paper. After drinking, they are placed in specially modified jars, and put into a refrigerator for winter diapause, or hibernation.

Zoo gardeners grow early blue violet (Viola adunca) plants for the larvae to feed on after their hibernation. Following winter dormancy, the butterfly larvae are fed these Viola adunca leaves until they pupate. They are then returned to Cascade Head and two other sites on the Oregon Coast, where they emerge as adult butterflies.

This conservation technique, called population supplementation, is intended to prevent inbreeding depression often seen in small populations.

We hope that improved habitat at Cascade Head will allow the butterfly to quickly recover its numbers and put us out of the butterfly rearing business. In the meantime, this U.S. Fish & Wildlife Service-funded cooperative project may be the butterfly's best hope for short-term survival.