Fender’s blue butterfly (Icaricia icarioides fenderi) was listed as an endangered species primarily because of its extreme rarity due to upland and wet prairie habitat loss and fragmentation. Sites with the butterfly across its range in the Willamette Valley, Oregon, have been monitored on a mostly annual basis for almost 25 years. The total number of butterflies across the valley and within individual sites can fluctuate substantially between years, which is likely driven by factors including weather and habitat conditions, along with survey method and technique.
Two critical elements of Fender's blue butterfly habitat are host plants for larvae and nectar plant species for adult butterflies. Kincaid’s lupine (Lupinus oreganus = Lupinus sulphureus ssp. kincaidii) is the primary larval host plant for Fender’s blue butterfly, and is listed as threatened. Alternate host plants include sickle-keeled lupine (L. albicaulis) and spurred lupine (L. arbustus). These hosts are found at a limited subset of sites across the butterfly’s range. Fender’s blue does not migrate, and completes its entire life cycle (of one year) near its lupine hosts, usually travelling less than 2 km from its home lupine patch. Adult butterflies typically live and are in flight for about 10 days, the period in which they mate and lay the eggs for the next generation.
Since 2016, IAE has coordinated annual surveys for Fender’s blue across its Willamette Valley range. The total number of Fender’s blue butterflies in existence fluctuates on an annual basis, driven by variation in individual populations, which could double or halve between years. Data suggest that roughly 3,000 Fender’s blue butterflies existed in 1993, which then fluctuated between a maximum of approximately 6,300 individuals and a minimum of 1,750 individuals between 2000 and 2011. Since 2012, newly discovered populations, populations expanded through habitat restoration, and changes in sampling method may explain the substantial growth in numbers of Fender’s blue butterflies; as the overall estimated population in 2016 was a peak of nearly 29,000 butterflies. Poor weather conditions in the winter and early spring of 2016-2017 contributed to a significant reduction in the estimated population size; declining by 50% to roughly 13,100 butterflies in 2017. The population recovered slightly in 2018, to an estimated 13,700 butterflies. While these numbers are far lower than recent years, they still represent a positive trend in the long term trajectory of the species. Factors like weather are out of our control, but we do see a positive response of butterfly populations to habitat restoration that reduces invasive weeds, increases lupine host plants and native nectar plants. IAE is proud to work at sites across the Willamette Valley to restore habitat for Fender’s blue.
IAE is grateful for funding provided by the U.S. Fish and Wildlife Service, and assistance from the Oregon Parks and Recreation Department. Monitoring all 90+ Fender’s blue sites during the 4-6 week flight season would not be possible without our dedicated team of surveyors, including Lee Bennion, Greg Fitzpatrick, Paul Hammond, Gary Pearson, Dana Ross, and Duncan Thomas. IAE staff implementing surveys this year included Monika Lapinski and the author. We are also grateful to all public (US Fish and Wildlife Service, Bureau of Land Management, Army Corps of Engineers), conservation (Greenbelt Land Trust, The Nature Conservancy) and private landowners who support the Fender’s blue butterfly survey process. Our continued appreciation also goes to Tyler Hicks for his collaboration on data analysis.