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The Secret Life of a Mistletoe Butterfly

By candace fallon on 11. August 2017
candace fallon

It’s the summer of 2009, and I’m slowly meandering down a shady Forest Service road, butterfly net in hand and royal blue hard hat on my head. Suddenly, I see a flicker of movement near a small puddle just ahead. I freeze, and then slowly start to creep forward. A few calculated (and then not so calculated) swings later, I have my target in hand. A small brown hairstreak butterfly. Exultant, I grin at my coworker and do a little dance, and then recompose myself and continue along the transect. Hazy beams of light pierce through the tall western hemlock trees overhead. A few bees and flies buzz around some roadside daisies and Columbia lilies, but otherwise it is quiet.


mistletoe butterfly
Johnson’s Hairstreak (Callophrys johnsoni) Photo: © 2011 mcrainey /


That little hairstreak was the only one I found that summer, working as an intern on the Willamette National Forest in Oregon. I was part of a team testing a new survey protocol for the Johnson’s hairstreak (Callophrys johnsoni), one of only two butterfly species in the Pacific Northwest that feeds on dwarf mistletoes (Arceuthobium spp.) in its larval stage. It leads a highly secretive lifestyle, appearing to spend most of its time in the tree canopy and occasionally descending to the forest floor to bask in the sun and seek nutrients from flowers, puddles, and springs. Originally described in 1904 by Henry Skinner from specimens collected in British Columbia, Johnson’s hairstreak is now known from British Columbia south to central California and east to western Idaho. However, its stronghold appears to be mistletoe laden west-side forests of the Oregon and Washington Cascades.


Very different than the shrub-like plant associated with holiday romance that grows in the east, western dwarf mistletoes are in the genus Arceuthobium. This parasitic plant is regarded as a pest species by the timber industry, yet it provides a wide range of resources to a diversity of wildlife. Photo:


Mistletoe is a parasitic flowering plant that attaches to the stems and branches of other plants. There are many different species found throughout the world, but Johnson’s hairstreak relies on various types of dwarf mistletoes. If you have ever walked through an old growth hemlock forest in the western Cascades, you have probably seen evidence of its existence. This tree parasite causes branch swelling and witches’ brooms—deformed branches that form dense clusters in the canopy. Dwarf mistletoes are native components in many forests, and they provide important food sources and nesting sites for a wide variety of invertebrates, birds, and small mammals. In the Pacific Northwest, these mistletoes are found on a wide range of conifers including Douglas-fir, western hemlock, ponderosa pine, and true firs. Dwarf mistletoe spreads by seeds, which are covered in a highly sticky substance that helps them adhere to potential host trees. Upon contact with an appropriate host, the mistletoe’s seeds will germinate and send small root-like structures down into the tree branch, seeking water and nutrients. It can then take several years before visible shoots emerge, and a few more years after that to produce flowers. Once it has flowered and fruited, its seeds can be dispersed in one of two ways—by hitching a ride to a new site via a bird or other animal, or by using hydrostatic expulsion to shoot seeds at speeds up to 60 miles per hour and at distances of up to 50 feet!

The visible shoots that appear after a few years are what Johnson’s hairstreak caterpillars depend on for survival. Dwarf mistletoes tend to be mostly leafless, instead composed of knobby succulent growths that provide the perfect camouflage for Johnson’s yellowish-green caterpillars. These caterpillars will go through several instars, or molts, as they grow, eventually pupating to wait out the winter. Adults can be found throughout the summer and even into early fall, depending on elevation.


johnsons hairstreak life cycle
Larval stages of Johnson’s hairstreak. Photo: Ray Davis / US Forest Service


In the eight years since I first worked with this species, researchers and agency biologists have learned a lot more about where Johnson’s hairstreak occurs and how best to survey for it. To assess the current state of our knowledge regarding this species, Xerces has been working with the Interagency Special Status Species Program (ISSSSP) in Oregon and Washington to develop a conservation assessment for Johnson’s hairstreak. Conservation assessments are great tools for evaluating potential project impacts and working with land managers to develop site-specific management guidelines and other recommendations. At their core, these assessments include information on the taxonomy, range, known distribution, and habitat needs of a species, but they may also provide information on critical research needs, monitoring recommendations, and other details that are relevant to land managers and others.

Now that we know where we stand, we have a much better idea of our conservation and research priorities. As usual, the list of questions is long. Is Johnson’s hairstreak a rare species, or just rarely encountered? What factors limit this species’ populations? What are its microhabitat needs? How will climate change and associated changes in wildfire frequency and severity affect this butterfly and its host plant? And, ultimately, how do we as conservation biologists accurately assess the population status of a secretive species that’s associated with a parasitic plant that lives in the tree canopy?

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