Documentation of The Foodplant Relationship Between Atlides halesus corcorani and Phoradendron juniperinum

Introduction
The relationship between herbivorous insects and their respective foodplants ranges from a species acting as a generalist versus a species acting as a specialist (Miller and Hammond 2003, 2007, Miller et al. 2020). The generalist life strategy is less common than the specialist life strategy. A generalist herbivore can successfully develop on any one of numerous plant species belonging to five or more plant families. In the extreme case, the insect may utilize more than 100 plant species across more than ten plant families. A specialist herbivore may be classified according to the taxonomic level of the foodplant(s). The first and broadest level of herbivore specialization would place all suitable foodplant species in a single plant family, such as Brassicaceae or Fagaceae. The second level of herbivore specialization would place all suitable foodplant species in a single plant genus, such as Streptanthus or Quercus. The third level of herbivore specialization would name the only suitable foodplant species.

The Lepidoptera is the Order of insects consisting of butterflies and moths. Worldwide all but a handful of perhaps 250,000 species are herbivores in their caterpillar life stage. Likely, 85% of these species are herbivore specialists either at the plant family level, the genus level, or the species level. In Oregon, the number of butterfly species totals 177 (Miller, in prep). Among these butterflies, the family Lycaenidae accounts for sixty species including the Great Blue Hairstreak, Atlides halesus corcorani.

The Great Blue Hairstreak, hereafter referred to as Atlides, is primarily a Californian species but in Oregon it ranges mostly west of the Cascade Mountains as far north as Polk County (Pyle and LaBar 2018; Figure 1). Note the east-west expansion of the range nearest to the Oregon-California border where Atlides occurs as far east as the Warner Mountains.

Figure 1. The Great Blue Hairstreak butterfly, Atlides halesus corcorani, reared from a caterpillar found and raised on Phoradendron juniperinum (juniper mistletoe) collected near Lakeview, Oregon, July 16, 2024. Warner Mountains, Lake County, Oregon, 2024. Photo by Jeffrey C Miller.

The Warner Mountains extend north from California into Lake County, Oregon and offer a very biodiverse environment rich in both widespread and endemic plants and butterflies. Our interest in Atlides and the Warner Mountains stems from foodplant specialization and the geographic position of the Warners Mountain Range. Prior to this study, Atlides, whose caterpillars are plant genus specialists, were known to feed exclusively on species of Phoradendron, and in Oregon, only on Phoradendron villosum (oak mistletoe). This level of plant species specialization creates a quandry; when the distribution map for oak mistletoe is overlayed onto the distribution map of Atlides, therange for Atlides extends beyond the range of the oak mistletoe by over 100 miles, from roughly Soda Mountain east to the Warner Mountains (Figure 2-right). However, two non-documented foodplants that are congeneric to oak mistletoe are present but localized. Our supposition was that one of these congeners, or both, act as alternative suitable foodplants for Atlides. Otherwise, the only way Atlides is observed in the Warner Mountains is due to annual northerly dispersal from California, and we believed that scenario to be unlikely.

Therefore, we proposed investigating the prospective role of Phoradendron juniperinum (juniper mistletoe) as a suitable foodplant for Atlides (Figure 2-left). We selected ‘juniperinum’ over ‘densum’ because of its range into the Warner Mountains, albeit based on a single pre-existing mistletoe record reported in 1927.

Figure 2. Left – Western juniper, Juniperus occidentalis, tree with the epiphytic parasite juniper mistletoe, a newly documented foodplant for the Great Blue Hairstreak butterfly in Oregon. Warner Mountains, Lake County, Oregon, 2024. Photo by Dennis A. Albert. Right – Map of the distributions for the butterfly Atlides halesus corcorani (black) and its sole foodplant documented for Oregon, Phoradendron villosum (Orange). Prospective congeneric foodplant species are also shown, Phoradendron densum (lite green) and juniper mistletoe (red circles). Map by Jeffrey C. Miller.

Methods
The caterpillar survey occurred from July 14-16, 2024, and covered the entire Warner Mountain region. Our field survey process was to locate ‘spheres’ of juniper mistletoe hanging amongst the juniper trees, record the latitude-longitude coordinates, remove the mistletoe from the tree, examine the mistletoe in the field for Atlides caterpillars, search the ground for any errant caterpillars that may have been dislodged during the mistletoe removal, then place the mistletoe into containers for inspection for additional caterpillars of Atlides. Caterpillars were then placed in screened cups with a sprig of foodplant and held at room temperature for the duration of the rearing process. Photographs (Figure 3) were taken to document feeding, larval growth, and final development into a chrysalis and eventually an adult (see Figure 1).

Figure 3. Atlides halesus corcorani: Left – lateral view of last instar caterpillar feeding on Phoradendron juniperinum; Center – frontal view of last instar caterpillar on juniper mistletoe showing a cephalad, middorsal, elongated white mark useful for identification of the species; Right – chrysalis following caterpillar feeding on juniper mistletoe. Warner Mountains, Lake County, Oregon, 2024.

Prior to our study only one record for juniper mistletoe was known for the Warner Mountains region. This sole record was in Bullard Canyon 2.4 miles east of Lakeview. In addition to confirming the single historical record, we found juniper mistletoe at eight new sites (Figure 4-left, red dots; Appendix 1).

All of our butterfly records pertain to the observations of caterpillars on juniper mistletoe; we did not observe any adult Atlides butterflies in the field during the term of our study, although prior to our study thirteen records were known for the Warner Mountains region. Eleven of the thirteen records were located on the west facing slope of the mountain range within one mile of highway 395, one of these being the historical Bullard Canyon site known for both the butterfly and the lone juniper mistletoe record. Only two of the historical records for the butterfly are from the interior of the mountain range east of the summit of the Warner Mountains along Crane Creek (Figure 4-right, black dots).

We found fifteen Atlides caterpillars at four sites within the Warner Mountains region which resulted in adding three new locations on the distribution map (Figure 4-right, red dots; Appendix 1). The field collected caterpillars ranged from early development to late development, caterpillar length ranged from 4 mm to 2.5 cm, essentially second instars to fifth instars. All fifteen of the caterpillars developed into a last instar caterpillar, but at this stage, five of the caterpillars produced parasitoid flies instead of continuing development into the chrysalis (pupal) stage.

Figure 4. Map of the Warners Mountains, Lake County, showing distributions of: Left – juniper mistletoe, now known to serve as a suitable foodplant for the butterfly Atlides halesus corcorani, with a single record in the Warner Mountains prior to the present study (black dots) and new records (red dots); Right – records for the Great Blue Hairstreak, Atlides halesus corcorani prior to the present study (black dots) and new records for the butterfly (red dots). Warner Mountains, Lake County, Oregon, 2024. Historical plant records obtained from the Oregon Flora Project website. Maps created by Jeffrey C. Miller.

Over a period of one month in the rearing containers, four caterpillars successfully pupated and produced adult butterflies. The adult stage was important for verifying the preliminary identification of the caterpillars. The remaining six chrysalids have exhibited some form of failed or suspended development as of January 27, 2025. These individuals either contain a parasitoid, have become desiccated and are deceased, or entered into a state of diapause, in which case they are still viable. These chrysalids will be monitored into the spring of 2025 to determine their fate.

The field observation of Atlides caterpillars feeding on juniper mistletoe, and their successful development on juniper mistletoe, is the first documentation that juniper mistletoe in Oregon is a suitable foodplant. An earlier citation (A. Shapiro website) from California maintained that juniper mistletoe could be a suitable foodplant, but no details were provided to support this assertion.

We have begun studies to determine whether Phoradendron densum may also serve as a suitable foodplant for Atlides. We are focusing our study on the 110-mile-wide area to the west, between Soda Mountain and the Warner Mountains.
–Dr. Jeffrey C. Miller and Dr. Dennis A. Albert

References
Miller, J.C. and P.C. Hammond. 2003. Caterpillars and Adult Lepidoptera of Northwest Forests and Woodlands. USDA, USFS, FHTET-2003-03.

Miller, J.C. and P.C. Hammond. 2007. Butterflies and moths of Pacific Northwest forests and woodlands: rare, endangered, and management sensitive species. USDA, USFS, FHTET-2006-07.

Miller, J.C., A.R. Moldenke, and D.A. Albert. 2020. Insects as plant taxonomists. In: Flora of Oregon vol.2, pp 23-55. Brit Press.

Oregon Flora Website: https://oregonflora.org
Pyle, R.M. and C.C. LaBar. 2018. Butterflies of the Pacific Northwest. Timber Press Field Guide. 461 pp.

Acknowledgments
We would like to thank the Native Plant Society of Oregon for funding our study. Oregon Flora was a critical source for information on plant distributions. The author’s field-based identifications of butterflies were verified by Paul C. Hammond and Dana N.R. Ross. Special thanks to Tim Jones, botanist for the Paisley Field Office of the Fremont-Winema National Forest, who assisted us with permit applications and the logistics of working in the Warner Mountains.