Straits acmon blue (Icaricia acmon sp.)

Close up of an acmon blue butterfly species perched on prairie soil
This is an example of an Icaricia acmon species.  (U.S. Forest Service - Pacific Northwest Region)
Category: Butterflies and moths
Common names: Fuca blue
Ecosystems: Marine shorelines
Vulnerability to climate change (More details)


If you see this species, please share your observation using the WDFW wildlife reporting form. Providing detailed information such as a photo and exact coordinates will improve the confidence and value of this observation to WDFW species conservation and management.

The population size of Straits acmon blue is considered a segregate of the more widespread Icaricia acmon. This segregate is limited in distribution along the Strait of Juan de Fuca. It in Washington is critical and is considered to have a declining trend. This lycaenid butterfly is recognized as a "Species of Greatest Conservation Need" due to its rare and restricted hostplants [food for caterpillars (larvae)] and habitat types, small number of isolated populations, highly limited range and distribution, and threats to its habitat. 

Research is needed on this species to understand its life history and quantify specific habitat requirements including vegetation structure, food plant size and density, and key habitat features.

Description and Range

Physical description

The Lycaenidae butterfly family consists of small and often brightly colored species with the common names: copper, hairstreak, elfin, and blue.  

Ecology and life history

This acmon blue segregate is restricted to a few coastal sand spits and beaches along the southern shores of the Straits of Juan de Fuca, in Clallam County, where it uses the semi-shrubby, prostrate, black knotweed (Polygonum paronychia) for its host. Beyond their host need and sand spit and beach occurrence, little is known of their habitat requirements.

This lycaenid butterfly has two generations per year (spring and late summer). All lycaenids are sedentary butterflies and do not migrate; instead, the species inhabits sites year-round (as egg, larva, pupa and adult), typically moving within only a few hundred yards of their natal locations. Adults emerge from their chrysalids (pupae) during species-specific time periods; the Straits acmon blue’s adult period is May through June and August.

The Straits acmon blue’s primary nectar plant is unknown. Both males and females feed by using their long proboscis to sip floral nectar. Males of most species require salts, which they obtain from evaporated puddles and moist soil and animal urine and feces. 

Males begin emergence first, followed by females; late season individuals are primarily or solely females. Weather influences butterfly emergence and the flight period duration, with wet or cold conditions potentially delaying emergence, and warm, dry conditions promoting earlier emergence. 

Male lycaenids seek mates using patrolling patterned flight or perching on vegetation in select spots and darting out to inspect passing butterflies. Females search for egg-laying sites by slowly flying and hovering above hostplant vegetation, and then landing and crawling to inspect vegetation before depositing eggs singly.

Larvae are slug-like in appearance and highly camouflaged in their host species. Many lycaenid larvae engage in mutualistic relationships with ants, known as myrmecophily, which typically consists of ants tending and milking larvae, obtaining nutrition in the form of a nectar-like substance (honeydew) in the process, and also protecting larvae from predators and parasitoids; in some situations, the ants move butterfly larvae or pupae into ground chambers, including their nests. The overwintering stage varies by lycaenid species: this stage unknown for Straits acmon blue.

Geographic range

The range of the Straits acmon blue is restricted to its coastal habitat along the Straits of Juan de Fuca in Clallam County, Washington. The distribution of this species is limited in part by a combination of their dependence on rare hostplant occurrence within rare habitat types. Its distribution and abundance in Washington is characterized by small numbers of small isolated populations. 

For a map of range-wide conservation status and distribution of Icaricia acmon species, check out NatureServe Explorer.

Climate vulnerability

Sensitivity to climate change


Restricted to coastal sand spits and beaches, both the butterfly and its habitat are  highly vulnerable to saltwater inundation due to sea level rise and increased storm frequency. Winter storm events on SAB sites have been observed inundating and eroding habitat in recent years. Intrusions of seawater are likely lethal to larvae which are located at the soil surface, and overtime will reduce requisite vegetation, including SAB host plants (black knotweed) and nectar sources. Temperature influences butterfly behavior (e.g., mating, foraging, egg-laying time), adult life span, and larval development. Cool, wet spring conditions limit first generation SAB adult activity and therefore fecundity. Warming temperature may also affect phenological timing between SAB and key plant species (host and nectar plants), especially in the late-summer second generation, and possibly cause low availability of nectar resources and pre-mature desiccation of larval forage, leading to reduced fitness or starvation of adults or larvae.  (See Species Fact Sheet for more information on SAB, and Scrub & Herb Coastal Vegetation habitat and North Pacific Sand Dunes assessments (including Table 5-3 and Figure 5-8) for more information on habitat sensitivity to climate change).

Confidence: High

Exposure to climate change


  • Sea level rise
  • Increased storm frequency and intensity
  • Increased temperatures
  • Increased invasive weeds
Confidence: High


This species is identified as a Species of Greatest Conservation Need (SGCN) under the State Wildlife Action Plan (SWAP). SGCN-classified species include both those with and without legal protection status under the Federal or State Endangered Species programs, as well as game species with low populations. The WDFW SWAP is part of a nationwide effort by all 50 states and five U.S. territories to develop conservation action plans for fish, wildlife and their natural habitats—identifying opportunities for species' recovery before they are imperiled and more limited.

Conservation Threats and Actions Needed

  • Climate change and severe weather
    • Threat: Populations located adjacent to marine waters that are rising.
    • Action Needed: Evaluate landscape and develop a plan to increase habitat area and habitat heterogeneity in currently occupied sites and within occupied landscapes.
  • Invasive and other problematic species
    • Threat: Invasive plants, those currently here, and many yet to come in the future, out-compete natives and otherwise make habitat unsuitable.
    • Action Needed: Using herbicide and mechanical methods to maintain open conditions of vegetation.

See the Climate vulnerability section for more information about threats posed by climate change to this species. Climate vulnerability is a way to assess the degree to which a habitat or species is susceptible to, and unable to cope with adverse impacts of climate change.



Ballmer, G. and G. Pratt. 1991. Quantification of ant attendance (Myrmecophily) of lycaenid larvae. Journal of Research on the Lepidoptera. 30(1-2): 95-112.

Fleckenstein, J. 2014. Rare alpine butterflies in the Olympic Mountains. Final report to the US Forest Service and Bureau of Land Management. Natural Heritage Program, Washington Department of Natural Resources. Olympia. 14 pp.

James, D. and D. Nunnallee. 2011. Life Histories of Cascadia Butterflies. Oregon State University Press, Corvallis. 447 pp.

Pyle, R.M. and C.C. LaBar. 2018. Butterflies of the Pacific Northwest. Timber Press. 461 pp.

Pyle, R. 1989. Washington butterfly conservation status report and plan. Washington Department of Fish and Wildlife, Olympia. 216pp.

Other resources