At a glance: Blue butterflies owe their vivid color mostly to structural coloration—microscopic wing scales that reflect blue wavelengths. Below are 9 standout “blues,” each with how to identify, where to find, host plants, flight season, and conservation notes, plus field tips for watching and photographing them responsibly.
1) Blue Morpho (Morpho spp.)
Key ID: Dazzling electric blue upperside with black margin; underside brown with large eye spots. Huge: 13–18 cm (5–7 in) wingspan. Females duller.
Range & habitat: Lowland and foothill tropical forests of Central & South America; edges and light gaps.
Host & diet: Larvae on various host trees (often Fabaceae); adults feed on rotting fruit/sap.
When to see: Year-round in the tropics; most obvious on sunny late mornings.
Notes: Often rests with wings closed; the iconic “blue flash” appears during takeoff/glide.
2) Fender’s Blue (Icaricia icarioides fenderi)
Key ID: Male sky-blue upperside; female brown with white-rimmed spots; ~2.5 cm (1 in) wingspan.
Range & habitat: Willamette Valley, Oregon (USA)—open prairie remnants with lupines.
Host: Kincaid’s lupine (Lupinus oreganus)—critical for oviposition and larvae.
When to see: Spring to early summer (local).
Conservation: Endangered; recovery hinges on prairie restoration, lupine re-establishment, and invasive control.
3) Silvery Blue (Glaucopsyche lygdamus)
Key ID: Cool silvery-blue male upperside; neat black spots on underside; small size.
Range & habitat: Widespread in North American grasslands, barrens, and open sandy sites.
Host: Mostly Fabaceae (and regionally some Polygonaceae).
When to see: Single spring/early summer brood, usually May–June.
Notes: Best found around low floral patches in calm, sunny conditions.
4) Common Blue (Polyommatus icarus)
Key ID: Males vivid bright blue with narrow black border; females often brown with orange lunules and white fringe.
Range & habitat: Europe to Western Asia; tolerant of meadows, road verges, brownfields.
Host: Various clovers/medics (Fabaceae).
When to see: Multi-brooded late spring to autumn.
Notes: Communal roosting at dusk offers great photo opportunities.
5) Eastern Tailed-Blue (Cupido comyntas)
Key ID: Tiny tail filaments on hindwings; males blue, females grayish-blue; underside with clear black spots.
Range & habitat: North America, common in lawns, lots, edges, and weedy fields.
Host: Legumes (clovers, vetches, etc.).
When to see: Spring through summer (multiple broods).
Notes: Look for low nectar sources; approach slowly from behind and above.
6) Karner Blue (Plebejus samuelis)
Key ID: Males bright blue; females brown with bluish suffusion; ~2.5 cm (1 in).
Range & habitat: Northeastern & Upper Midwest USA—oak-pine barrens and sandy openings.
Host: Wild lupine (Lupinus perennis)—sole larval host.
When to see: Bivoltine (spring and summer flights).
Conservation: Endangered; success depends on keeping sites open, sunny, and lupine-rich via fire/grazing/mowing regimes.
7) Adonis Blue (Lysandra bellargus)
Key ID: Males an intense, ultra-clean sky blue with crisp white fringe; females brown.
Range & habitat: Southern Europe, tied to chalk/limestone grasslands.
Host: Typically horseshoe vetch (Hippocrepis comosa) and other Fabaceae.
When to see: Regional multi-brooded flights; numbers track grazing pressure and sward height.
Notes: Search warm, short-turf slopes with abundant flowers.
8) Boisduval’s Blue (Icaricia icarioides complex)
Key ID: Pale to bright powder-blue males with white fringe; females brown.
Range & habitat: Western USA—coastal dunes, foothills, and montane openings.
Host: Primarily lupines/legumes.
When to see: Spring–summer (local variation).
Notes: Often patrols along sunny south-facing edges and roadcuts.
9) Green-underside Blue (Glaucopsyche alexis)
Key ID: Male upperside blue; distinct green-tinted underside; females largely brown.
Range & habitat: Europe to Western/Central Asia in sunny flower-rich grasslands.
Host: Favours Fabaceae (varies locally).
When to see: Spring peak.
Notes: Side- or back-lighting brings out the green undersurface sheen.
Field Tips: Watch & Photograph Without Harm
Light: Early/late side-light enhances metallic blues; midday blows highlights.
Approach: Move slowly from rear/above while the butterfly is nectaring or basking.
Do no harm: Stay on paths, avoid trampling host plants (especially lupines), and don’t handle or net for photos.
Log useful data: Date, location, habitat, host plant, and clear dorsal/ventral shots help with ID and contribute to citizen-science databases.
Why “blue” looks so blue (quick science)
Most “blue” butterflies aren’t blue because of pigment. Their scales form micro-/nano-structures that selectively reflect blue light via interference and scattering—hence the vivid, angle-dependent “flash.”
Conservation: What actually helps
Habitat management: Keep grasslands open, sunny, and flower-rich with light grazing/mowing timed outside peak flights.
Host plant recovery: Re-establish lupine/clover belts; control shading invasive shrubs.
Community science: Share records to regional platforms with photos + host info to improve monitoring.
Quick Reference Table
| Species | Signature Blue | Fast ID | Primary Hosts | Best Season |
|---|---|---|---|---|
| Blue Morpho | Electric, mirror-like blue | Huge; brown eye-spotted underside | Various trees (often Fabaceae) | Year-round (tropics) |
| Fender’s Blue | Sky-blue male | Tiny prairie blue; females brown-spotted | Kincaid’s lupine | Spring–early summer |
| Silvery Blue | Icy silvery male | Neat black underside spots | Legumes (some Polygonaceae) | Late spring–early summer |
| Common Blue | Bright sky blue male | Females brown with orange lunules | Clovers/medics | Late spring–autumn |
| Eastern Tailed-Blue | Blue/gray-blue | Hindwing tails | Legumes | Spring–summer |
| Karner Blue | Bright blue male | Two broods; sandy barrens | Wild lupine | Spring & summer |
| Adonis Blue | Intense pure sky blue | Chalk grassland specialist | Horseshoe vetch, etc. | Multi-brooded; regional |
| Boisduval’s Blue | Powder-blue male | White fringe; western USA | Lupines/legumes | Spring–summer |
| Green-underside Blue | Blue top, green bottom | Strong blue-green contrast | Fabaceae | Spring |
