New York Natural Heritage Program
Northern Oak Hairstreak
Satyrium favonius ontario (W. H. Edwards, 1868)

General Description [-]
An average size brown hairstreak, occurring in early summer, with a green slug-like caterpillar occurring on oaks in the spring. In New York, this is most likely to be confused with the gray hairstreak. The Northern Oak Hairstreak is a much browner species and the adults are illustrated in virtually any butterfly guide.

Characters Most Useful for Identification [-]
Note the inwardly pointed "v" mark next to the largest white spot on the hindwing beneath and the prominent "M" (or "W") formed by the inner most line. These two markings together should be diagnostic. There is never any blue above. Live individuals always land with the wings closed. The flight season is also a clue to species, as this species will only be seen in or near New York in June and about the first half of July.

Best Life Stage for Proper Identification [-]
This should be identified as an adult, but an expert on hairstreaks might be able to identify the larva. See Allen et al. (2005).

Behavior [-]
Adults seem to come to flowers most often late in the day and are thought to spend most of their time in the oak canopy. Adults occasionally are caught in blacklight traps in places where they are otherwise not known to occur.

Diet [-]
The larvae are oak feeders, but exactly which species of oaks they use is largely unknown. They do not seem to be particularly associated with scrub oak, but they might use it where it is available. Larvae feed on the new spring growth only and the adults will visit a number of species of flowers, but are most often seen on common milkweed (Asclepias syriaca) and dogbane(Apocynum sp.). Dale Schweitzer has found them on maleberry (Lyonia ligustrina) flowers in New Jersey.
Northern Oak Hairstreak Images
click to enlarge
The Best Time to See
The adults occur for less than a month in any given year, from about mid-June into mid-July. They often start a few days earlier than the other single-brooded early summer hairstreaks, but commonly occur with the others. The eggs overwinter on oaks and the larvae feed on the new growth in spring, probably finishing about the first of June in most years. There is only one brood in all parts of the range of this species. The adults visit flowers most often late in the day, usually after 16:00 hours.
Present Reproducing Larvae present and active Eggs present outside adult
The time of year you would expect to find Northern Oak Hairstreak present (red shading), reproducing (blue shading), larvae present and active (green shading) and eggs present outside adult (orange shading) in New York.