Biting Insects


Seasonal Cycles of Biting Insects

Isle Royale’s climate is kept at a cooler temperature than the surrounding mainland by the cold water temperatures of Lake Superior. A cooler climate alters the length and warmth of summer seasons and shortens the activity time of all insects including mosquitoes and black flies. These species tend to hatch around late May into early June. Typically, they are worse in late June and early July. By late August and early September, insect numbers typically decline.

Check current conditions for seasonal bug reports.

Prepare For Biting Insects

  • Cover exposed skin by wearing long sleeved-shirts, long pants, and long socks.

  • Use mosquito netting to cover head and neck.

  • Consider bug spray and insect repellent.

A female adult mosquito sitting on a human arm.
Isle Royale is home to 64 species of mosquitoes.

NPS Photo


Size: 0.125 – 0.3125 inches (3 - 8 mm)
Family: Culicidae
Season: Late spring through early autumn.

Only female mosquitoes bite in order to produce eggs. Eggs are laid in standing water such as beaver ponds and boreal bogs and hatch within two weeks. On Isle Royale, different species emerge at different times throughout the spring and summer months, and are more active during the cooler hours of dawn/dusk, near lowland wet areas, and in calmer winds.

Artwork showing a black fly.
Black flies are small, usually black or dark in color, and have a hump shape to their backs.

©Sturgis McKeever, Georgia Southern University,

Black Fly

Size: Approximately 0.12 inches (2-5 mm)
Family: (Simuliidae simulium)
Season: Late spring through summer.

Only feeding during the daytime, evening hours typically bring relief from these biting bugs. Black fly bites are usually painful and leave a triangle shaped bite wound that itches for 1-2 days afterwards.

: Deer fly sitting on human skin.  It has transparent wings with black dot in the middle of each wing.  The body is mostly yellow with green on back.
Deer flies can be identified the distinctive bands on their wings.

©Sturgis McKeever, Georgia Southern University,

Deer Fly

Size: Less than 0.5 inches (10-12 mm)
Family: (Chrysops callidus)
Season: Arrives with summer heat and remains through warm weather.

Deer flies have an audible buzz to their flight that can be heard as they circle their intended victim. Deer flies will immediately bite upon landing.

Close up of American Horse Fly.  It has large green eyes, red legs and a black/brown body.
Only female horse flies bite in order to produce eggs.

©Sturgis McKeever, Georgia Southern University,

American Horse Fly

Size: Larger than 0.5” (13 mm)
Family: (Tabanus americanus)
Season: Arrives with summer heat; males survive through the summer and females survive into the autumn.

Horse flies are larger than deer flies, but share similar characteristics in audible flight, seasonal existence, and painful bites. The wounds from their bites tend to remain open and bleed for several minutes due to their saliva having an anti-coagulant property that prevents clotting.

Biting stable fly sitting on a green leaf.
Stable flies are one of the few biting members of the house fly family.

©Whitney Cranshaw, Colorado State University,

Stable Fly

Size: Approximately 0.3 inches (8 mm)
Family: (Stomoxys calcitrans)
Season: Emerge during the warmest months and subside in the autumn.

Stable flies can be identified by their grey bodies with a greenish-yellow sheen and the visible proboscis they use to bite. Having a fierce bite, they often attack ankles and lower legs of humans and other mammals. Stable flies can reproduce several generations each summer.

A view of tall grasses and bare tree snags.
Ticks are often found in tall grass and shrubs where they will wait to attach to a passing host.

Paul Brown

Wood Tick

Size: Approximately 3/16 inch.
Family: Ixodidae

Ticks are blood feeding parasites. Infrequent sightings of wood ticks, also known as dog ticks, have been reported at Isle Royale. At this time, there is thought to be a small and non-sustaining population of wood ticks. Report all tick sightings to park staff.

There are no deer ticks at Isle Royale.


Last updated: August 6, 2022

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