Horse Fly (Tabanidae)
Identification
Horse flies are robust, sturdy flies with large compound eyes that are often iridescent or striped. The body is typically gray, brown, or black. Wings are clear or lightly tinted.
They are larger and heavier-bodied than house flies or stable flies. Deer flies (also in the same family) are usually smaller and often have dark-spotted wings.
Biology & Life Cycle
Only adult females bite — they need a blood meal to develop eggs. Eggs are laid in masses on vegetation near water or moist soil. Larvae develop in wet soil, mud, or shallow water, often in marshes, pond edges, or stream banks. Most species have one generation per year. Adults are active from late spring into early fall in Wisconsin.
Habits & Behavior
Horse flies are strong, persistent fliers. Females seek large mammals (including people, horses, and cattle) during the day, especially in sunny conditions near wooded edges, wetlands, or pastures. They cut the skin with blade-like mouthparts and lap up the blood. Bites are painful and can leave a raised welt. Males feed on nectar and do not bite.
Signs of Activity
Large, fast-flying flies that persistently circle people or animals outdoors
Painful bites during the day, especially near wooded or wet areas
Higher numbers on warm, sunny days from June through August
Health & Property Risks
Horse fly bites are painful and can cause significant irritation, swelling, and secondary infection from scratching. In rare cases they can mechanically transmit certain pathogens, though this is uncommon in Wisconsin. Heavy populations can stress livestock and make outdoor activities unpleasant.
Prevention Tips
Avoid peak activity areas (wetland edges, wooded pastures) during midday when possible
Wear long sleeves and pants in light colors
Use EPA-registered repellents (some provide partial protection)
Keep moving — horse flies are less successful on moving targets
Screens and fans can help around outdoor seating areas
Treatment Approach
There is no highly effective chemical control for adult horse flies on a large scale.
Focus on personal protection and avoiding peak areas
Traps (such as Manitoba traps or other visual/attractant traps) can reduce local numbers around livestock or high-use areas
Managing breeding sites is difficult because larvae live in natural wet soils and wetlands
Professional help is mainly useful for livestock operations or properties with severe, persistent problems.
When to Call a Professional
Call if:
Horse fly pressure is unusually high and affecting outdoor use or livestock
You need help evaluating whether traps or other localized measures are worthwhile
Bites are causing significant reactions or secondary issues
Local Notes
Horse flies and deer flies are common across Racine County and southeastern Wisconsin from late May through September, especially near wetlands, ponds, streams, and wooded edges. Activity peaks on warm, sunny days. They are a seasonal outdoor nuisance rather than a structural pest.
Quick Facts
Family: Tabanidae (includes horse flies and deer flies)
Adult size: About ½ to 1¼ inches (some species larger)
Key feature: Large, often iridescent compound eyes
Behavior: Females bite; males do not
Activity: Daytime, late spring through summer