The pale giant horse fly (Tabanus bovinus) and the dark giant horse fly (Tabanus sudeticus) are among the largest and most noticeable biting flies in the UK. Both species belong to the horse fly family (Tabanidae) and are well known for their painful bites to humans and livestock. They are often confused due to their similar size and behaviour, but there are clear differences in appearance and ecology.
This article explains where these species occur in the UK, how they behave, how to tell them apart, and how to reduce the risk of bites.
Occurrence and Activity in the UK
When are giant horse flies active?
In the UK, both Tabanus bovinus and Tabanus sudeticus are active only during the summer months. Their activity is strongly weather-dependent.
Typical activity patterns include:
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Active from late May to August
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Peak activity:
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Tabanus bovinus (pale giant horse fly): mainly May–June
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Tabanus sudeticus (dark giant horse fly): mainly June–August
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Active only during daylight hours
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Most aggressive on warm, sunny, windless days
They are rarely encountered in cool, overcast or windy conditions.
Feeding Behaviour
What do pale and dark giant horse flies feed on?
Adult horse flies feed on:
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Nectar and plant sugars (both males and females)
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Blood (females only)
Female flies require a blood meal to produce eggs. They bite mammals to obtain this blood, including:
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Horses and ponies
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Cattle and deer
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Humans and other large mammals
Although horse flies normally feed on living hosts, Tabanus sudeticus has occasionally been observed feeding on carcasses.
In general:
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Tabanus bovinus shows a preference for cattle
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Tabanus sudeticus more frequently attacks horses, ponies and deer
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Both species readily bite humans
Prevention and Control
Can giant horse flies be controlled?
There are no practical methods to control horse fly populations in the UK. This is mainly because:
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There are no targeted insecticides for large-scale control
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Horse flies breed in wetlands and natural habitats that cannot be treated
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Traps exist for research purposes but are not suitable for general use
As a result, prevention focuses on avoiding bites rather than eliminating the flies.
How to avoid bites
The most effective way to avoid bites is to use high-quality insect repellents on skin and clothing. Although repellents are primarily designed for mosquitoes, many of them also deter horse flies to some extent.
Additional practical tips include:
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Wearing long sleeves and trousers in high-risk areas
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Avoiding wetlands, marshes and woodland edges on hot days
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Seeking shade or cooler areas, as horse flies avoid shade
Treating Horse Fly Bites
Bite, not sting
Horse flies bite, they do not sting.
Their mouthparts:
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Cut a triangular wound in the skin
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Release saliva containing anticoagulants to prevent clotting
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Allow the fly to lap up blood
Because of this, horse fly bites are usually more painful and may bleed slightly.
How to treat a bite
Horse fly bites should be treated like other biting-fly wounds:
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Clean the bite with soap and water
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Apply soothing or anti-itch products
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Use a cold compress to reduce swelling
If symptoms are severe – such as large swelling, fluid discharge, increasing pain or signs of infection – medical advice should be sought.
Pale Giant Horse Fly or Dark Giant Horse Fly?
Size comparison
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Dark giant horse fly (Tabanus sudeticus)
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Typically around 2.5 cm long
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The largest horse fly species commonly encountered in the UK
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Pale giant horse fly (Tabanus bovinus)
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Usually 2.0–2.5 cm long
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Slightly smaller but still very large
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Because of their similar size and behaviour, the two species are frequently confused.
Visual Differences Between the Two Species
Pale Giant Horse Fly (Tabanus bovinus)
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Overall brownish appearance
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Slightly smaller and lighter than T. sudeticus
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Abdomen marked with brown or pale reddish bands
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Light brown or greyish triangular markings along the back
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Triangles almost reach the band above them
Dark Giant Horse Fly (Tabanus sudeticus)
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Generally darker, often black and yellow
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Slightly larger and more robust
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Abdomen marked with dark or black bands
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Small yellow triangular markings along the back
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Triangles do not reach the band above
Additional differences can be seen in antenna shape and abdominal segment colouring, but these usually require close inspection.
Eggs and Larvae
Very little is known about the early life stages of these species in the UK.
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Tabanus sudeticus is believed to lay eggs in wet, marshy areas
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The larval habitats of Tabanus bovinus are poorly documented, but larvae are thought to live near lake and river margins
Like other horse flies, the larvae are predatory and develop in moist soil or shallow water.
Other Notable Characteristics
Flight and sound
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Tabanus sudeticus produces a distinct loud buzzing sound in flight
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The buzzing usually stops just before landing
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Landings are light and often unnoticed until the bite occurs
Behaviour around horses
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Horses may react strongly to bites, sometimes developing large swellings
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Giant horse flies are not the only biting flies affecting horses; deer flies and rain horse flies also contribute
Flight ability
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Both species are fast, agile flyers, making them difficult to swat or avoid
Preference for sun
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Like all horse flies, they avoid shade and cool conditions
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Activity is strongly linked to direct sunlight and warmth
Males
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Male flies do not bite
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They feed only on nectar and plant juices
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Males tend to remain in wooded areas and may show territorial behaviour
Summary
The pale giant horse fly (Tabanus bovinus) and dark giant horse fly (Tabanus sudeticus) are among the most formidable biting flies encountered in the UK summer. While they cannot be controlled directly, understanding their behaviour, seasonal activity and preferred conditions makes it easier to reduce exposure.
Avoiding peak activity periods, using effective repellents and recognising the differences between the two species can help limit the discomfort caused by these large and persistent flies.