Why Horse Flies, Midges and Black Flies May Suddenly Explode After a Wet UK Spring
- Horse Leads
- May 17
- 3 min read
Many horse owners across the UK have noticed something unusual this year: despite moving into late spring, the expected surge of biting flies has not fully appeared yet. The most likely reason is the prolonged unsettled weather pattern we have experienced through much of the season.
Cool temperatures, repeated rain fronts, strong winds and cold nights can significantly delay the emergence and activity of many biting fly species. However, delayed does not mean absent. In fact, when warmer and more stable conditions finally arrive, it can create the impression that all the flies appear at once.
With warmer weather now forecast, it is worth understanding how the main culprits behave around horses and why sudden population increases can happen very quickly.
Black Flies
Black flies, sometimes called buffalo gnats, are small dark biting flies that are commonly found near rivers, streams and fast-flowing water. Unlike many other flies, their larvae develop in moving water rather than stagnant ponds.
Wet springs can actually support large black fly populations because they increase water flow and create ideal breeding conditions. What suppresses adult activity is prolonged cold, wind and unstable weather.
Once temperatures rise consistently above around 15°C and conditions become warmer, more humid and less windy, adult black flies can emerge in large numbers.
Black flies often target:
ears
around the eyes
belly line
sheath and udder areas
Some horses react mildly, while others develop significant irritation, swelling and crusting. Horses that are sensitive may become very uncomfortable very quickly.
Wind is one of the biggest limiting factors for black flies, which is why sheltered fields and still conditions often produce the worst activity.
Midges
For many horses, midges are the most problematic biting insect because they are responsible for sweet itch reactions.
Midges thrive in:
damp pasture
wet vegetation
woodland edges
muck heaps
standing moisture
Like black flies, unsettled weather delays activity because midges are weak fliers and dislike wind and cool temperatures. However, a sudden spell of warm, humid and calm weather can trigger a rapid increase in numbers.
Midges are usually most active:
at dawn
at dusk
on warm still evenings
This is why many horses appear comfortable during breezy daytime conditions but become distressed in the evening.
The severity of sweet itch is not necessarily linked to huge numbers of midges. Some horses develop allergic reactions after only a few bites, leading to mane rubbing, tail rubbing and broken skin.
Horse Flies
Horse flies, also known as clegs in some areas, behave very differently from midges and black flies.
They are:
strong fliers
visual hunters
attracted to movement, heat and dark colours
Their larvae develop in wet ground, marshy pasture and damp soil. A wet spring can therefore create excellent breeding conditions, while delayed warm weather postpones adult emergence.
When a hot, sunny spell finally arrives, horse flies can suddenly appear in noticeable numbers.
Unlike midges, horse flies are often most active during:
bright sunshine
warm humid afternoons
still weather
They are aggressive daytime feeders and can cause significant distress to horses, particularly those standing still in fields.
Why Sudden “Explosions” Happen
This year’s weather pattern may have created what entomologists sometimes describe as compressed emergence.
Instead of gradual increases through spring, prolonged poor weather can hold back activity until conditions suddenly become favourable. When that happens, large numbers of adult flies may emerge over a relatively short period.
The key triggers include:
several warm days in a row
mild overnight temperatures
humidity
lighter winds
Warm nights are particularly important. Once overnight temperatures consistently remain above around 10–12°C, fly activity often increases dramatically within a few days.
Practical Management for Horse Owners
While it is impossible to eliminate flies completely, management timing becomes especially important during sudden warm spells.
Helpful strategies include:
using fly rugs before peak emergence
stabling sensitive horses at dawn and dusk
increasing airflow in stables
avoiding sheltered wet grazing areas during peak midge activity
using masks and belly protection for black fly irritation
effective fly sprays
This year may simply be a later season rather than a quieter one. If the forecast warm weather arrives alongside still, humid conditions, horse owners across the UK may notice a very sudden increase in fly activity over a short period of time.
The Ecosystem
Although flies are a major nuisance for horses, they also play an important role in the ecosystem by providing food for birds, bats, dragonflies and other insect-eating species. Many fly species also contribute to pollination and help break down organic matter in the environment. So we don't want to kill them, just move them along!
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