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Biting Midges, "No-See-Ums" & Sweet Itch In Horses

- Use insect repellents to avoid bites -

Summary: Biting midges, or no-see-ums, are tiny flies in the family Ceratopogonidae. Like black flies and mosquitoes, biting midges cause intensely itchy skin lesions and an allergic reaction. Insect repellents offer the best protection. Sweet itch (summer itch) is an allergic skin condition in horses caused by the bite of these flies.

Jack DeAngelis, PhD
OSU Ext. Entomologist (ret.)

Biting midge identification

Biting midges are tiny (0.6-1.5 mm) flies in the family Ceratopogonidae (see photo right). These flies are variously called biting midges, gnats, "sand flies", no-see-ums, and punkies*. The midges are nearly invisible to the unaided eye and can pass through normal window screens and even some mosquito netting. When under attack by these flies outdoors people often say something like "I'm getting bit but I can't see what's biting me!". While there are many non-biting midges, or gnats, these are the only midges that bite. At rest biting midges fold their wings across the back like scissors. The wings may be spotted and somewhat hairy.

biting midge adults

These tiny biting midges (flies) are nearly invisible to the unaided eye.

*True sand flies belong to the family Psychodidae; the term "gnats", is generally used for another family of flies; however, among horse owners biting midges are frequently called "gnats". Sweet itch is also known as Queensland itch, kasen (Japan), dhobie itch (Philippines) and summer itch.

Biting midges develop in damp, organic soils

Biting midges develop in wet, marshy, or swampy soil but even a small spring can produce large numbers of these flies. The larvae are aquatic or semi-aquatic and midge infestations tend to be located near poorly drained or constantly wet soil.

Infestations can be very "spotty" across fields, high in some wet areas while absent from drier spots. Biting midges are poor fliers so are not able to move far from their breeding habitat. Short of draining the wet ground it is nearly impossible to eliminate midges. Instead it is best to use repellents to protect both people and horses from bites (see below).

Using repellents

When outside and in areas where biting midges are known to be a problem insect (mosquito) repellents are the best defense against these biting flies. Repellents containing DEET or picaridin will give the longest-lasting protection and midge-proof bed netting should be used if midges are gaining access to sleeping areas. The new "personal foggers" such as ThermaCELL (tm) are also effective against biting midges (see Mosquito/Biting Fly Protection).

Sweet itch in horses

Sweet Itch is a skin condition in horses caused by the bites of biting midges. The skin ailment is the result of an allergic reaction to the bites of the midge. Horses vary in their response to bites -- some react strongly with open, weeping sores while others hardly react at all. The sores are caused by constant rubbing and biting to relieve the itch. You should move horses, if possible, away from pastures of known midge infestation. There are very effective allergy treatments for the allergic reaction itself so check with your vet.

There are three popular choices for treating horses with a fly (insect) repellent -- sprays, wipes and "spot-on" applications. The spot-on treatments are similar to the new flea control medications used for dogs and cats where drops of insecticide repellent are applied to the skin. Sprays are easier to use but some horses won't allow you to get near them with a spray bottle so wipe-on or spot-on repellents may be your best bet. All of the repellents are effective, the trick is to find the application method that works for you and your horse.

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Jack DeAngelis, PhD,  , email:  [email protected]

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