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Identification & Distribution:

Tabanus bromius is a medium-sized species with a body length of 13-15 mm. Both sexes (female below first, male second) have a dark, hairy abdomen with three longitudinal rows of yellowish- or greyish-brown spots. The overall appearance is rather variable ranging from a yellowish form (resembling Tabanus glaucopsis) to a blackish form (resembling Tabanus maculicornis). The mesonotum of Tabanus bromius is gray with five indistinct longitudinal lines.

 

The female of Tabanus bromius has the front corners of tergite 2 pale yellow with yellow hairs (see first picture below). This distinguishes it from the female of Tabanus miki which has an orange area with black hairs.

 

The alula is broad (see second picture above). This differentiates Tabanus bromius from the similar but darker Tabanus maculicornis which has a longer, narrower alula. Their eyes are bare, and in both sexes are greenish with a characteristic single straight violet-red eye band shown in the images below (female first, male second). Its visibility or otherwise depends to some extent on the viewing angle, and it not generally visible in dried specimens.

 

The male has no distinct zone of small facets along the hind edge of the upper part of the eye. This distinguishes Tabanus bromius from male Tabanus miki, which has a tapering zone of small facets along the upper margin of the eye. The subcallus is entirely dusted (see second picture above) which distinguishes it from Tabanus glaucopsis which has a glossy subcallus.

Distribution & Seasonal Occurrence

The band-eyed brown horsefly flies from late May to early September, with numbers highest in July and August. In Britain Tabanus bromius is found widely in various habitat types including woodland and downland, but less so in coastal marshes. It commonly feeds on the blood of cattle and ponies, but may take an occasional human meal. Tabanus bromius is distributed widely over most of Europe into Russia.

 

Biology & Ecology:

Resting behaviour & Swarming

They may often be seen resting on fence posts where they are remarkably well camouflaged from the predatory activities of birds.

One behavioural characteristic of Tabanus bromius is that they often enter buildings. The images below shows two female Tabanus bromius after they entered our cottage in a Sussex village on one hot day in June.

 

Whether such behaviour is the result of a sudden mass-emergence, or of ideal weather conditions for 'swarming' on that day, is unclear, but it is not uncommon phenomenon with tabanid species. They may have entered the house because of the higher carbon dioxide concentration compared to ambient, although they showed no inclination to obtain a blood meal from the occupants.

Blood feeding

Baldacchino et al. (2013)  recently looked at the feeding patterns of tabanids in France. In alpine pastures 50% of blood meals of Tabanus bromius were on cattle, 38% were on deer and 12% on horses (n=8). In a wildlife park 85% of blood meals of that species were on red deer, 9% on bison and 6% on wild boar (n=32). None of the blood meals in alpine pastures or the wildlife park were from commonly available small ungulates such as roe deer and izard, indicating that Tabanus bromius preferentially feeds on large ungulates. Just considering large ungulates, the feeding pattern of Tabanus bromius was more or less in line with host availability, indicating that amongst large ungulates Tabanus bromius is an opportunistic feeder.

In Britain, Colyer & Hammond (1951)  reported females biting on the flanks and bellies of cattle in meadows. Grayson (1997)  reports that female Tabanus bromius attack man in ones and twos and encircle the legs and torso several times before alighting on the legs or torso. This species was so numerous at some sites that a succession of attacking females were encountered over periods of half an hour or more. Our own experience is that Tabanus bromius is rather reluctant to bite man - one suffers very few bites even when odour baited traps are catching hundreds a day.

Nectar feeding & puddling

Male Tabanus bromius are most often seen taking nectar from flowers, especially Angelica sylvestris, or taking moisture/salts on the ground as can be seen in the pictures below. Drake (1999)  observed many individuals feeding at the wet mud of puddles in wheel ruts in the sun. He also saw a few Tabanus bromius dipping at these wheel ruts. Dipping is the curious behaviour of tabanids in which they dive at the water surface then shoot away again after touching it very briefly. On one occasion Drake observed a female bromius settle on the water surface of a recently dug pond only to be almost immediately set upon by predatory Notonecta water bugs.

 

Kniepert (1980)  showed that both the males and females of many species take nectar from flowers. Whilst only about 11% of female Tabanus bromius had fresh or digested blood in the midgut (n = 5632), over 47% had taken nectar (n=5007). Krčmar & Maríc (2010)  have shown that at least one blood meal is essential for females of this species for egg maturation.

Breeding sites

Oldroyd (1939)  cited workers from Europe giving a wide range of breeding sites for Tabanus bromius. These included soil under turf in meadows, in fresh mole hills in meadows, in mud beside streams and in damp moss on tree stumps. The larvae are voracious predators of earthworms and insect larvae.

Trapping & odour attractants

Some work has been done to test the effectiveness of different odour attractants for Tabanus bromius. In Croatia Krčmar et al. (2005)  showed that 1-octen-3-ol and aged horse urine are very effective attractants for Tabanus bromius. Krčmar et al. (2006)  compared the response of tabanids to different natural attractants. Aged cow, horse, sheep and pig urine all gave greatly increased catches in a canopy trap relative to an unbaited trap, with aged cow urine performing better than the other attractants. Krčmar (2007)  showed that 4-methylphenol (one of the active chemicals in cow urine) increased the catch of Tabanus bromius in canopy traps by twenty times.

Several workers have referred to Tabanus bromius being the commonest horsefly in their part of Europe, at least when assessed by odour baited traps. For example, Krčmar et al. (2005)  found Tabanus bromius was the most abundant species with 53.14% in the sample collected by canopy traps. In Britain Goffe (1931) considered it easily the commonest species of the genus in Hampshire, but rare in the Midlands. Stubbs & Drake (2014)  consider it is now a local species in Britain which has declined substantially possibly as a result of the widespread ploughing and destruction of meadows.

We have found the same using cow urine/acetone - baited NG2F traps (pictures above and below) in several habitat types in southern England (Brightwell & Dransfield, 2014 ). Catches were highest in mixed deciduous/coniferous woodland (below, first) and downland (below, second).

 

Tabanus bromius was still predominant in farmland (below left) but was uncommon on coastal grazing marshes (below right) where other species predominate.

 

 

Disease transmission

Tabanus bromius is an important nuisance biter of cattle and horses, mainly because of its high relative abundance compared to many other species. It is a potentially important mechanical vector of equine infectious anaemia for the same reason. A painful bite leads to interrupted feeding and transfer of infectious blood to nearby animals. The risk of a horse fly transmitting disease from a sub-clinically infected equid to an uninfected animal is very low (DEFRA, 2011 ). The risk of transmission from an infected horse with clinical signs is much greater. It is considered that there is medium risk of transmission, if the correct conditions occur, and if clinically infected horses are not being quarantined or destroyed.

This species was also shown by Böse et al. (1987)  to be a vector of Trypanosoma theileri, a ubiquitous but apparently harmless protozoan parasite of cattle.

References

  • Baldacchino, F. et al. (2013). Blood-feeding patterns of horse flies in the French Pyrenees. Veterinary Parasitology Full text 

  • Böse, R. et al. (1987). Transmission of Trypanosoma theileri to cattle by Tabanidae. Parasitology Research 73, 421-424. Abstract 

  • Brightwell, R. & Dransfield, R.D. (2014). Survey of Tabanidae (horseflies) in southern England 2014. A preliminary survey of tabanids using odour-baited NG2F traps. 14 pp. Full text 

  • Colyer, C.N. & Hammond, C.O. (1951). Flies of the British Isles. Warne, London.

  • DEFRA (2011). Qualitative analysis of the risk of introduction of Equine Infectious Anaemia (EIA) into Great Britain from an EIA endemic area through temporary movement of UK origin horses (Roberts, H. & Paterson, A.) Veterinary Science Team, 17 Smith Square, London, SW1P 3JR, United Kingdom. Version 1.0, Released 20 June 2011, p.15. Full text 

  • Drake, M. (1999) Notes on predation. Larger Brachycera Recording Scheme Newsletter 17Full text 

  • Grayson, A. (1997). Personal notes on attacks by female tabanids. Larger Brachycera Recording Scheme Newsletter 15Full text 

  • Kniepert, F.W. (1980). Blood-feeding and nectar-feeding in adult Tabanidae (Diptera). Oecologia 46, 125-129. Abstract 

  • Krčmar S. et al. (2005). Response of Tabanidae (Diptera) to natural and synthetic olfactory attractants. Journal of Vector Ecology 30(1), 133-136. Abstract 

  • Krčmar, S. et al. (2006). Response of Tabanidae (Diptera) to different natural attractants. Journal of Vector Ecology 31(2), 262-265. Full text 

  • Krčmar, S. (2007). Response of Tabanidae (Diptera) to canopy traps baited with 4-methylphenol, 3-isopropylphenol, and napthalene. Journal of Vector Ecology 31(2), 188-192. Full text 

  • Krčmar, S. & Maríc, S.V. (2010). The role of blood meal in the life of haematophagous horse flies (Diptera: Tabanidae). Periodicum biologorum, 112(2), 207-210. Full text 

  • Oldroyd, H. (1939). Brachycera. In: F.W. Edwards et al. British blood-sucking flies. British Museum (Nat. Hist.), London.

  • Stubbs, A. & Drake, M. (2014). British Soldierflies and their Allies. BENHS, Reading.