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Square-spot deerflyOn this page: Identification Distribution & Seasonal Occurrence Blood feeding Breeding sites Trapping & odour attractants
Identification & Distribution:
Chrysops viduatus is a medium-sized deerfly with a body length of 8.5-10 mm. The female Chrysops viduatus has only a small quadrate spot at the base of the second tergites (see pictures below). This may be slightly bilobed, triangular or heart shaped and does not usually extend to the hind margin of the tergite. The hind tibiae are entirely reddish-yellow. The wing is clear behind the sub-apical spot up the wing margin.
The male (not pictured here) is similar to the male of Chrysops relictus, but has no grey-brown shading near the hind margin of the wing. Chrysops viduatus has the second abdominal segment yellow with a large well-defined quadrate black spot. The middle tibiae are reddish yellow much like Chrysops relictus, but the fore and hind tibiae are generally dark. The facial bare spots are large, almost meeting in the middle line.
Distribution & Seasonal Occurrence
Chrysops viduatus occurs in wet meadows, mires, fens and wet woodlands. It may be very common in valley mires. It is on the wing from late May to September, peaking in July.
Biology & Ecology:
As with Chrysops relictus, there is very little detailed information on the host range of Chrysops viduatus. It is known to feed on large mammals including cattle, horses and deer, but is also very ready to bite man if available.
Grayson (1997) reports that attacks are usually as noted for Chrysops caecutiens, but he had had them alight on various parts of his body from the knees upwards. He also reported seeing a couple of female Chrysops viduatus taking a blood-meal from a horse's neck.
Trapping & odour attractants
Some work has been done to test the effectiveness of different odour attractants for Chrysops viduatus. In Croatia Krčmar et al. (2006) showed that canopy traps baited with aged cow, horse, sheep, or pig urine caught more Chrysops viduatus (a total of 28) than did unbaited traps (0), but differences were not statistically significant. Since the control (unbaited) traps failed to catch any of that species, no index of increase could be calculated. Similarly 4-methylphenol (one of the active chemicals in cow urine) increased the catch of Chrysops viduatus from 0 to 18 but again the difference was not significant. (Krčmar, 2007).
Using cow urine/acetone - baited NG2F traps in southern England (Brightwell & Dransfield, 2014), we have only caught Chrysops viduatus in one site in wet grassland at Knepp Castle Estate in West Sussex.