meEntomologist - 110 (l), 33-36 (1991)

The ubiquitous occurence of posticatum (Diptera) in rivers around Oxford

TREVOR WILLIAMS, Institute of Virology and Environmental Microbiology, Mansfield Road, Oxford. OX1 3SR

Key words: Simuliumposticatum, Oxon, distribution, abundance.

Introduction Blackfly (Simuliidae) are haematophagous Diptera. The larval and pupal stages attach to plants or stones and filter feed detritus, usually in fast-flowing rivers and streams. Simuliumposticatum Meigen 1838 (syn. S. austeni Edwards 1915) occurs in Britain and is commonly known as "The Blandford ". The bite of this fly causes a severe reaction in some people and is of undoubted public health importance around the town of Blandford on the banks of the River Stour, . In Britain, S. posticatum is a univoltine species. Eggs are laid from July onwards in the soil of riverbanks and hatch in the following February and March when water levels have risen substantially (Ladle, Bass and Cannicott, 1985). Larvae attach to aquatic plants and go through at least six instars before pupating in late April. Adults emerge mid-May to June (Hansford, 1978). It is in the Stour and associated tributaries that the abundance of S. posticatum is believed to be greatest. This species has recently attracted attention for the navel control measures which have been used against populations of the around Blandford (see for example: Gdan,9th March 1990). Indeed, the first use of the microbial , Bacillus thuringiensis var. ismelensis in water courses in Britain was a trial on a tributary of the Stour against S. posticatum. A significant reduction in the number of S. posticatwn larvae was achieved with little detrimental effect on non-target organisms downstream of the treatment sites (Jon Bass, pers. comm.). The distribution of many blackfly species is poorly described. Lack of geographically diverse records means that the maps published by the Freshwater Biological Association (Davies, 1968) can be misleading for all but the most common species (Crosskey, 1982). This note describes the distribution and relative abundance of simuliid species in the Oxford area, with particular reference to S. posticatum.

Sampling Weed samples were taken between 5 - 26th April 1990 from 10 rivers, at a total of 21 sites within a 20 mile radius of Oxford. The table below gives the sites visited along with the number of S. posticatum taken and other species identified in the sample. Map references shown in the table relate to 0.s. map number 164. The density of S. posticatum pupae and large larvae (only later larval instars can positively be identified) is expressed as numbers per gram wet weight ofvegetation. The WIlhelmia and Ornatum groups are complexes each containing three species, Trevor Williams fue larvae of which cannot reliably be separateduntil they pupate. Therefore, where members of these groups are identified to species it is because fue pupae were presento

Results and Discussion Blackfly larvae were found at 18 ofthe 21 sites sampled. S. posticatum was present at all18 of these gires, the numerically dominant species at 16 gires, and was more abundant than all other simuliid speciespul together at 14sites. Clearly, for this springtime period, S.posticatum must playa significant role in the ecology ofmany ofthe lowland rivers examined. The samples were of a qualitative nature and should not be considered in relation to absolute Simuliid population densities published elsewhere (Colbo, 1987). The fact that densities of larvae are presented as number per gram of wet vegetation means that they must be &lultip1ied by a factor of approximately17 (Ranunculus spp. contain approximately 94 % water by weight) to compare with Simuliid densities per gram dry weight of plant material published by Ladle, Bass and Jenkins (1972)for example. Ifthis is done, ! densities of as much as 425g-t dry wt. at Shipton-on-Cherwell are probably realistic estimates. Local pupal densities as high as 322g-1dry wt. on filaments of Ranunculus sp. were algOrecorded. These compare with peaks of 55 and 18üg-t dry wt. recorded by Hansford (1978)for S. posticatum pupae in the Blandford area in 1973and 1974respectively. His methods were of a similar qualitative nature to my own. The afea of freshwater around Blandford, however, may actually be capable of supporting a far larger population of S. posticatum than found at the small, local gireson the River Cherwell.

The distribution of simuliid irnmatures is dictated by fue presence of suitable gires. These must be fairly fast flowing with well oxygenated, unpolluted water, such as found in upland streams, in fue shallow riffle arcas of clean rivers, or by weirs. (The three gireswhich yielded no simuliids were deficient in one ofthese factors.) The population distribution ofl;lrvae and pupae in lowland rivers is therefore highly contagious and such contagion can result in locally high population densities. Por giressuch as Shipton-on-Cherwell, there must come a point when intraspecific competition for space on aquatic plants will be as important a factor limiting local abundance as fue availability offood, disease, or predation by fish. Future work will focus on development of a quantitative sampling method which can be used to relate weekly counts of larvae on artificial substrates to the true size of fue larval/pupal population presentin fue rivers. These observationsdo, however,give an insight into fue ubiquity of S.posticatum in fue rivers ofOxfordshire, including fue Thames. As such waterwaysbecome cleaner and given fue provision of fast flowing sections by weirs etc. , it may prove interesting to investigate fue distribution and abundance of this potentially important species where high densities of humans and coincide.

Acknowledgement 1am grateful toJon Bass ofthe F.B.A. at Monk's Wood, for bis time and help explaining Simullid identification. '1A S.posticatum in rivers aroundOxford

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't~ Trevor Williams

Surnmary The distribution of Simulium posticatum larvae in 10 rivers around Oxford was investigated in April, 1990.Samples of aquatic vegetation were taken and all blackfly stagespresent were identified. S. posticatum was detected at 18ofthe 21 sites sampled and was usually dominant over all other blackfly species during this springtime periodo

References

Colbo, M.H. (1987) Problems in estirnating blackt1y populations in their aquatic stages. In: Blackjlies: ecology, populalion managementand annotated ~rld list (Edite

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