Emergence and lateral dispersal of adult Plecoptera and Trichoptera from Broadstone Stream, UK

1. Emergence and inland dispersal of adult stoneflies (Plecoptera) and cadd isflies (Trichoptera) from Broadstone Stream, an acidic and iron-rich strea m in southern England, were studied over 10 months in 1996-1997. Fifteen py ramidal emergence traps were placed randomly in a 200-m stretch. Three Mala ise traps were placed above the stream and six more on each side (one woode d, one open) along a transect at distances of 1, 15, 30, 45, 60 and 75 m fr om the channel. 2. More than 16 000 stoneflies, belonging to 11 species, and just under 400 caddisflies (22 species) were caught. Four dominant stoneflies (Leuctra fu sca, Leuctra nigra, Leuctra hippopus and Nemurella pictetii) accounted for 96% and 95% of the catches in the emergence and Malaise traps, respectively . Two caddisflies (Plectrocnemia conspersa and Potamophylax cingulatus) acc ounted for 63% of the catch in the Malaise traps. Few caddisflies were take n in emergence traps. 3. The emergence periods of L. fusca, L. nigra and L. hippopus were well-de fined and unimodal, whereas that of N. pictetii was prolonged and erratic. Overall, more females (1285) emerged than males (740). 4. Female stoneflies and caddisflies were in the majority in the Malaise tr aps above the stream. On land, significantly more females than males of L. fusca, L. nigra and P. cingulatus were caught. The sex ratio of the remaini ng species did not deviate significantly from 1:1. 5. The three Malaise traps placed above the stream caught most of the stone flies though there was also dispersal away from the channel, the numbers ca ught declining with distance. Exponential models explained between 67% and 99% of the variation in numbers of individuals with distance from the chann el in the four common stoneflies. Half the individuals went less than 11-16 m from the stream, while 90% travelled less than 51 m. Significantly more L. nigra and N. pictetii were caught in the woodland than on the open side, whereas L. hippopus showed no overall preference for either side.