Studies of fluctuations in insect populations VII The Button Top Midge (Rhabdophaga heterobia) at Syston, 1934-39

The following is largely the author's summary. This study of Rhabdophaga heierobia H. Lw., in a field of commercially grown willow (Salix triandra) in Leicestershire is a continuation of the third study in the series [R.A.E., A 23 153], which covered the years 1928-33, and contains the data for the years 1934-39 and a review of the results obtained during the whole period. The changes in the combined population of R. heterobia and its parasites are discussed. Birds, especially tits, are shown to play an important part in reducing the total population during the winter months [cf. 17 405]. A drop in 1930 was probably caused by a drought in the late summer of 1929, and the hot summers of 1933 and 1934, acting through plant growth, caused similar reductions in 1934 and 1935. Since 1936, the population has steadily fallen because the cultivation of the willows was discontinued in that year, and they have since been exterminated as a result of grazing. A positive correlation was found between the numbers of R. heterobia and of its parasites, but there is no correlation between the size of the combined population and the percentage parasitism. The percentage changes from year to year in the numbers of the combined population and in those of the parasites were almost identical. The absolute number of parasites did not appear to lag behind that of the host, but observations were made only on the third generation of the latter. Relative parasitism as high as 64 per cent. exerts no control on the host; the parasites do not become more efficient at high population levels than at low ones and so cannot have any balancing effect. In addition to the six parasites already recorded [23 154], a predacious Cecidomyiid, Lestodiplosis sp., was reared in small numbers. R. heterobia and its parasites emerge readily in response to increased warmth in March. Under normal conditions, the dates of the first appearance of R. heterobia varied from 13th April to 16th May, but the weeks of peak emergence showed less variation. The emergence of the parasites usually follows closely that of the host, but in some years the weather appears to act differentially on host and parasites as regards dates of emergence.