Studies of Fluctuations in Insect Populations

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Abstract
Including earlier published reports, fluctuations in the population of this gall midge on Salix triandra and in its parasites are analyzed for 1928-39. Birds greatly reduced the total population over winter. A drop in population in 1930 was probably caused by a drought in the late summer of 1929. The hot summers of 1933 and 1934 also caused reductions in population the following years through effect on plant growth. After 1936 the population became steadily reduced as the willow became extirpated through grazing. The number of parasites varied directly with the population of midges and did not exert any greater influence when the host population was high. Relative parasitism as high as 64% exerted no control over the host population. First emergence of the midge in the spring varied from Apr. 13 to May 16, with warmer weather hastening the time of emergence. The parasities appeared soon after the host but changes in weather exerted some differential effect on the two.
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