Fluctuations of diptera larvae in soil from a spruce forest in relation to climatic factors

Abstract

Diptera larvae were extracted by the wash and flotation method according to Healey and Russell-Smith (1970) as modified by Altmuller (1979). The larvae had been kept in alcohol for at least 2 months. After this time their specific gravity changed, and an additional flotation with MgSO4-water solution of SG = 1.26 g/cm3 was very effective. This new method is described. The working procedure can be reduced to about 50%–70% of the time Altmuller (1979) and Hovemeyer (1984) needed. Many tests have shown that some small sciarid larvae settle out with the needle and humus particles in solution of SG = 1.26 and cannot be found, so that it is necessary to apply a correction factor. Samples taken each August from 1979 to 1984 were extracted and floated off by the methods described. Larvae from seven families of Diptera nematocera and from six families of Diptera brachycera were found. Numbers of sciarid larvae varied between 2500 (1979) and 100/m2 (1981), of the cecidomyid larvae between 6700 (1983) and 400/m2 (1980) and of the brachyceran larvae between 650 (1979) and 50/m2 (1984). The correlation coefficient (= r) of larval population size with temperature and precipitation, respectively, was calculated and hence the coefficient of determination as a percentage (r2 x 100), which represents the proportion of the fluctuations in population size that is accounted for by weather factors. All families were influenced negatively by the temperature and positively by the precipitation in August, i.e., the higher the precipitation and the lower the temperature in August the higher the larval abundance in August. The development of the larvae begins in the previous year, and the influence of monthly climatic factors during the whole period of development was tested. It was shown that: Precipitation is correlated with the abundance of (1) sciarid larvae in June and August of the previous year (positively); (2) brachyceran larvae in June (August) of the previous year and in (February), March, April and August of the same year (positively); and (3) cecidomyid larvae in July of the same year (negatively). Temperature is correlated with the abundance of (1) sciarid larvae in August of the previous year (negatively); (2) brachyceran larvae in August of the previous year (negatively); and (3) cecidomyid larvae in September of the previous year, in January and July of the same year (positively). Monthly data for precipitation and temperature, taken together, showed maximum positive correlations with abundance as follows: (1) for sciarid larvae, with data for August of the previous year; (2) for brachyceran larvae, with data for August of the previous year; (3) for cecidomyid larvae, with data for July of the same year.