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Itasca, Lake (Minn.); Chironomus plumosus; Insect surveys; Insects--Development


During the period 15 June to 3 September 1965, 626 Ekman dredge hauls were made in Lake Itasca, Minnesota. The numbers and weights of Chironomus plumosus L. at 6 m, 7 m, 8 m, Siefert's Hole (9.5 - l 0.4 m), and Peace Pipe Vista (11.5 - 13.7 m) depression were determined. In addition, the numbers and weights of several other benthic dipteran larvae, Cryptochironomus, Palpomyia, and Procladius, normal associates of C. plumosus, were determined. A loss of 98,407 larvae/ha/day occurred in the 6-9 m stratum during the 12 weeks. There was a decrease in numbers of larvae of 79.3% and 84% at Siefert's Hole and Peace Pipe Vista depression, respectively. The summer decrease in biomass was 4.23 kg/ha/day in the 6-9 m stratum, 1.72 kg/ha/day in Siefert's Hole, and 0.98 kg/ha/day in the Peace Pipe Vista depression. The individual larvae were gaining weight during the 12 week period when the biomass was decreasing. The ANOVA of numbers of larvae during the last seven weeks of the sampling period showed significant effects of depth and time on numbers, but no significant interaction. Fisher's coefficient of dispersion was used to determine the distributional patterns of larvae al the various sampling depths; few collections varied significantly from random distribution, in several samples from Siefert's Hole and the Peace Pipe Vista depression the larvae showed a clumped distribution. The number of larvae surviving to 3 September were estimated to represent a potential pupal crop of 87.31 kg/ha in the 6-10.5 m stratum.

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