GROWTH, ABUNDANCE AND DISTRIBUTION OF LARVAL TABANIDS IN EXPERIMENTALLY FERTILIZED PLOTS ON A MASSACHUSETTS SALT-MARSH

Tabanus nigrovittatus and Chrysops fuliginosus, 2 of the 4 spp. of tabanid larvae [the 2 remaining species are T. lineola and C. atlanticus] found on Great Sippewissett Marsh [USA] comprised approximately 90% of the total larval tabanid population and were both predominantly found in areas where the short form of Spartina alterniflora was the dominant plant. The average larval densities for T. nigrovittatus and C. fuliginosus found in the control plots of 1972-3 and the 1974 plots are comparable to the maximum values for T. nigrovittatus obtained elsewhere on the E coast of North America. Larval tabanid populations were drastically reduced in plots treated with a sewage sludge fertilizer but were unaffected by treatments with a phosphate and a urea fertilizer. Analysis of monthly larval size classes for the 1972-3 sampling period indicates that the larval life of T. nigrovittatus probably lasts 2 yr. The summer growth rate is about 4 mm/mo. T. nigrovittatus feeds on most soft-bodied insect larvae found on the marsh and also on the amphipod Orchestia grillus. Larval densities were reduced in laboratory experiments from 80-240/m2 to 40-120/m2 through cannibalism, the lower figure approximating the densities of larval tabanids found in the field.