Population, biomass and secondary net production of above ground insects in a tropical grassland

Abstract

Population density, biomass and net secondary production of aboveground insects in a tropical grassland dominated by Desmostachya bipinnala were determined from 20 August to 3 December, 1975. Fifty one species of adult insect belonging to 23 families of 9 insect orders were recorded. The most dominant family was Acridide (both in terms of number and biomass). Out of the identified Orthoptera, Hieroglyphus banain is the most abundant species in terms of dencity while Gastrimargus transversus is the dominant species in terms of biomass. Of all the insects, Eurema hecabe is the most abundant species in terms of density. Herbivores constitute 84.37% of total population, predators 11.05%, 0.47% and omnivores 3.31%. The nymphs constituted 72.41% of the total orthopteran population. The density of grasshoppers ranged from 0.33 m^-2 (20 August) to 1.66 m^-2, (5 November), But the peak biomass was attained on 17 September (258.69 mg m^-2). Pieridae comes next to Acridide in density (peak o.42 m^-2) while Libellulideae follows Acrididae in biomass (peak 26.22 mg m^-2). The total density of insect population ranged from 1.50 m^-2 to 3.08 m^-2 and the biomass ranged from mg m^-2. The density and biomass showed a linear relationship in Coreidae and Pieridae, but a curvilinear relation when data for all insects were pooled. Four different methods were employed for estimating the net secondary production. The maximum estimate of production (724.51 mg m^-2) was obtained by trough-peak analysis at species level.