RESUMEN
BACKGROUND & OBJECTIVES: The predatory behaviour with reference to the frequency-dependent prey-selection of the water bugs Sphaerodema annulatum Fabricius and S. rusticum Fabricius was studied in the laboratory using the IV instar larvae and pupae of Armigeres subalbatus as prey to ascertain their efficacy as predator of mosquito immatures. METHODS: Field collected adult morphs of the water bugs were allowed to predate on larvae and pupae provided in different ratios and densities as per the model of Greenwood and Elton' for a fixed time period. The data obtained on their predation rate were analysed with respect to the model parameters, lnV--the frequency independent component and b--the frequency dependent component of selection. RESULTS: It was found that the prey-selection was dependent on the relative numbers of prey available, favouring apostatic selection. The b values and lnV values for S. annulatum were 0.54 +/- 0.01 and 0.92 +/- 1.04 respectively whereas the corresponding values for S. rusticum were 0.71 +/- 0.03 and 0.17 +/- 1.57 respectively. INTERPRETATION & CONCLUSION: The selection of preys by the water bugs was dependent on the relative number of the prey forms and thus they are expected to predate on the form more abundant in a heterogeneous prey population and adversely affect the adult emergence.