The functional response of two aquatic predatory insects and its implications for the biological control of Coptodon rendalli fry

Abstract

Precocious breeding is a major challenge in the culturing of Coptodon rendalli. Laboratory experiments were undertaken to evaluate the biological control potential of two predatory aquatic insects, Anisops debilis and Gerris swakopensis against tilapia fry. The functional response of these insects was determined at different prey densities during a 24 hr experiment. A. debilis and G. swakopensis exhibited type III and type II functional responses, respectively. The attack rate (2.2046) was higher in A. debilis than G. swakopensis (0.00239). Handling time was lower in A. debilis (0.5420) than G. swakopensis (1.6153). Another experiment was done at different prey densities under light and dark conditions. Anisops debilis exhibited a type II functional response under light conditions and a type III functional response under dark conditions. The attack rate of A. debilis declined from 2.5950 under light conditions to 1.2372 under dark conditions. The handling time for A. debilis was lower under dark conditions than under light conditions. G. swakopensis maintained a type II functional response under light and dark conditions. The attack rate for G. swakopensis increased under dark conditions while the handling time declined. The relative control potential (RCP) was also calculated. The RCPs for the 24 hrs experiment, light and dark conditions were 4.7941, 5.4431 and 3.5622, respectively. This probably indicates that A. debilis may be effective in controlling tilapia fry.