Selective predation by the backswimmer, Notonecta1

In laboratory experiments Notonecta hoffmani was size‐selective, although prey size alone did not explain its preference between prey species. When Notonecta was fed mosquito larvae, its preference first increased and then decreased with increasing prey size. Its consistent preference for the larger size class of a given species of zooplankton presumably reflects the fact that all zooplankton were small relative to the predator. Preference was determined mainly by differences in the relative attack rates on the different prey, but in two instances relative success rates of attacks were equally or more important. Although differences in the frequency of encounter influenced attack rates, the main determinant appeared to be the differences in the predator’s response to different prey once they were encountered. The results suggest that the behavior underlying Notonecta ’ s preferences is stereotyped and inflexible. The results help explain the effects of Notonecta on field populations of prey.