Intrageneric Predation by Notonecta (Hemiptera: Notonectidae) in the Laboratory and in Nature

Abstract

When five New England species of Notonecta were offered amphipods, corixid nymphs, mosquito larvae, and notonectid nymphs as prey, all five species commonly preyed on smaller congeneric nymphs, even when alternate prey were available. Notonecta apparently cannot distinguish among species of congeneric prey, attacking nymphs of their own species at the same rate as those of another species. Preference for first-instar congeneric prey was like that for 4-5-mm mosquito larvae and corixid nymphs, but preferences for third-instar (or older) congeneric prey became increasingly less than that for mosquitoes and corixids. The predation rate on congeneric prey in laboratory trials depended on the relative sizes of the interacting notonectids; the larger the difference in their sizes, the more likely that intrageneric predation would occur. Intrageneric predation rates in pond enclosures, however, were generally independent of the relative sizes of the interacting notonectids. Apparently the relationship between prey size and predator-prey encounter rates was different in the two environments. Nevertheless, estimated daily rates of intrageneric predation by Notonecta undulata Say and N. irrorata Uhler in the field were high enough to have a significant effect on the survival of young congenerics where these species breed, and possibly to prevent smaller species from establishing populations at such sites. N. insulata Kirby, a species which spends more time foraging below the surface and deeper in the water column than other species, produced the lowest rates of intrageneric predation in both the laboratory and the field.