Some Demographic Consequences of Food Shortage for the Predator, Notonecta hoffmanni

Abstract

The freshwater hemipteran predator, Notonecta hoffmanni, has one generation per year. Eggs are first laid in late winter and begin hatching in April; most nymphs hatch in mid—June. A short period of heavy mortality of instar I nymphs in eary summer is followed by a more gradual decline in the numbers of older individuals until autumn. The mortality of young nymphs shifts the age—distribution towards older individuals early in summer and increases the changes for the first nymphs hatched in spring to survive and breed successfully. Three alternative hypotheses about the causes of nymphal mortality are proposed and I argue that the primary cause is a reduction in food availability resulting in increased cannibalism. Reduction in spatial refuges also influences cannibalism rates. Field and laboratory experiments show that delayed reproduction by young adults is related to food availability. I suggest that a second generation of N. hoffmanni is possible if sufficient food were available when young adults mature in August.