Effect of Notonectids (Hemiptera: Notonectidae) on Mosquitoes (Diptera: Culicidae): Predation or Selective Oviposition?

The density of the aquatic, predatory bugs Notonecta hoffmani and N. kirbyi was manipulated in stock troughs in the Santa Ynez Valley, Calif., to assess the predator's effect on mosquito larvae. Each trough was divided in half by a wooden partition, and notonectids were removed from one half and retained in the other. Over a 3-month sampling period very few large mosquito larvae or mosquito pupae were found on the side of the trough where notonectids were present, whereas large densities were observed on the side where notonectids were absent. To ensure that this was not an artifact of the side of the trough chosen for notonectid addition or removal, the predators were changed from one to the other. The same result was obtained. Although it is well known that notonectids are voracious predators of mosquito larvae, the experimental results can be adequately explained by selective mosquito oviposition. This hypothesis was supported by laboratory experiments in which adult mosquitoes laid the fewest eggs in tubs containing notonectids. Laboratory and field experiments also demonstrated that notonectids may disrupt mosquito egg rafts, but no evidence of a reduction in subsequent hatching success was obtained. These results point out the danger of making conclusions about predation merely by observing predator and prey distributions. They also draw attention to the need for more information about the effect of predators on prey behavior, particularly if a predator is being considered as a biological control agent.