Limnol. Oceanogr., 44(2), 1999, 454–458

The phantom midge, Chaoborus, is a dipteran, the larvae of which occur in many freshwater habitats and are known to be vulnerable to predation by fish. The hypothesis was tested that ovipositing imagines of Chaoborus use chemical cues to detect predatory fish and avoid depositing eggs into waterbodies containing fish kairomones. The experimental setup consisted of cages with two small water-filled plastic containers, in which Chaoborus imagines could oviposit. One container was connected to a flow-through system holding fish kairomones, while the other container (the control) was connected to a flow-through system without fish kairomones. Three different Chaoborus species were tested: two ponddwelling species, Chaoborus crystallinus and C. obscuripes, which do not coexist with fish, and a lake-dwelling species, C. flavicans, which does coexist with fish. The imagines of the two pond-dwelling species showed an ovipositing preference for fish-free water. No significant difference in ovipositing preference was found for the lake-dwelling species (C. flavicans). The present experiment suggests that in addition to direct predatory effects, adult oviposition behavior may contribute to the lack of local coexistence between fish and ponddwelling Chaoborus. Predation is often thought to be limited to direct, lethal effects of predators, although more recent data suggest that indirect effects can be equally important. Indirect effects include trophic linkage effects and those caused by behavioral and chemical interactions (Kerfoot and Sih 1987). Many indirect effects in freshwater communities are mediated by chemical communication, and a large number of organisms have been shown to respond in a variety of ways to chemicals or kairomones released by their predators (Havel 1987; Dodson 1989; Adler and Harvell 1990; Larsson and Dodson 1993). Many aquatic insects have terrestrial adult stages that must return to the water to oviposit, and the selection of oviposition sites can be an important feature in determing freshwater community structure. The presence of fish could indirectly structure these communities by influencing adult choice of oviposition sites. Therefore, selection should favor adults with oviposition behavior that yields the highest juvenile survival. This particular research topic has largely been ignored by aquatic ecologists (but see Resetarits and Wilbur 1989; Kats and Sih 1992).