Natural control of Culex quinquefasciatus larvae in residential ditches by the copepod Macrocyclops albidus.

Natural populations of three larvivorous copepod species live in residential roadside ditches in Louisiana: Macrocyclops albidus, Acanthocyclops vernalis, and Megacyclops latipes. Macrocyclops is most common and killed an average of 27 first-instar Culex quinquefasciatus larvae/copepod/day in the laboratory. Although severe pollution from septic tank effluent in some parts of the ditches creates havens for Cx. quinquefasciatus production by excluding predatory copepods and fish (Gambusia affinis), Macrocyclops and the fish substantially reduce Cx. quinquefasciatus larval survival when present where pollution is not so severe. At natural abundance, Macrocyclops reduced the survival of Cx. quinquefasciatus larvae (during their first four days) to 2.6%, compared with 46% survival in controls without Macrocyclops. During one year of field observation, Macrocyclops was common in the spring but disappeared during the summer when fish (which prey on copepods) appeared in many ditches, reduced water flows led to more severe pollution, and water temperatures in very shallow water were sometimes higher than Macrocyclops could survive. Macrocyclops reappeared in many ditches during autumn and winter, when water temperatures and pollution declined and fish disappeared. Introduction of Macrocyclops to ditches in October accelerated its reappearance during autumn and winter and reduced the number of sites with Cx. quinquefasciatus larvae to one-quarter the number in control ditches. The most effective way to control Cx. quinquefasciatus is to eliminate pollution so predators like fish and copepods can live throughout the ditches, but timely introduction of fish and copepods could also contribute to control. More experience will be necessary to ascertain whether copepod introductions are cost effective.