Natural control of Culex quinquefasciatus larvae in residential ditches bythe copepod Macrocyclops albidus

Natural populations of three larvivorous copepod species live in residentia l roadside ditches in Louisiana: Macrocyclops albidus, Acanthocyclops verna lis, and Megacyclops latipes. Macrocyclops is most common and killed an ave rage of 27 First-instar Culex quinquefasciatus larvae/copepod/day in the la boratory. Although severe pollution From septic tank effluent in some parts of the ditches creates havens for Cx. quinquefasciatus production by exclu ding predatory copepods and fish (Gambusia affinis), Macrocyclops and the f ish substantially reduce Cx, quinquefasciatus larval survival when present where pollution is not so severe. At natural abundance, Macrocyclops reduce d the survival of Cx. quinquefasciatus larvae (during their first four days ) to 2.6%, compared with 46% survival in controls without Macrocyclops. Dur ing one year of field observation, Macrocyclops was common in the spring bu t disappeared during the summer when fish (which prey on copepods) appeared in many ditches, reduced water flows led to more severe pollution, and wat er temperatures in very shallow water were sometimes higher than Macrocyclo ps could survive. Macrocyclops reappeared in many ditches during autumn and winter, when water temperatures and pollution declined and fish disappeare d. Introduction of Macrocyclops to ditches in October accelerated its reapp earance during autumn and winter and reduced the number of sites with Cs. q uinquefasciatus larvae to one-quarter the number in control ditches. The mo st effective way to control Cx. quinquefasciatus is to eliminate pollution so predators like fish and copepods can live throughout the ditches, but ti mely introduction of fish and copepods could also contribute to control. Mo re experience will be necessary to ascertain whether copepod introductions are cost effective.