CHARACTERIZATION OF FACTORS MEDIATING OVIPOSITION SITE CHOICE BY CULEX TARSALIS

Fermented infusions of organic matter were tested for their effects on Culex tarsalis oviposition. Bermuda grass infusion and polluted water collected from a natural oviposition site (La Brea tar pits, CA) enha nced oviposition rates, but an alfalfa infusion and water from a 2nd n atural oviposition site (Prado Basin, CA) did not. Bermuda grass infus ion was fractionated by dialysis and filter sterilization. Crude Bermu da grass infusion, and fractions of the infusion containing large mole cules (> 12,000 daltons), particulates, and microorganisms significant ly increased oviposition rates compared to distilled water controls. T he fraction containing small molecules was no better than a distilled water control, suggesting that small molecules are not involved in ovi position stimulation in this species. However, using the egg raft coun ting bioassay, the possibility that the small molecules fraction conta ined oviposition attractants could not be ruled out. Overall, our expe riments suggest that results obtained with the egg raft counting bioas say, which has been used frequently to screen for oviposition attracta nts, should be interpreted with caution. High oviposition rates in thi s bioassay may be due to responses to factors such as nonvolatile, con tact oviposition stimulants rather than to volatile attractants.