Effects of Circadian Rhythms and Other Bioassay Factors on Cat Flea (Pulicidae: Siphonaptera) Susceptibility to Insecticides

Variability in insecticide susceptibilities during nylon-disk bioassays of adult cat fleas (Ctenocephalides felis) due to the time of day the bioassay was conducted, illumination, density of insects, sex ratio, and${\rm CO}_{2}$exposure was investigated. Also examined were illumination effects on flea circadian rhythms, as recorded by time-lapse video under 4 lighting conditions (12:12 L:D, 12:12 L:D with photophase at night, continuous light, or continuous red light simulating darkness), and respiration in bioassay tubes.The time of day when the bioassay was started (00:30, 7:00, 12:30, 18:30), dark or light exposure, density (5, 10, 30 fleas/bioassay tube), and sex of the flea had no effect on mortality. After 24 hr, fleas that had been immobilized with${\rm CO}_{2}$during sexing were more susceptible than unsexed fleas not exposed to${\rm CO}_{2}$. At 4 and 16 hr, there was no difference. Unsexed fleas immobilized with${\rm CO}_{2}$had intermediate susceptibility.Fleas showed 24-hr circadian rhythms. When under 12:12 L:D and continuous red light, peak activity was at 21:00 while respiration peaked at 18:00. Activity shifted 6 hr earlier under 12:12 L:D reverse photoperiod. Under continuous light or dark, endogenous rhythms occurred. Respiration of fleas in darkness was usually lower than in light. Though activity of fleas varied, insecticide susceptibility was not affected by these conditions. Other factors must contribute to high variability in cat flea susceptibility.