Diapause Termination, Gonoactivity, and Differentiation of Host-Seeking Behavior from Blood-Feeding Behavior in Hibernating Culex Tarsalis (Diptera: Culicidae)

Diapausing Culex tarsalis were collected from mine tunnels in Boulder Co., Colorado, during late September and early October 1980 and studied in the laboratory. Exposure to a long photophase (L:D 15:9) resulted in diapause termination in essentially all mosquitoes by day 7 at 25 °C. Warming alone (at 25 °C) for up to 8 days did not terminate diapause in a significant proportion of females maintained at typical autumn day lengths. Diapause was terminated in Cx. tarsalis by topical application of methoprene, a juvenile hormone (JH) mimic, suggesting that the physiological pathway for diapause termination involves secretion of JH by a reactivated corpus allatum. Host seeking and blood feeding were shown to represent discrete stages in the biting cycle of Cx. tarsalis, and these were differentiated in diapausing females by varying the size of the feeding chamber. Diapausing females could be induced to take a blood meal only when the host-seeking step in the biting cycle was bypassed by placing the females in close proximity to a host. Gonotrophic dissociation occurred in a significant proportion of females only when 3 conditions were met: females had to be in a state of diapause when fed, and fed females had to be incubated at a cool temperature (15 °C) and under short-day conditions during the time of blood meal digestion. The demonstration that gonotrophic dissociation can occur under experimental conditions probably has no relation to what occurs in nature. Warm Indian-summer conditions notwithstanding, typically short autumn day lengths are sufficient to suppress host-seeking behavior in diapausing populations and host seeking is a requisite for blood feeding in nature.