Effect of temperature on the development of Culiseta melanura (Diptera: Culicidae) and its impact on the amplification of eastern equine encephalomyelitis virus in birds.

Eastern equine encephalomyelitis virus (EEE) is perpetuated in a maintenance cycle that involves Culiseta melanura (Coquillett) as the enzootic vector and passerine birds as the amplifying hosts. Amplification of virus in any given year requires an influx of nulliparous Cs. melanura in the presence of susceptible avian hosts. We conducted laboratory experiments at constant temperatures from 10 to 34 degrees C to develop thermal heat summation models to predict emergence in nature. Embryonic development progresses slowly at 10 degrees C, and the time to eclosion decreased significantly as temperatures increased to 28 degrees C. High temperature were lethal and eggs failed to hatch at 32 degrees C. The thermal minimum (t0) for embryonic development was 9.38 degrees C, and 38.46 degrees-days (DD) were required for egg hatch. The time for larval development decreased with increasing temperatures. Cs. melanura larvae develop in subterranean habitats (crypts) where water temperatures remain below 20 degrees C throughout the summer. Under controlled conditions, egg hatch to emergence took 8 mo at 10 degrees C, 3 mo at 16 degrees C, and 1 mo at 22 degrees C. The thermal minimum for larval development (t0) was 8.5 degrees C, and 467.29 DD were required from eclosion to adult emergence. Our findings indicate that Cs. melanura is well suited to develop in cold water crypts where the larvae are collected most frequently. The mosquito appears to be bivoltine in the northeast with an overwintering generation of larvae that emerges as a spring brood of adults and a summer generation of larvae that emerges in fall. Higher than normal water temperatures hasten development of the summer generation and increase the probability for amplification of EEE by bringing large numbers of nulliparous mosquitoes into contact with recrudescing virus for subsequent transfer to the growing population of susceptible juveniles. Water temperatures in the crypts also may regulate the northern limit for virus amplification each year.