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MODELING THE IMPACT OF VARIABLE CLIMATIC FACTORS ON THE CROSSOVER OF CULEX RESTAUNS AND CULEX PIPIENS (DIPTERA: CULICIDAE), VECTORS OF WEST NILE VIRUS IN ILLINOIS

The aim of this study was to model the impact of temperature on the timing of the seasonal shift in relative proportion of Culex restuans Theobald and Culex pipiens L. in Illinois. The temporal pattern of West Nile virus (WNV) and St. Louis encephalitis virus transmission in the midwest exhibits a late summer to early fall peak in activity, which parallels the temporal increase in the abundance of Cx. pipiens. The daily number of egg rafts oviposited by each species has been monitored at multiple surveillance sites in Urbana-Champaign in central Illinois for more than 13 years. The time when the two Culex species are in equal abundance (crossover) varies considerably from year to year. Our investigation of several thermal measures indicated that this variation was related in large part to climatic conditions with warmer (cooler) temperatures correlated to earlier (later) crossover dates. Models based on degree days and the number of days in which the daily maximum temperature exceeded an upper temperature threshold explained more than 60% of the variance in crossover dates. In contrast, models based on the number of days in which the daily minimum temperature exceeded a lower temperature threshold explained no more than 52% of the variance. An evaluation of these models demonstrated that they provide relatively simple and accurate estimates of crossover date from daily temperature data, a necessary component for developing an overall climatic index for the risk of WNV transmission in Illinois.

Received January 23, 2005. Accepted for publication May 7, 2005.

Financial support: This work was supported by the National Oceanic and Atmospheric Administration (NOAA) under contract number EA133E-02-CN-0027, Centers for Disease Control and Prevention grant PHS U50/CCU52051, and the Illinois Waste Tire Fund, Illinois Department of Natural Resources.

Disclaimer: Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of NOAA or the Illinois State Water Survey.

^* Address correspondence to Robert J. Novak, Medical Entomology Program, Illinois Natural History Survey, Champaign, IL 61820. E-mail: rjnovak{at}uiuc.edu

Authors’ addresses: Kenneth E. Kunkel, Illinois State Water Survey, Champaign, IL 61820, E-mail: kunkel{at}hercules.sws.uiuc.edu. Robert J. Novak, Richard L. Lampman, and Weidong Gu, Medical Entomology Program, Illinois Natural History Survey, Champaign, IL 61820, E-mails: rjnovak{at}uiuc.edu, rlampman{at}inhs.uiuc.edu, and wgu{at}inhs.uiuc.edu.