Seasonal replacement of Culex restuans by Culex pipiens in Northwest Ohio: A case study in habitat partitioning between mosquitoes

Background/Question/Methods

Two closely related species of Culex mosquitoes exist throughout much of North America. Culex restuans is a native species whereas C. pipiens is European and apparently arrived centuries ago. Earlier studies have shown that larvae of C. restuans are numerically dominant in spring and early summer but that C. pipiens becomes dominant by mid summer. The timing of this numerical transition is the “crossover” date. Earlier studies have shown that crossover date was determined by temperature regimes, but no studies have focuses on adult mosquitos or explored possible interactions among populations for signs of competition. These were the goals of the present study. Adult mosquitoes of both species were captured in 14 New Jersey Light Traps by the Lucas County Sanitary District, Toledo, Ohio, between May and October, 1980-2011. The ratio of C. restuansto the sum of both species was calculated for each day (pooling traps) for each year and a linear regression of this ratio over Julian day estimated the crossover day for each year (i.e., day at which this ratio = 0.5).

Results/Conclusions

The mean crossover date was 201±28, similar to a previously reported value for larvae in Central Illinois (219). The total cumulative degree-days (above 0 C), total cumulative precipitation, and total number of mosquitoes of each species were calculated for each month of each year. Pearson correlations examined relationships among crossover date, species abundances and climatic factors by month (all N=31). Crossover date was correlated only to cumulative C. restuans abundances in May, June and July (r=0.569, 0.588 and 0.610, respectively), and no climatic factor. Abundances of C. restuans were not related to any factor in May, but were correlated to seasonal precipitation (r=0.382 and 0.325) and annual degreedays (r= -0.313 and -0.367) in both June and July. Abundances of C. pipiens were correlated with cumulative annual precipitation in May, June and July (r=0.0.465, 0.453 and 0.310), as well as C. restuans in June and July (r=0.964 and 0.960). These results show no indication of negative relationships between the two species, both of which seem to respond to somewhat different environmental cues: C. restuans negatively affected by mid-season temperatures and C. pipiens positively affected by precipitation. Crossover date appears to be determined primarily by the overall abundance of C. restuans as higher numbers simply take longer for C. pipiens to match.