Monthly time series of mosquito abundance (Tarsalis females, normalized counts [mosquitos/trap day] were assembled from several mosquito traps that have been maintained for several years in Kern County in the southern San Joaquin Valley. There are several mosquito species and both male and female genders present in these data sets, but we have chosen Tarsalis because of its involvement in the encephalitis life cycle.

Chris Barker shared with us Microsoft Excel spreadsheets of mosquito data for 1973-2000 at various sites in Kern County. We began by focusing on the species tarsalis (female) since that species/gender was in greatest abundance throught the period of record. We plotted the period of record for the 54 available stations. We also computed the trend of the data and plotted the trend line and detrended mosquito counts for the 54 stations. The images below show a few sample plots.

The data set considered here covers the period 1973-2000. 28 of the 54 mosquito trap records had 10 or more years of data, so we initally focus on these in testing for possible links to climate variables. Inspection of the Tarsalis abundance series (see Figs above) reveals large interannual variability and in many cases, a remarkable trend over the duration of the sampling period. Most of these trends show decreasing abundances with time (see Fig below), perhaps because environmental conditions and light levels at the trap sites has changed. Because of this, the linear trend was removed to yield detrended residual anomalies (see Figs above), from which the initial set of climate analyses was conducted.

From the 54 stations, we chose 28 stations that had a period of record greater than 10 years for further analysis. Below are maps of Kern County (Kern River enters plots from the west) showing the total trend for the 28 select stations for the female tarsalis for the seasons AMJ and JASO.

The analysis with climate variables has only recently begun, but we show here a series of correlations of Tarsalis abundance correlated with the Palmer Drought Severity Index (PDSI), air temperature, and precipitation. Each of the climate measures is from the National Climate Data Center "climate division" average monthly data, in this case for the San Joaquin Drainage division, which covers the Kern County region. Previous studies have shown that divisional data provides a reasonable index of the local short period climate variability. For these initial analyses, the divisional temperature and precipitation data was aggregated into seasonal (DJF, MAM, etc) averages, while the Tarsalis data was aggregated into early (AMJ) and late (JASO) season average abundances. Since PDSI is a weighted aggregate of previous months' precipitation and to a lesser degree previous months' temperature, it was not necessary to form seasonal averages of it; instead we chose January and April PDSI. The PDSI and precipitation correlations generally proved to be largest, while temperature correlations were relatively weak (see Figs below). Early season (AMJ) Tarsalis abundance correlated with PDSI and precipitation at a higher level than did JASO Tarsalis abundances. Both winter (January or DJF) and spring (April or MAM) PDSI and precipitation exhibited relatively high correlations with Tarsalis abundances, especially with those in the early season (AMJ). In general moister conditions (positive PDSI and positive precipitation anomalies) associate with higher mosquito abundances and vice versa for drier conditions, as indicated by the positive correlation coefficients. For temperature (see Fig below), higher temperatures in winter had some tendency to associate with higher AMJ Tarsalis abundances.

Using the 28 select stations, we also correlated the female tarsalis detrended counts with various climate parameters (Palmer Drought Severity Index, Surface Air Temperature, Total Precipitation). Maps of the correlations coefficients for these are shown below.

Further work is planned in which other regional variables (streamflow, soil moisture) and broader scale climate measures (atmospheric circulation and sea surface temperature) will be related to will be tested for links to Tarsalis abundances. Other mosquito species will also be examined. Pedictive capabilities will be examined. More mosquito trap data will eventually be assembled, so these will also be examined.