West Nile – Light Penetration Study | Brian Altonen, MPH, MS

West Nile – Light Penetration Study

WEST NILE SURVEILLANCE (continued)

Subcanopy Lighting as an indicator of risk areas and vector-related ecological behavior

The variety of mosquito species in the county provided an opportunity to summarize their ecological behaviors. During a period of four and half years of trapping, species counts and location were kept in a database for ad hoc analyses to be developed once the need or opportunity for such a review came to bear. The fact that the county lacked a nidus or nest where west nile could be a result of the local ecology suggested to most working in this field that west nile could not be easily borne by the local ecological settings. To many it seemed more likely that the positive testing species (host and vector) being captured in this region were very much a consequence of crow migration behaviors. Yet, the long history of mosquito behavior (ecology) in this region suggested to those familiar with these local natural settings that west nile could become as much an issue locally in disease ecology as the lyme disease was just a few years before.

To document what species were trapped in the county, all of the data was simply reviewed and summarized to produce the following bar chart demonstrating the diversity of mosquito species found in the region, at least according to the preliminary notes kept by earlier field workers (some of these species may in fact be erroneously identified). A number of the species seemed uncommon and unusual at first, but when an ecological study was performed on all trap sites using a standardized landuse/plant ecology form to keep ecological notes, and a standardized form for describing each trap site, it became clear that the most important feature in understanding vector ecology and place related to the types of plants in the immediate region of where they rest, mate, lay eggs, and find their blood source.

For this reason a photometer was employed to study all of the trap sites. Measuring the light penetration of the canopy, we were able to related specific species with specific canopy light penetration defined trap locations. This penetration, measured in microjoules/per sec., was used to determine where future traps would be placed, and the lighting requirements for specific species expected to be potential vectors as a natural part of the local ecosystems.

Steps taken in the analysis of trapsite canopy light penetration data

There were 3 essential steps to documenting the data needed for analyzing species presence and activity in relation to subcanopy light penetration. First, a form was developed onto which all data known about a particular site could be documented and these notes extended when necessary. Second, another form was developed to detail the phytoecologic features of the trap site by identifying the primary plants growing in the immediate region, at least down to the genus level, but usually down to a species level. Third, two or more sets of light penetration readings were taken for each of the sites; each reading was concluded with a lateral reading taken to document ambient light value, this reading was often taken close to ground level adjacent to a protruding rock, cut tree truck, or log lying on the ground.

In some cases, light readings had to be taken beneath a canopy and in partial and direct sunlight close to the trap. This was to get a better idea on the effects of scattered light on the quality of the light readings immediately adjacent to the trap itself. On occasion, light readings were repeated due to weather conditions on the day of the initial readings, but often readings were taken on similar days and repeated in order to determine the best values to use in the final analyses.

Trapsite areas often varied considerably in size, both lengthwise and sidewise (or, North-south versus East-West). In urban settings where traps were placed adjacent to buildings, backyards, and between very close urban residences, the measurement was pretty much restricted to the immediately area in one direction, but allowed to be taken the typical length from the trap in the other alley-way direction.

Usually, photometric readings were taken using either on a 3 by 3 grid or circular radius pattern with at least six measures taken along the circumference edge. In some cases, stabilizing values were measured outside the grid or circle as well, typically in four or more directions, to again get an idea of light scattering effects and whether or not light readings did have significant points in the area where they might deviate from the norm.

Several major types of lighting of sites could be recognized based on common sense and typical plant profiling procedures. There were sites that had minimal light penetration of the canopy, which had specific types of photometric readings that varied little between ground and chest height. Some settings had partial light penetration of the canopy and could easily have a groundlevel reading that was significantly lower than the chest-high reading normally measured and applied to analytical reviews. Some settings had significant light penetration to the point where scattering seemed to be of minimal significance. Finally, there were sites that for the most part were either completely exposed to sunlight or so exposed that the photometric readings reached the range of readings typically associated with an open field readings even on a partially cloudy day.

This analysis resulted in the following differentiation of sites into different forms based on the photometric readings:

Conclusions about trap site types based on light penetration studies

To test the methodology and values used for analyses of trap site productivity in relation to light penetration readings, trap capture counts (at first, regardless of species caught, but later more focused on specific species) were correlated with various ways of analyzing the grid and circle generated matrices of trapsite photometric readings. Some correlations relied upon the average of all photometric readings, including centroid (trapside) readings and others took into account only the non-trapside grid readings. This was then correlated with each species count to produce a number of valuable outcomes.

Canopy Light Penetration Readings, Performance of each of the different methods evaluating for analyzing these outcomes

“Ttl_Prs” data referred to the use of photometric readings that were calculated based on two measures of the site values done synchronously, one after the other about 15 to 30 seconds apart. This was performed to account for ongoing changes in cloud cover, wind-related changes in canopy penetration, and changes in how the sensor detected light penetration based on standing position and direction I was facing while taking the measures. The C-5 readings included the centroid (“C”) or trapside value, 2-5 readings did not. The Centroid readings at the top of the table are just what the name implies–only the trapside centroid value is used for an analysis. AVG_AMBIENCE values are derived from treating all measures taken at the site equally. Meanx2Ambd3 refers to assigned a 2x value to ambient average to which the overall mean is added, with this total divided by three; this assigned more value to the ambient readings versus the complete site average. AVG1, 2 and 3 measures are what the name implies: an average of 1, 2 or 3 complete sets of measurements, with either 5 measurements taken minimally in the surrounding grid or circle, or 6 measurements. This was applied in order to account for settings where physical features prevented a sixth measure from being taken.

All significant values produced by this analysis are indicated in brownish red or red. The higher the t-value the better the outcome, the lower the p-value the better the outcome, with p usually representing significance of 95% for values < 0.05, and 99% for values < 0.01. Obviously, with regard to the p-outcomes, p-values significantly less than 0.01 were considered highly significant.

The conclusion drawn from this data is that paired readings taken almost simultaneously (15 to 30 secs apart) produce results that are more reliable than identical readings taken on different visits. When using this method, the addition of the centroid value to the equation produces slightly more reliable results. If a fieldworker were to choose taking only the centroid value for light penetration, of to rely upon an average of all measures taken regardless of placement and numbers, the significance and reliability are still less than 5%, but between 1% and 2% thereby increasing the chance for measurement errors impacting the results. Relying upon one or two visit related averages on similar days weatherwise, also produced fairly reliable results. The third visits increased the likelihood that something was intervening with the measurement process, such as human judgement based error in relationship to determining whether or not the conditions at the site were the same as the two readings before.

Impacts Per Mosquito Species

In terms of species related specificity, mosquito species varied greatly in the trap site light ranges they preferred. Analyzing each specific trapped species total counts (all mosquitos captured at a site, followed by each of the particular species) in relation to the light penetration values per site, we were able to determine what species preferred well-lit sites over poorly lit sites and vice versa.

On a per species basis, canopy light penetration seemed to impact some species more than others. This may be because some species are more adapted to enduring significant light change in order to find the next blood source, or it may have some underlying physiological and biological reasoning yet to be uncovered. In either case, Aedes trivittatus trapping most closely linked to trap site lighting conditions than other species. Ochlerotatus japonicus demonstrate borderline behaviors related to changes in light statistically. The vector for the Dutchess County region, Culex pipiens-restuans also demonstrated statistical significance in regard to its behavior relative to trapsite canopy coverage; this means that this species is best trapped for surveillance purposes under the right lighting conditions.

The statistical relationship between mosquito species capture and canopy light penetration

Relating this to site ecology, we can state the following.

Relating species to light penetration in a sub-canopy trapsetting

The conclusion drawn here essentially states that ecology is more important than canopy light penetration alone. If we vary the conditions by correlating light-test outcomes with ecology features, this relationship to light is not that strong or valid to use. This could be implying that there is a canopy tree species relationship that is only important when tree type and form impact light penetration behaviors. Mroe than likely, it correlates more with the trapsite capture history analyses which states that Cx. pipiens-restuans capture relates more with land use ecology features (i.e. the strong relationship to urban and periurban trap settings on vacant and waste lands), than simple ecology-linked trapsite lighting features. Other potential vectors like Anopheles (associated with malaria spread), or the frog-specific Uranotaenia are more canopy and light sensitive. Most other species of mosquitoes do demonstrate a fair amount of selectivity for their ecosystem-related activity, with Aedes species perhaps the most tolerant of light penetration of a forest or field edge canopy.

The Mosquito Species-Canopy Species Correlation

One of the final outcomes ofthis study worth mentioning is the relationship between different species specific phytoecologic settings with species specific mosquito habitat selection. Certain mosquito species may favor an association with certain tree settings in certain ecosystems. Using the data related to the above studies, the following correlations could be statistically shown to exist.