Vector Ecology and Surveillance

Trap Performance and Vector Ecology

There are four major types of traps used to capture mosquitoes in 2002 and 2003. Three of these were very commonly used and are covered bleow. The fourth, a wooden box painted black on its interior called the resting box. This is essentially a 2.5′ hollow wooden cube with one face open, which is sealed when the capture begins and then fumigated with the goal of causing the resting adults within it to become deceased. This device was used to capture mosquitoes during (as the name suggests) their resting stage of the day amd was only occasionally from 2002 on due to its poor outcomes in 2001. To improve performance, this trap was usually placed areas with the largest density of mosquitoe swarms.

: An ABC Trap

ABC Traps. The basic trap used by most studies is the ABC Trap, which utilizes either Carbon Dioxide gas provided by a pressurized tank or Carbon Dioxide produced as an evaporant of dry ice released from a bag suspended adjacent to the trap to attract mosquitoes. The basic trap has a light that can be programmed to produce either a steady light of a blinking light, based on how the attachments are placed in the electronic hardware of this device. Some trappers favor a blinking light, others a steady light. For the most part, this setting is up to the individual, and is based on a little bit of field experience and mostly guesswork and luck when it comes to determining which setting will be best. The primary preiod for applying this trap is following sunset, overnight, until sunrise.

Gravid Traps. The second type of trap commonly used is the Gravid Trap, which is designed to provide a pool stinky or swamp-like water into which it is assumed breeding mosquitoes will aggregrate not only to mate, but also to lay their eggs.

This trap also has its personally-defined features, features defined by its field workers. One series of arguments states that the smell of the water contained in the trap and the presence of water are the most attractive thing about this set-up for attracting mating females. With this argument it is common to find the “cleanest” of these traps produced using starch and brewer’s yeast, to which other less attractive things are added such as swamp-based detritus, urine, sour milk, a little bit of tomato juice, rotting cabbage leaf cuttings, etc. The second argument for preparing the water for these traps is to use “the real McCoy”, by either collecting it from some local smelly marsh-setting or manufacturing it your self by collecting swamp or lakeside marsh-detritus like cattail roots, rotting bulrush stalks and bases, mud, blackened swampwater bottom leaf remains, wild animal excrement and remains, etc. etc. The primary period for applying this trap is late day, evening,, overnight and early morning.

Faye Prinz traps. The third type of trap is a Faye Prinz trap, which is a suspended device about arm’s length in both directions in width, and approximately 8 inches deep and in height when viewed from the front. The Faye Prinz trap differs from the other two traps in that it is intended to be a daytime trap. There are three panels that make up this device and a series of mirrors designed to trap mosquitoes even during the day. The notion is that the placement of two of these panels (wings or flanges) on the sides allow them to serve as an acoustic lens, attracted mosquitoes to the returning wing sounds, the acoustic center of which is at the center near the back mirror. Once in this central area, this mirror supposedly keeps them in close proximity to the fan below for a short time, enabling the fan to pull them into the net down below.

This bag into which the mosquitoes are fed and the held is later picked up at the end of a day, when the other two devices are being set for the overnight trapping event.

Source: http://www.johnwhock.com/images/712.jpg

All three of these trap types have a small fan that is run by a 9v battery designed for outdoor use. The purpose of these fans is to capture the mosquitoes and afterward prevent them from flying out of the bag once they have passed through the trapping device and entered it. This means of course that often the final determining factor as to whether or not trapping is a success depends upon the longevity of the Carbon Dioxide source in use, the length of battery time, wind direction and strength, and various environmental features capable of attracting or preventing mosquitoes from flying into the immediate trapping area.

Note: For more on traps see http://www.fairfaxcounty.gov/hd/westnile/wnvtraps.htm or http://www.fallschurchva.gov/Content/Government/Departments/Public
Safety/EmergPrep/MosquitoTraps.pdf

Trap Productivity

The effectiveness of trapping is dependent in part upon nature and in part upon human knowledge about vector ecology, the skills needed to set these traps and keep trap failure rates down, and the ability of people engaged in surveillance to be able understand when and where the times and conditions are just right. Later in this review of West Nile Ecology work, the various features in a natural setting that help to make trapping more successful in terms of numbers, and extremely successful in terms of identifying positive testing west nile sites, willbe reviewed as well.

In 2003, the following illustrated the totals of captured mosquitoes on a per day basis, for the 11 primary species and two non-specified genera observed in the research areas. It is important to note that with this graphing method, the numerous peaks that are seen are typically artifacts of human behavior, based on when and where the traps are set through the course of a week.

Captures per day by species

When this data is summed up for the year, and a 3-day rounded average applied, we see the following trend in mosquito population growth as a whole in the county.

3-Day Rounded Averages for all Species captured in 2003, per Day

When we look at individual trap site performance for the season, we can also get some input into where particular species of mosquitos tend to mate. (The first three letters for each trapsite name/identifier represents a city or town where it is place, the number is assigned when the particular site is first located and assigned a gps value in traditional lat-long, degree decimals form (i.e. xx.xxx or xx.xx) for the database.)

Performance Trap Locations in 2002

Performance of Major Trap Locations in June and July of 2003

In the development of a database for keeping data on mosquito trapping, several land use features were incorporated into the descriptive data for the various trapsites selected. These descriptors were pretty much based on standard landuse classification routines and at times were not always the best way to describe a particular pice of property upon which a trap was set. Nevertheless this data is of some use on a more regional or interstate and national basis due to the use of some standard terminology.

Two methods of coding trap site environment, ecology, and use

For the ecological studies reviewed in the all subsequent webpages, several other methods for classifying vector ecology regions were tried. The first method was an attempt to improve upon the original classification system used by paying more attention to specific ecological and topographic features that may be related to the various types of mosquito populations that exist, along with their natural and human ecological behaviors and activities.

Additional Method Applied to define Hydrogeographic Status of trapsites

To classify a site, a scale of 1 through 4 was utilized to determine how much the condition or state defined by the term fit the overall site description. These scores were then added up and put through a series of calculation to determine the significance of each of these 8 features for each of the trap sites. This resulted in the following method for mapping site ecology.

A first attempt at site ecology mapping

A more rigorous attempt was made to document site ecology by way of engaging in a phytoecological study of each site. In later portions of this review of the west nile work, the signifiance of the methods used are detailed quite extensively.

End Note: The key to understanding the abbreviations and nomenclature used to detailed vector genus and species.

The following abbreviations appear throughout these presentations and are used to refer to particular mosquito species that were caught (the 3-letter abbreviations) or the specific genus (2-letters), with species unidentifiable due to age or condition of the specimen removed from the trap net.