Mosquito Sampling and Surveillance
Mosquito surveillance is a continuing process of inspection to monitor changes in mosquito population density, diversity, ecology and behavior. The evaluation of the incidence of mosquitoes within an area of interest, the relative abundance of various species, and the information in relation to breeding sites and habits of the resident species, can be used to decide on the control strategies and timing of control, as well as to justify applications, withhold control measures and conduct post treatment assessments.
A. Larval mosquito surveillance - Larval surveys are conducted with a standard pint-size dipper and are used to locate the exact areas in which the mosquitoes breed and to estimate their relative abundance. Surveillance is of special value in control operations to determine areas to be treated, to evaluate treatment efficacy and for resistance monitoring.
1. Inspection sites include rural and urban sites:
a. Rural - swamps, salt & freshwater marshes, woodland pools, flooded fields/ pastures, roadside ditches, storm water retention structures, tree holes, rice fields.
b.Urban - flower pots, tires, trash containers holding water, gutters, tree holes, septic ditches, roadside ditches, swales, non-functional swimming pools, etc.
2. Data recorded should include number of larvae per dip, mosquito species, stage of development, type and size of breeding area, map location of positive larval sites and GPS coordinates.
3. Data are analyzed to evaluate treatments and to decide on control strategies.
B. Adult mosquito surveillance - Data are collected to estimate adult mosquito population species, densities, age structure, distribution, to decide on control strategies, to time control measures, to evaluate treatments and for arboviral surveillance purposes. Methods of collection include:
1. Light traps - Various types of mechanical devices that use a light to attract flying mosquitoes and a suction fan to draw them into a container.
a. Some types are permanently stationed and run several times per week (New Jersey light trap).
b. Some are portable and run periodically for special data gathering purposes (CDC miniature light trap).
c. Portable light traps are generally operated using attractants such as CO2 (dry ice, compressed cylinder gas) and octenol.
d. Light trap collections are sorted by species, gender, number and location.
e. Data are used to decide on control strategies and form a baseline for treatment efficacy assessment.
2. Landing rates - Inspectors use themselves as bait and record the number and species of adult mosquitoes that land on them in a given period.
a. Landing rates are taken daily and are used in conjunction with other surveillance data to determine if treatment is necessary.
b. The data also are used to determine the proper control strategy and provide a baseline for treatment efficacy assessment.
3. Resting stations - Resting stations may be natural or man-made sheltered situations such as rotted tree stumps, barns, culverts and resting boxes (open-end red/black painted wooded boxes, 1 ft3), from which mosquitoes are collected by aspiration.
a. Resting stations are used in the surveillance of selected adult mosquito species.
b. This sampling method is used in specialized situations such as surveillance for Culiseta melanura, vector for eastern equine encephalitis (EEE), Anopheles spp. vectors of malaria and certain arboviruses.
4. Gravid traps - Traps that use an ovipositional attractant (hay infusion, fish emulsion oil) to sample female mosquitoes which have at least taken one blood meal and are seeking a site to lay eggs.
a. Gravid traps are used primarily for mosquito species such as Southern house mosquito (Culex quinquefasciatus), the primary vector of St. Louis Encephalitis (SLE) and West Nile virus (WN), and the Asian Tiger Mosquito (Aedes albopictus}, also a vector of WN (see Appendix C for a list of common names of important mosquitoes from Louisiana).
C. Ovipositional sampling - Ovipositional traps are various containers (glass/plastic cups with paddle boards or paper strips for females to lay eggs on) designed to sample populations of selected container breeder mosquito species, specifically Aedes albopictus (vector of WN) and Aedes aegypti (vector of dengue and yellow fever).
D. Operational factors
1. Collection of mosquitoes is carried out by one or a team of trained inspectors that also performs systematic surveillance for new or previously unidentified mosquito breeding sites and adult mosquito populations.
2. A mosquito identification specialist
(biologist/entomologist) normally has the task of identifying mosquitoes to species, assessing the physiological age of the females (gonotrophic cycle; important in assessing capacity to transmit diseases), etc.
3. Frequency of collection will vary from several times a week to once a month, depending on time of the year, sampling method, purpose of surveillance, mosquito species of interest and availability of personnel.
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