Weather Surveillance
The weather is the most dominating factor affecting mosquito production. All mosquitoes
spend their larval and pupal stages of development in water, consequently rainfall is critical to
their existence. In addition, temperature governs their rate of development and is a secondary
variable in their life cycle. Over forty species of mosquito are found within Illinois, and each has
environmental conditions best suited to its development. The Aedes vexans and Culex pipiens
with related species are the primary mosquitoes found within the District. In general, heavy
rains followed by warm temperatures create the potential for a major population of the
floodwater annoyance mosquito, Aedes vexans. In contrast, dry conditions creating areas of
“stagnant” water are ideal for prolific production of many disease vector mosquitoes as Culex
pipiens and related species.
The District monitors rainfall, temperature, and other weather data at six locations throughout
the district. In addition, rainfall and temperature data is obtained from the National Weather
Service at O'Hare Airport.
Mosquito Larvae/Pupae Surveillance
Larval/pupal mosquito surveys provide critical information for our control efforts. The surveys
are conducted on a regular basis to determine the extent, type, and concentration of mosquito
populations within the district. Potential mosquito breeding sources are inspected regularly
throughout the season. Larval samples are taken from sites found breeding, and are identified
by the laboratory staff to species. All potential sources are marked on maps and numbered for
reference. Individual source histories are maintained in a computer database. In addition,
follow-up post-treatment inspections and evaluation help insure the success of control
procedures.
Adult Mosquito Surveillance
Adult mosquito populations within the District are monitored through a network of light traps,
gravid traps, and oviposition traps. These traps take daily samples which are identified by the
laboratory staff to determine the number and types of mosquitoes present.
Disease Vector Surveillance
The potential for the occurrence, amplification, and spill-over to the human population of
mosquito-borne disease is a combination of numerous factors. Two mosquito-related factors, in
combination, are important in the West Nile Virus(WNV), St. Louis Encephalitis(SLE), and
Eastern Equine Encephalitis(EEE) cycles. These factors are the mosquito vector population
levels and the infection rate within the vector mosquito population. The District has been
tracking vector (Culex spp.) population levels for many years. Recently, the use of gravid traps
has been added to our vector surveillance.
Gravid traps are specialized traps that collect live gravid adult female mosquitoes, primarily
Culex species. Gravid mosquitoes are female mosquitoes that have taken a blood meal and
are ready to lay eggs. Because they have taken at least one blood meal, it is more likely that
they have been exposed to WNV, SLE, or EEE. The District uses gravid traps to sample the
adult female Culex mosquito population to test for the presence of disease viruses as well as to
observe vector mosquito population levels. The samples from these traps are tested in-house
for the presence of a disease organism (in the case of WNV, SLE, or EEE a virus) with the
"dipstick test" (VecTest WNV/SLE/EEE Antigen Panel Assay). This test can be performed in-
house with results known within 15 minutes. The samples are then sent to the Illinois Natural
History Survey(INHS) for confirmation using the more sensitive RT-PCR test.
