What is That in my Tree?

“What’s that in my tree and why is it there?”  The contraption often found hanging from a tree or fence is actually an important tool that the Contra Costa Mosquito and Vector Control District uses to protect public health. It is a mosquito trap. Mosquito traps are specialized devices that the District uses to monitor changes in mosquito populations and to collect mosquitoes for disease testing.

The traps are located in particular areas where the District conducts surveillance to detect the risk of West Nile Virus. They do not target a specific home or homeowner; they are simply placed in locations where the District has the best chance at catching mosquitoes in order to protect public health. When the District needs to trap on or near private property, District employees will attempt to contact the property owner. If no one is home, the District employee will leave a door tag explaining the purpose of the trap and how to contact the District if the homeowner has any questions. The traps themselves are also labeled with the District’s contact information.

In some cases, traps are located in specific areas every year. In other cases, traps are set up when a member of the public requests a mosquito inspection. A District employee will return later to collect the trap. The District’s entomologist and his staff then count the number of mosquitoes in the trap and identify them to determine if they are one or both of the mosquito species known to transmit West Nile virus in Contra Costa County. If they are, the District’s scientific programs staff may also test the mosquitoes to see if they are infected with the virus.

The information the District gleans from the traps is very important because it helps the District’s vector control supervisors determine where to focus efforts to prevent mosquitoes from transmitting disease to the citizens of Contra Costa County.

The District uses two primary types of traps: the New Jersey Light Trap (NJLT), and the Carbon Dioxide Trap. The NJLT was invented in New Jersey nearly 100 years ago. It features an energy efficient compact fluorescent light bulb to attract night-flying mosquitoes and a fan to blow them into a collection jar. These traps need electricity, so they must be plugged in to a standard electrical outlet. This means they can only be used at specific locations. There is a built in light sensor which turns the trap on at dusk and off at dawn for additional energy savings.  The District operates approximately 25 NJLTs and collects samples from them once each week, year-round.  Some of these traps have been in the same locations for more than 20 years, which makes them very useful for monitoring long-term changes in mosquito populations and comparing current counts with past averages.

Carbon dioxide traps, also known as Encephalitis Virus Surveillance (EVS) traps, are portable and battery powered so they can be used almost anywhere. They attract mosquitoes using a small amount of dry ice, which gives off carbon dioxide as it evaporates. Mosquitoes are naturally attracted to the carbon dioxide humans and animals exhale in their breath. The EVS traps also have a small light source to lure mosquitoes to the trap and a fan to blow them into a collection container. These traps capture both day and night flying species, and tend to capture higher numbers than NJLT. Because the dry ice evaporates overnight, the EVS traps are generally set for a 24 hour period and retrieved the following day.  This means most of the trapped mosquitoes are still alive when the traps are returned to the District’s lab, making them most effective for virus testing. 

The District operates about 25 of these traps at specific ‘fixed’ locations once a week from April through October, and an additional 20-30 EVS traps at ‘random’ or ‘targeted’ locations. These locations are selected based on dead bird reports, mosquito complaints from the public, or other indications of increased disease or nuisance risk.  Because these traps are collected on a daily basis, that allows District managers to quickly assess a particular location and decide whether additional control efforts are necessary. 

If a trap is located on your property, you don’t have to do anything to the trap. In fact, because they are used for scientific surveillance, it is important that members of the public leave these traps alone and do not damage them or take them apart. After all, that funny looking contraption hanging from a tree or fence is an important tool to protect you and your neighbors from the mosquitoes that can impact your health and quality of life, right here in Contra Costa County.