A tree hole filled with water produces thousands of mosquitoes each year if not managed correctly

Anyone who thinks mosquitoes are only a sign of summer have never met the Western tree hole mosquito. The Aedes sierrensis mosquito is an active biter day and night and tends to make its yearly appearance at the start of spring. But with Contra Costa County's fairly warm, dry winter thus far in 2013, this small, black mosquito with bright white striped legs is already alive and well and preying on Contra Costa County residents.

It's important to note that the Asian tiger mosquito, a primary transmitter of dengue fever, is currently in Southern California. It is also a day-biting mosquito and has very similar markings to the Western tree hole mosquito. However, the Asian tiger mosquito has not been found in Contra Costa County. The Contra Contra Costa Mosquito & Vector Control District is aware of the Asian tiger mosquito and working to prevent it, should it make an effort to take up residence in our county.

 

Several holes in this tree fill with rain water each year and allow mosquitoes to flourish

As for the Western tree hole mosquito, the reason it is making an early appearance this year is the same reason many trees and plants are already sprouting buds; nature suspects spring has sprung. Spring's arrival or weather mimicking that occurrence triggers the female mosquito's search for a blood meal which enables her to produce eggs. The female tree hole mosquito can lay her eggs in a variety of containers, but prefers to lay eggs along the rims of holes that often develop in older trees - thus the source of her name. When it rains and those holes fill with water the eggs begin to develop into larvae, followed by pupae that emerge as adults capable of biting people or animals.

While the aggressive tree hole mosquito can and does bite humans, it usually doesn't make men or women sick. Instead, it's a health risk for man's best friend. This mosquito is the chief transmitter of dog heartworm. Typically, it's most active from February or March until about June.  But as long as there is rain in the county that enables the mosquitoes to develop into adulthood, this mosquito will continue to put dogs at risk.

To reduce the risk of tree hole mosquitoes, the Contra Costa Mosquito & Vector Control District recommends dog owners consult a veterinarian for appropriate dog heart worm prevention. At the same time, the District recommends residents fill tree holes that hold water with a small amount of absorbent that can be found at home and garden stores.

"These products are called horticultural hydrophilic gel products and they go by the names Agrosoke™, Broadleaf ® P4 ®, Soil Moist, or Water-Keep. They're designed to fill in the hole and prevent mosquitoes from developing in that location for several years," says Mosquito Program Supervisor Sheila Currier. "It usually only takes one teaspoon of one of these products to be effective in each hole. They're designed to expand and fill the hole the first time it rains after application. But if you can't find one of these products, pouring sand in the hole can provide a short term solution," adds Currier.

Currier says she often finds these mosquitoes in trees that grow along creeks and other waterways. Because the trees are close to bodies of waters, residents suspect the mosquitoes are coming from the creek. Currier says they are often surprised to learn the mosquitoes are actually coming from the trees.

While these mosquitoes are most common in central and western Contra Costa County, they can be found anywhere in the county. If a resident is being bitten day and night, Currier suggests the resident place a dead mosquito in a clear plastic bag, and then  contact the District for a free inspection. The dead mosquito in the bag serves as a sample, so the District's public health inspector can identify the mosquito which provides a good idea where the mosquito is coming from. And locating the source of the mosquito is a crucial step in preventing future mosquitoes and protecting public health along with man's best friend.