Officials Concerned Post-Rainy Season Mosquitoes Will Proliferate, Increasing Cases of West Nile Virus

Published : Tuesday, March 26, 2019 | 4:41 AM

With drought concerns out of the way, Pasadenans can now turn their concerns to mosquitoes because that’s exactly what healthy winter rains generate in Southern California.

“As an agency we are pretty happy the rain has relieved us of the drought,” said San Gabriel Valley Mosquito and Vector Control District spokesman Levy Sun. “Unfortunately, it has filled up a lot of containers in people’s backyards and patios, and our fear is that, once the weather warms, we will see an increase in mosquitoes.”

San Gabriel Valley Mosquito and Vector Control is responsible for disease suveillance and mosquito control in Pasadena.

The concern at Vector Control, said Sun, is that the long rainy season, coupled with warming temperatures, will create a perfect storm of reproduction that, in turn, will increase the instances of West Nile Virus, the most common, mosquito-borne illness in the region.

Standing water left behind by weeks of rain make ideal breeding grounds for mosquitoes.

Other potential threats are yellow fever, dengue fever, and Zika. “Luckily, we have only seen cases of West Nile virus in recent years and not the others,” said Sun.

Approximately two weeks after it rains, Pasadenans will begin to see swarming insects, reminiscent of mosquitoes, but which are “midges” and nothing to worry about, he explained.

These are followed by large Crane flies that can be some three- to four-inches in length and, like the midges, absolutely harmless.

If the weather heats up in the wake of these early insect arrivals, mosquitoes are in the offing.

Mosquitoes are broken down into a pair of distinct populations. The first are referred to as “Culex” mosquitoes, the primary drivers of West Nile virus, according to Sun.

Culex lay eggs like “egg rafts,” said Sun, and most are flushed away, but those in containers with no in- or out-flow, survive to hatch.

The second type of mosquito that is cause for concern goes by the name of “Aedes,” and it poses a special problem as it is invasive, or not native, to local ecosystems. Their eggs stick to the sides of containers, just above the water line.

That means a cursory dumping of the water in containers will not eliminate the eggs of the Aedes. “These can stick to dry containers and stay viable for at least a year,” Sun explained. “The rain reactivates them.”

Zika is spread by the Aedes variety of mosquito, which has become a real problem since first appearing in the San Gabriel Valley since 2011.

The Aedes mosquito is more resilient than the Culex because, as an invasive species, it has no natural predator to thin its numbers. They are aggressive daytime biters. The environment and food source that sustains them is the backyard container, said Sun.

“These mosquitoes primarily feed on people,” he explained. “We get a break from the Culex because they feed on birds, but the Aedes don’t care about birds. They want human blood.”

What’s a mosquito-borne-disease-fearing citizen to do?

“First, they need to immediately get rid of stagnant water around the home once a week,” Sun stated. “I say weekly because it takes that long for these mosquitoes to get from egg to adult.”

Thinning out vegetation around the home will also help because thick croppings of green shrubbery and trees provide the flying pests a safe harbor.

In the realm of how to protect the human body from mosquitoes, Sun said Vector Control recommends the application of repellent. There are four Centers for Disease Control recommended topicals: Deet; Picaridin, oil of lemon eucalyptus, and IR3535.

You must apply to all exposed skin as the Aedes mosquito is irrepressible when it comes to targeting those parts of your body that are unprotected.