Mosquitoes with West Nile Virus likely more Abundant if Weather Along Front Range Stays Warm

This spring’s cooler than normal temperatures, high level of precipitation and the current warming weather trend could mean that the area’s mosquitoes that carry the West Nile virus will be in larger numbers than last year, according to Chet Moore, an infectious disease researcher at Colorado State University.

Last year’s numbers of mosquitoes were low because the weather stayed cool. Cool temperatures slow the development of larvae and cause female mosquitoes to diminish their efforts to bite humans and animals. Moore’s lab at the CSU Foothills Campus works with Larimer County to test samples of mosquito populations for the virus.

“A spring with lots of snow and rainfall followed by warm weather tends to be a sign that these mosquitoes will be in higher numbers because the weather pattern provides them with more habitat,” Moore said. “Warm weather leads to runoff and flooding, which leaves standing water for mosquitoes to lay their eggs.”

Hot and dry weather also means an increase in irrigation, leading to standing water in irrigated land.

A mosquito lays her eggs in standing water and if the water sits for about five to six days, the larvae develop into adult mosquitoes. Some large areas of standing water can produce as many as 1 million new mosquitoes; even a birdbath, without preventative measures, can spawn hundreds of newly hatched mosquitoes every four to six days.

Not all mosquitoes carry the virus; in the United States, there are several species of mosquito that are considered by scientists as important in transmitting West Nile virus, and in the West, it is the Culex tarsalis and Culex pipiens mosquitoes that carry and transmit the virus.

And while temperatures play a large role in determining how much of a problem virus-carrying pests may be, scientists still are working to unravel some of the mysteries of West Nile virus. Bird populations, for example, also may impact the number of mosquitoes carrying the virus. Birds carry the virus after being bitten by an infected mosquito, then pass it on to uninfected mosquitoes when they bite, helping spreading the disease through the mosquito population that in turn infect humans and other animals. Some species of birds, including crows, magpies, jays and many hawks and owls, are extremely susceptible to West Nile infection and suffer high mortality.

The virus has only existed in Colorado since 2002, but researchers don’t know if the virus survives in infected mosquitoes over the winter to re-emerge each spring, or if migrating birds return the virus to the area as they return each spring. The average lifespan of a mosquito is about a week, and although adult female mosquitoes can overwinter, the virus does not seem to survive the winter in mosquitoes. During the winters of 2003-04 and 2004-05, Moore’s group tested more than 10,000 overwintering mosquitoes from the Front Range and did not find that a single one had West Nile virus. To date this spring, no mosquitoes with the virus have been found by the lab, which will begin testing within the next several days. However, each spring the virus eventually reappears in mosquito, bird and human populations in the area.

Only the female mosquitoes bite and transmit the virus. Male mosquitoes feed on plant nectars. Females must take a blood meal to produce eggs and need to mate with a male only once in their lifetime. Once mated and fed, they produce as many as 200 eggs every three to four days in ideal conditions.

In their lifetime, some mosquitoes can easily travel up to 30 miles, sometimes covering up to as much as 15 miles in a night. Fortunately, the Culex vectors of West Nile virus fly only about half those distances. When cooler temperatures strike – in the low 50s, which occurs often at night in the spring and fall– mosquitoes are unable to fly, which contributes to diminishing mosquito bites in the fall and increasing bites when temperatures warm.

“Early prevention is the key to lowering mosquito populations,” Moore said. “Taking steps in the spring can reduce the numbers of mosquitoes through the summer.”

Moore’s top tips for avoiding a bite from a mosquito:

– Water management. Fill low areas where water puddles with dirt and treat standing water in ponds, birdbaths and other areas with larvicide according to directions on the label. Clean birdbaths and other yard decorations that incorporate water about once a week and also treat those with larvicide donuts if needed. Several larvicide products are safe for birds and other animals—be sure to check the label before using any product.

– Avoid the outdoors from dusk to midnight, which is a prime time for the mosquitoes carrying West Nile virus to feed. The species of mosquitoes carrying the virus are active later into the night than some other species in the area, which feed mainly in the early evening hours.

– Wear long sleeved shirts and long pants in light colors. Lighter colored clothing repels mosquitoes more than dark colored clothing.

– Use a repellent when necessary. Repellents with DEET are most often recommended; a relatively new product called picaridin is almost as effective. Picaridin is Centers for Disease Control and EPA approved and it is not as oily as DEET because it is made with an alcohol base. The strong odor of lemon eucalyptus may also be effective compared to other “natural” repellents. Follow the label when applying repellents because over applying it may irritate the skin. Citronella has not been proven to be very effective against mosquitoes.

-There is no scientific relationship between wearing perfume, aftershave or other scented products and mosquito bites, although some scents may attract mosquitoes and some may repel them. Some people are more attractive to mosquitoes based on their body temperature, moisture on the skin and specific compounds in their sweat. There also is no scientific evidence that consuming garlic, vitamin B or other foods work in keeping mosquitoes away.