Ticks and Mosquitoes: What the Mild Winter Really Means

There has been a lot of discussion about how a mild winter, an early spring and a bumper crop of acorns might affect the number of mosquitoes and ticks we’ll see this spring and summer. Will there be more blood suckers out there? Will we be at higher risk of Lyme disease? Like many things in science, the answers are not clear cut.

First of all, the mild winter across much of the United States will have little impact one way or the other. Winter temperatures have to be extremely cold for a long time to affect mosquitoes and ticks. So our mild winter does not mean that there will be more of these blood suckers out there. But the early arrival of spring-like temperatures does mean that mosquitoes and ticks are becoming active earlier. (See our primers on ticks and mosquitoes.)

For example, one of the most common mosquitoes in the southeastern U.S., the Asian tiger mosquito (Aedes albopictus), overwinters as an egg. It lies dormant until the days get longer and the temperatures go up.

The early return of warm weather means that they’ll hatch earlier – which, in turn, could allow mosquitoes to begin breeding earlier. This may lead to more generations of mosquitoes over the course of the spring and summer, resulting in larger-than-usual populations later in the summer.

But the mosquito population will be influenced by an unknown variable: rainfall. A dry spring or summer would limit mosquito populations, because they would have fewer places lay their eggs. Naturally, there is a caveat: if people are watering their lawns and flowers, and leave standing water in containers (like the plates under their plants), mosquitoes will likely take less notice of the drought than humans will.

Ticks are a different story.

Early emergence will likely not result in a population increase for ticks, because their reproduction strategy is fairly predictable. It doesn’t hinge on temperature or environmental factors, but instead on the availability of a blood host for it to feed on.

There are reports that unusually large crops of acorns have boosted mouse populations, which will boost tick populations because it will be easier for them to find a host. The fear is that this will drive up the incidence of Lyme disease.

However, it’s important to note that a spike in Lyme disease is likely a concern only in areas where Lyme disease is already common. In the southeast U.S., ticks are less likely to pick up Lyme disease from a host – thus less likely to spread the disease.

This is because the black-legged or deer tick (Ixodes scapularis), which is the primary vector of Lyme disease, often feeds on lizards in the southeast. Reptiles are not efficient reservoirs for the spirochete that causes Lyme disease, so scapularis is less likely to become infected.

Furthermore, scapularis ticks in the southeast appear less likely to feed on humans compared to their counterparts further north. Researchers don’t know why.

The bad news is that this is changing, as rising numbers of Lyme disease cases in northeastern Virginia indicate that the northern strain of scapularis is moving south.

There is one piece of good news: a larger mouse population is unlikely to boost populations of the lone star tick (Amblyomma americanum), which is the tick we are most likely to encounter in North Carolina. This is because the lone star tick primarily feeds on larger animals. And since the lone star tick carries both Rocky Mountain Spotted Fever and Ehrlichiosis, we can all be glad there won’t be more of them than usual.

Note: Many thanks to Dr. Charles Apperson, William Neal Reynolds Professor Emeritus of Entomology at NC State, for taking the time to talk to me about mosquitoes and ticks. Apperson, whose work focuses on vector biology, ecoepidemiology and ecology of mosquitoes and ticks, was patient in his explanations, and kindly answered all of my questions (even the inane ones). Any errors in the above post are mine alone.