Larvae of Culex Mosquitoes. As seen on the picture, larvae make dense groups in standing water. A shift in the feeding behavior of those mosquitoes helps explain the rising incidence of West Nile virus in North America. It appears that the darker structure at the top center of the image is one pupa. Credit: James Gathany, CDC
Key points:
- A new study suggests light at night disrupts mosquitoes’ winter dormancy.
- With altered circadian clocks, the insects may not acquire enough energy reserves to last the winter.
- The lack of dormancy period also may extend biting season into the fall.
A new study on mosquitoes that transmit West Nile virus brings both good and bad news. The good news is urban light pollution seems to be disrupting the winter dormancy period of Culex pipiens—meaning they may not survive the winter if their plans to fatten up are foiled. The bad news, however, is the dormancy period may simply be delayed, meaning they’re biting humans and animals longer into the fall.
The study from researchers at Ohio state University, and published in Insects, is among the first to show that artificial light at night could have a significant impact on mosquito behavior, including effects that aren’t necessarily predictable.
“We’re finding that the same urban light at night can have very different effects under different seasonal contexts,” said Megan Meuti, senior author of the study and assistant professor of entomology at Ohio State University.
Diapause for female Northern house mosquitoes (Culex pipiens) is not quite a winter slumber, but rather a period of dormancy when the insects live in caves, culverts, sheds and other semi-protected locations. Prior to winter’s arrival, mosquitoes convert sugary sources, such as plant nectar, into fat. As days get longer, females begin foraging for blood meals to enable egg production. Some get infected with West Nile virus by feeding on infected birds, and later transmit the virus when they feed on people, horses and other mammals.
In the study, Meuti and her team compared daily activity and nutrient accumulation by mosquitoes reared in two lab conditions: long days mimicking the insects’ active season and short days that induced dormancy—with and without exposure to artificial light at night.
According to the results, exposure to light pollution suppressed the amount of water-soluble carbohydrates that were accumulated by mosquitoes in both long- and short-day conditions.
Patterns of accumulation of the sugar glycogen were reversed by exposure to artificial light at night: Under normal conditions, non-dormant mosquitoes had a lot of glycogen in their bodies, but diapausing bugs did not.
However, in mosquitoes subjected to light pollution, the long-day mosquitoes didn’t accumulate much glycogen and short-day mosquitoes showed an increase in glycogen accumulation.
The researchers observed consistent trends in activity-related effects of light at night, with slight increased activity among the dormant mosquitoes and slightly suppressed activity among long-day mosquitoes expected to be busy looking for food.
Though the findings weren’t statistically significant, the team says the combined observations suggest light pollution causes mosquitoes to ward off diapause—perhaps by scrambling signals from their circadian clock. The researchers plan to carry out field studies to see if these lab findings hold true in the wild.