After two African villages started using mosquito nets to fight malaria, the local mosquitoes seemed to change their biting habits to skirt the barriers, according to a French study.

Insecticide-treated bed nets are considered a central weapon in the global fight against malaria, which is transmitted by parasite-carrying mosquitoes and kills more than 650,000 people a year, according to the World Health Organization.

In the study, which appeared in the Journal of Infectious Diseases, French researchers zeroed in on mosquito behavior before and after all households in two African villages were given insecticide-treated nets.

They found that mosquitoes seemed to change their hours of “peak aggression” from 2 a.m. or 3 a.m. to around 5 a.m. three years after nets were put up. And in one village, the proportion of mosquito bites inflicted outdoors rose.

Outdoor bites accounted for 45 percent of all bites at the outset but rose to 68 percent one year later and 61 percent after three years.

The finding is “worrying since villagers usually wake up before dawn to work in crops, and as such they are not protected by mosquito nets,” senior researcher Vincent Corbel, of the Montpellier, France-based Institute of Research for Development, said in an email.

Still, the results come from just two villages in one country, Benin.

“We cannot extrapolate to a wider geographical area and/or a different entomological context,” Corbel warned.

Mosquito nets have been credited with spurring big drops in malaria deaths, and a report for the Cochrane Collaboration, an international group that publishes rigorous reviews, estimated that for every 1,000 children protected by an insecticide-treated net, five to six lives would be saved every year.

But in recent years, malaria cases have started to climb again in certain African countries, Corbel said. Experts have mainly been concerned about mosquitoes’ growing resistance to the insecticides used in bed nets and for indoor spraying.

A malaria researcher not involved in Corbel’s study said the results of the study should be interpreted with caution.

One reason is the difficulty in getting reliable measures of mosquito “biting behavior” over time, according to Thomas Eisele, from the Tulane University School of Public Health and Tropical Medicine in New Orleans.

In this study, Corbel’s team used the standard way of gauging mosquito activity – the “human landing catch” – which, as the name implies, means that a mosquito collector lets the pest land on his skin, then catches it.

The researchers had mosquito collectors do three different rounds, indoors and outdoors, at each village: once before the nets were given to all households, then again one year and three years afterwards.

Eisele said measuring biting behavior can be “fraught with error” and added: “This study was conducted over only a couple of years, which would likely be insufficient to detect evolutionary changes in biting behaviors within the same species.”

Corbel said the study challenges the “dogma” that malaria-transmitting mosquitoes in Africa bite exclusively at night.

“Long-lasting insecticidal nets were developed to protect people at night when they are sleeping,” he said, noting that if mosquitoes shift to early-morning and outdoor biting, the nets might not be enough to keep malaria under control.