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

Insecticide-treated bed nets are considered a central weapon in the global fight against malaria - an infectious disease transmitted by parasite-carrying mosquitoes. Malaria kills more than 650,000 people a year, according to the World Health Organization - most of them babies and children in sub-Saharan Africa.

In the new study, 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, the researchers reported in the Journal of Infectious Diseases. 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 said. Entomology is the study of insects.

A malaria researcher not involved in the study said the results 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 mosquitoes' activity - the "human landing catch" - which, as the names 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 nets were given to all households, then again one year and three years afterward.

"Obtaining measures such as biting behavior is fraught with error, in that it is difficult to do exactly the same way across replications," Eisele said in an email.

But most importantly, he 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 of Anopheles."

The mosquitoes studied were of the Anopheles funestus species, one of the two main malaria vectors in Africa.

Mosquito nets have been credited with spurring big drops in malaria deaths, Corbel noted.

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 each year.

But in recent years, malaria cases have started to climb again in certain African countries, Corbel pointed out.

Experts have mainly been concerned about mosquitoes' growing resistance to the insecticides used in bed nets and for indoor spraying. Earlier this year, the World Health Organization said resistance had been detected in 64 countries.

The WHO also issued a plan that said each country at risk must analyze the extent of its insecticide resistance and design a preemptive strategy, rather than waiting for resistance to increase.

Corbel said the current findings challenge 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," Corbel said.

But if mosquitoes shift to early-morning and outdoor biting, those nets might not be enough to keep malaria under control, according to Corbel.

He said more research should be done into the possible effects of different mosquito-control methods on the insects' behavior.