A mounting number of citations on a popular disease-tracking website suggests that mosquitoes may be moving into new ecological niches with greater frequency.

The website, ProMED mail, has carried more than a dozen such reports since June, all involving mosquito species known to transmit human diseases.

Most reports have concerned the United States, where, for example, Aedes aegypti — the yellow fever mosquito, which also spreads Zika, dengue and chikungunya — has been turning up in counties in California and Nevada where it had never, or only rarely, been seen.

Other reports have noted mosquito species found for the first time on certain South Pacific islands, or in parts of Europe where harsh winters previously kept them at bay.