Yale researchers have identified two separate invasions in California of Aedes aegypti – known colloquially as “the yellow fever mosquito” because it can carry yellow fever, dengue fever, chikungunya, and Zika, among other diseases.

Published in PLOS Neglected Tropical Diseases journal on Aug. 10, the research was conducted by graduate student Evlyn Pless and Jeffrey Powell, professor of ecology & evolutionary biology.

Their study built on the 2013 reports of the first A. aegypti presence in northern California. After the species was detected in southern California in 2014, the authors decided to investigate the species’ origins in California. They found that the invasion of mosquitoes in southern California was distinct from the one that brought the species to northern California.

After genotyping 516 mosquitoes from five sites in northern California and seven sites in southern California, Pless and the team concluded that the mosquitoes in northern California likely came from the south–central U.S. and those in southern California from the southwest U.S. or northern Mexico. The evidence suggests both invasions occurred a number of years before their detection.

The researchers also found that some of the populations in northern California had survived over winter, even though scientists have thought that the species cannot tolerate freezing temperatures. In other words, said the researchers, a disease–carrying, invasive species of mosquito has developed a lasting presence in northern California and may be able to colonize other areas that were originally considered too cold for them.

“California has one of the most extensive mosquito–monitoring systems in the U.S.,” wrote the researchers, “so the possibility that A. aegypti was in California years before detection may mean mosquito invasions have occurred elsewhere in the U.S. but escaped notice. Understanding and accounting for the invasion dynamics of A. aegypti will continue to be essential for detecting new invasions, monitoring vector presence, and preventing disease outbreaks in California and other regions.”

Knowing how to detect invasions of A. aegypti, monitor their presence, and prevent A. aegypti–borne outbreaks of disease is especially important today, given that incidences of dengue fever and Zika have recently increased while there are still no vaccines available for either disease, note the researchers.

This study was done in collaboration with Vicki Kramer at the California Department of Public Health, Vector–Borne Disease Section, and funded by a grant from the NIH National Institute of Allergy and Infectious Diseases.

The study was in the journal PLOS.