Mosquitoes can pass the Zika virus along to future generations in their eggs, researchers have found. That means winter may not stop Zika’s spread.

Infected female mosquitoes can transmit the virus along to their offspring, said a study in the Journal of Tropical Medicine and Hygiene. That means that even once it gets too cold or dry for adult mosquitoes, their eggs — which can easily survive the dry season or winter — can hatch the next spring when it rains, producing Zika-infected larvae that grow into infected mosquitoes.

The Aedes aegypti, the mosquito known to carry Zika, is a creature of warm weather. But the study found that the virus, like mosquito eggs, has no such climate restrictions. The transmission to offspring “is a mechanism to allow the virus to survive from one season to another,” said researcher Robert Tesh, who chairs the pathology department at the University of Texas Medical Branch in Galveston. The only way mosquitoes were previously known to transmit the virus was by biting an infected human. Common methods to kill adult mosquitoes, like spraying, do not work on eggs. Eggs can be destroyed by adding certain bacteria to the standing water where they were laid—but it’s difficult. “They’re so tiny,” Tesh said, “if they’re on a dark surface, you won’t see them.”