The season for mosquito-borne illnesses in western Massachusetts is all but over after much of the region received a hard frost over the weekend. Even with mild temperatures in the days since the frost, it was enough to kill off most adult mosquitos.

Dry weather has contributed to a lower than normal mosquito population this year says Chris Horton, superintendent of the Berkshire County Mosquito Control Project. But, Horton says, that low population produced high numbers of mosquito-borne illnesses. And, he says, the record heat over the summer could have contributed too.

“When you have extra hot conditions, the chance of virus being transmitted between mosquito and bird is higher.”

Both West Nile Virus and Eastern Equine Encephalitis (EEE) are spread from infected birds to humans through mosquito bites. All told, 22 human cases of West Nile have been confirmed in the state so far this year, and seven human cases of EEE. That’s well above last year’s numbers when the state saw six cases of West Nile and two  cases of EEE. Horton says now that a frost has hit much of the region, the risk of encountering an infected mosquito is essentially gone, but even with winter approaching, the mosquitos themselves are not.

“There are different species of mosquitos. Some will over-winter as adults, some will over-winter as larvae, and some could over-winter as eggs, and next spring, as soon as the snow melts, it’ll start all over again.”

Many schools and municipalities in the region had limited evening outdoor activities while the risk of mosquito-borne illnesses remained high earlier this fall. After Saturday’s frost, many organizations have lifted those limitations, including UMass Amherst.