With March being the driest on record and April below average so far for rainfall, swatting away mosquitoes may not become a frequent activity this spring and summer.

But then again, you never know.

`` You can never predict'' the type of season it will be for mosquitoes, said David Lawson, assistant director of Norfolk County Mosquito Control Project. `` It is always a week-to-week type of thing with the rain.

`` If conditions stay dry as they are right now, we predict a light spring brood. Things are very dry. Water levels are very low in wetlands. We expect mosquitoes to be on the lighter side,'' Lawson said. `` If we get five inches of rain in a week, the story completely changes.''

If it is anything mosquitoes enjoy, it is wetness, authorities say. They lay eggs in standing water. Aerial spraying of wetlands is scheduled to begin Tuesday to battle those nasty insects in Norfolk County. Foxboro, Wrentham, Plainville, and Norfolk are targeted among 24 communities.

The Norfolk County Mosquito Control Project will spray between dawn to dusk, weather permitting, with a granular form of Bti (Bacillus thuringiensis israelensis), a naturally occurring soil bacterium that kills mosquito larvae in low-lying swampy areas where they breed and become mosquitoes throughout the season.

As long as the sun is out, and there isn't much wind, the spraying will be carried out, Lawson said.

Aerial spraying should run about 2 1/2 weeks, but there are usually at least some delays because of weather, he said.

Ground spraying began last week with the same bacterium, Lawson said.

Requests for ground spraying have been about average, he added. Spots that the mosquito control unit knows usually have a lot of mosquitoes are also being sprayed.

In the larger wetlands that are inaccessible from roads, the applications will be done by three helicopters flying at tree-top level, officials said.

`` There will be one or two helicopters working in your town at a time,'' said John J. Smith, the director of the Mosquito Control Project.

Bti is virtually non-toxic to humans, plants and animals, officials say.

According to information on the federal Environmental Protection Agency Web site, www.epa.gov/pesticides/factsheets/larvicides4mosquitos.htm, microbial larvicides `` are essentially nontoxic to humans, so there are no concerns for human health effects with Bti and do not pose risks to wildlife, most non-target species, or the environment.'' The Bti is impregnated on coarsely ground corncob granules that fall straight into the wetland and should not drift beyond the targeted area, officials said.

Aerial Bti applications will be done only in wetlands that potentially harbor significant numbers of mosquito larvae, officials said. Mosquito control has taken samples of water.

`` We have scheduled this every year for about five years,'' Lawson said of the aerial spraying. `` It is a routine thing now.''

Maps showing the targeted wetlands by town and more information are available at

http://massnrc.org/ncmcp/ or call the Project office Monday through Friday between 7 a.m. and 3:30 p.m. at 781-762-3681.

Although the official deadline has passed, the project will try to avoid certain properties if the owners inform both town clerks in the communities and the Project office, Lawson said.