PORTLAND — It’s been almost two decades since DDT was banned in the United States, but federal biologists say Maine’s bald eagles may still be suffering from the pesticide’s lingering effect.

Although there are more eagles in Maine than 20 years ago, the birds’ reproductive rate is lagging far behind that of eagles in other states.

Biologists fear that byproducts of DDT and other contaminants still persist in the cold environment, making eagle eggs infertile and their shells dangerously thin.

“We can see the number of nesting pairs and occupied (nest) sites growing, but until the reproductive rate follows through, we have a hunch they’re not going to be self-sustaining,” said Charles Todd, a biologist at Maine’s Department of Inland Fisheries and Wildlife.

DDT was sprayed on Maine forests from 1957 to 1963 to kill the spruce budworm, an insect that can destroy whole stands of young timber. It was also used to control Dutch elm disease.

Farmers and home gardeners also used it on apples, blueberries and vegetables, and some towns used it to control mosquitoes.

Throughout the 1960s, fish in Sebago and Moosehead lakes carried dangerously high levels of DDT residue. Maine’s bald eagles never recovered from the years absorbing DDT and other chemicals that worked through the food chain.

DDT was banned in 1972, and Maine environmentalists began trying to help rebuild the eagle population. Maine was the first state to transplant eagle eggs from other states to nests here.

Aerial surveys of eagle nests have traced a gradual increase in the number of occupied nesting sites. In 1990, the survey recorded 123 occupied nests, up from 56 a decade ago.

But the standard used by biologists across the country to determine the stability of an eagle population is the number of young produced per occupied nest.

“Measuring reproduction sort of gives you the crystal ball image of the future,” Todd said. “The young that are raised this year are going to enter the breeding population five, six or seven years later.”

Even though the number of eagles is up from a decade ago, the reproductive rate is still a whopping 40 percent below that of Maryland, where in recent years eagles have given up much of their Chesapeake Bay habitat to developers, Todd said.

Todd and federal researchers interested in the bald eagle problem hope to get funding from the federal endangered species program for a study that will test their theory that chemicals are the problem.

Some work already is under way. Last spring, biologists began testing the dead and injured eagles that are routinely brought to Inland Fisheries and Wildlife for chemical contaminants.

This spring, they will begin collecting a few unhatched eggs from eagle nests, testing them for chemicals and measuring the thickness of the shells.