Every spring and summer for the more than four decades that Hal Ziserman has lived in rural Chester County, he's enjoyed "clouds of butterflies" in his wildlife-friendly garden.

But this year is different. "No butterflies," he said.

And not just in Chester County.

A census taken on July 4 at Hawk Mountain in Berks County recorded 736 butterflies, fewer than half the annual average and the second-lowest total in the 18-year history of the event.

Hawk Mountain is one of 500 sites across the country where counts are conducted for the North American Butterfly Association, based in Morristown, N.J. This year's totals will be due in September, but preliminary data suggest that the Hawk Mountain number may not have been a fluke.

Similar falloffs have been noted in the Blue Ridge Mountains in Virginia, Big Oaks National Wildlife Refuge in Indiana, and Pleasantview Hills, Idaho.

"It seems pretty clear that, over a very large portion of the United States, butterfly numbers are roughly half of what they were last year, which was not a very good year for butterflies," said Jeffrey Glassberg, president of the 5,000-member butterfly organization.

Nobody knows for sure what has caused the apparent drop, and there's not a lot of money out there to research the mystery. But Glassberg and others suspect weather may have been a factor.

Butterflies are essentially solar-powered, and extremely sensitive to environmental conditions. They need warm weather to emerge from their winter state, whether egg, caterpillar, or cocooned chrysalis, and to rev up for flying and courting. And, at least in the northeastern part of the country, there just wasn't a lot of sun in late April, May, and June.

"It's been a terrible year for butterflies," said Stephen C. Mason, a senior at Richard Stockton College in Pomona, N.J., who is studying butterflies in the Pine Barrens this summer with the Academy of Natural Sciences in Philadelphia.

Cool, wet weather in spring delays the butterfly life cycle, leaving larvae more vulnerable to predators and infection. Rain can knock caterpillars to the ground. The weather could also have caused fungal diseases on plants that are genetically programmed for picky butterflies to lay eggs on, caterpillars to eat, and adults to sip nectar from.

Ziserman, 84, a widower and retired businessman, lives with his daughter Rebecca Cesarz, 46, her husband, and her 10-year-old son on nine wooded acres in sleepy Glenmoore. Living here has taught them reverence for the butterfly world and all else.

A family of black vultures nests in the barn. Deer are photographed, not fenced out; squirrels are fed sunflower seeds and Rice Krispies; and great horned and screech owls are cherished guests. Cesarz even rescues baby mice from the family cats.

Every year, father and daughter plant butterfly magnets like joe-pye weed, purple coneflower, butterfly bush, and phlox. And every June through August, they're rewarded with plentiful cabbage whites, swallowtails, skippers, monarchs, and painted ladies.

"There are usually so many butterflies around, I can't get into the garden," Cesarz said.

Although the butterflies are beginning to come back, the summer scene has been downright depressing.

"Butterflies are a lot of the life in your garden, and it's been completely empty," Ziserman said.

The same was true this year at Tyler Arboretum in Media, where Alan and Barbara Mennig volunteer in the Butterfly House. The Malvern couple also participate in the Hawk Mountain census.

She's a greeter in "the house," which has as many as 150 butterflies in various stages of development. They're captured on the arboretum grounds, some with a net hoisted aloft by Alan. (All will be released on Aug. 30, when the Butterfly House closes for the season.)

In late March, the Mennigs typically see the colorfully named mourning cloaks emerge.

"It's always a thrill," said Alan, an accountant. "We say, 'Butterfly season is here!' "

Then come the common cabbage whites, the orange sulphurs, question marks and Eastern commas, the red admirals and great spangled fritillaries.

"This year, we just didn't see any," said Barbara, an elementary school teacher, who predicts it will take a year or more for the butterflies to rebound.

But who really knows?

Scientists caution that nature is constantly in flux, that trends are not determined in a year or two, and that conditions detrimental to one kind of butterfly can be a boon to another.

"Weather is only likely to impact a broad range of butterfly species when it is catastrophic, such as hurricanes or heavy flooding - and then it will only impact on a local scale," said Jason D. Weintraub, manager of the Academy of Natural Sciences' butterfly and moth collection.

Butterfly health is much more affected by what people do, Weintraub added, citing "widespread and often indiscriminate use of supposedly benign, 'ecofriendly' pesticides, such as Bacillus thuringiensis."

Bt, as it's known, is used to combat pests such as the gypsy moth, but Weintraub said it hasn't been adequately tested to determine its toxicity to nontargeted moths and butterflies.

Besides spraying, the butterfly association's Glassberg has concerns about farms that raise and sell butterflies to schools and for celebrations like weddings.

"These well-meaning but unsuspecting folks wrongly think they're increasing the number of butterflies out there and doing something good for the environment," said Glassberg, who warned that farmed butterflies, raised in close quarters and "intentionally dumped into the environment" after use, spread disease to natural butterfly populations and decrease their genetic fitness through breeding.

If you want to help butterflies, which themselves are pollinators and bird food, Glassberg recommends planting things they like in your garden - milkweed, for example. "You will actually increase the number of butterflies in the world," he said.