SOUTH PORTLAND — Fear of spreading the tree-killing winter moth has forced the South Portland Land Trust and the Community Garden Collective to scale back their popular spring plant sale this year.

The May 19 event won’t offer its usual array of favorite and unique plants culled from local gardens because experts say transferring garden materials from one yard to another is a primary way to spread the invasive and destructive insect.

Winter moth was first identified in Maine in 2011, possibly transported in garden materials from Massachusetts that contained contaminated soil, and has since spread along the coast from Kittery to Bar Harbor.

Hardest hit is neighboring Cape Elizabeth – where the tiny green caterpillar of the small gray moth has defoliated and killed 300 acres of oak trees – and it has spread into South Portland and Scarborough in recent years.

The land trust and garden collective will hold a smaller plant and bake sale, featuring seedlings started in clean soil by event organizers, at the former Hamlin School community garden at Ocean and Sawyer streets from 9 a.m. to noon.

The South Portland groups followed the path of the Cape Elizabeth Garden Club, which stopped holding spring plant sales last year and now sells flower bulbs in the fall to raise money for a college scholarship fund.

Male winter moths, like this one photographed in South Portland, are most noticeable because they fly, but the near-wingless female moths do the real damage to trees.

Organizers of the South Portland event were disappointed to cancel their garden-grown sale, usually held in the parking lot of the American Legion on Broadway. It typically featured about 1,000 plants dug from local gardens and was expected to raise as much as $4,500 this year, said Dugan Murphy, land trust program manager.

Some members prepared months in advance, separating and potting pieces of established perennials during the previous season. For many it was a community-building event that made yardwork more enjoyable for experienced gardeners and beginners alike.

“This is one of our members’ favorite events each year,” Murphy said. “But the threat that the winter moth poses is too great to ignore.”

ADVICE FROM EXPERTS

The organizing committee decided to cancel the garden-grown sale after consulting with pest, horticultural and other experts, including the University of Maine Cooperative Extension. The move may help to stop the spread of winter moth well beyond South Portland, said Peggy Stewart, plant sale chairwoman.

“The fact that we sell such a large number of donated plants actually made the decision easier,” Stewart said. “Every year we have people who buy lots of plants to bring to their camps. We have landscapers who buy in bulk. We just have no way of knowing where all these plants are going.”

Some local organizations decided to go ahead with garden-grown plant sales this spring after taking precautions to stem the spread of winter moths and other pests, including the Cumberland County Master Gardener Program and the garden club at Congregation Bet Ha’am in South Portland. The garden club created a pamphlet to give buyers advising them to wash off and properly dispose of dirt on any perennials before planting in their yards.

Sharing plant materials from infested areas is a particular concern because much of the winter moth’s life cycle takes place in the ground, near the trees that it feeds on. It prefers oak trees but also will infest maple, ash, elm, aspen, cherry and apple trees, and cranberry and blueberry bushes.

The moths are most noticeable in early winter, especially in December. When outdoor temperatures rise into the 40s and 50s, they emerge from the pupal state and crawl out of the ground to mate and lay eggs.

With a 1-inch wingspan, the moths seen flying around porch lights and street lamps are male. In heavily infested areas, they can form huge clouds that look like it’s snowing. The female moths have vestigial wings and are flightless; they crawl up tree trunks and lay their eggs in crevices in the bark.

STEMMING THE SPREAD

The moths die soon afterward. The eggs can survive the harshest winter weather. Defoliation follows the next spring, when trees first put out leaves and outdoor temperatures hit the low 50s. The green, hairless caterpillars hatch from the eggs and feast on leaves before dropping to the ground, burrowing in and starting the pupal stage to form cocoons.

If a tree loses a significant amount of foliage at the start of the season, it will try to put out a second set of leaves, diverting energy from the usual growth process and weakening the tree. While trees can survive if they maintain at least half of their foliage, repeated and complete defoliation will kill a tree within a few years.

Government agencies and community groups have been fighting winter moth infestations in Maine for several years. Common tactics include spraying horticultural oil on trees; banding trunks with sticky tape that traps the female moths as they climb to lay eggs; and releasing parasitic flies that kill the moths in their pupal state.

Experts also recommend planting native trees that aren’t susceptible to winter moths and other pests, such as hickory, sycamore, sassafras, black gum, catalpa, hornbeam and linden trees.

Organizers of the South Portland plant sale would like to resume their garden-grown offerings in the future.

“We hope (to) limit further damage and devastation brought on by the winter moth within our immediate area (and) give us a better chance of bringing back the sale in future years,” Murphy said.