Enemy follows invasive tree pest across the sea

By Beth Daley, Globe Staff

For decades, a tiny fly-like insect accidentally imported here from Europe has found a home in the region’s birch trees, creating a halo of brown, disfigured leaves that can turn homeowners’ away from the popular landscaping tree.

A birch tree with leafminer damage (Forestryimages.org)

But now, a team of researchers at the University of Rhode Island say the pest, called birch leafminer, is all but eradicated in the Northeast, thanks to a tiny European parasitic wasp they began releasing in 1989 that kills the leafminer. A URI recent survey showed that the leafminer was under control in Rhode Island, Massachusetts, Connecticut, New York, Pennsylvania and northern New Jersey.

Such biological control is periodically used in the Northeast to control invasive pests, from the lily leaf beetle which attacks Asiatic lilies in gardens to pests that attack apple crops. The concept, however, can be controversial because of the unintended consequences the deliberately released species can bring. For example, the cane toad was introduced in Australia in 1935 to control beetles on sugar cane crops but its population has grown unchecked, making the sometimes dinner-sized plate toads a national nuisance.

URI officials say the parasitic wasp they introduced in 1989 is a natural enemy of the birch leafminer and creates no other harm to the environment.

“We have not seen any (birch) damage in Rhode Island in four years,” Casagrande said, “and it is highly unlikely that we’ll see problems with it again.”

The birch leafminer arrived in the U.S. in 1923, probably in a shipment of plants and soil sent to Connecticut and soon spread throughout the Northeast and into the Midwest. The insect is not fatal to birches - it disfigures the tree making it unattractive.

In the 1970s, several European wasps were released to fight the pest, and one of them became established in the Mid-Atlantic States. Later, groups from URI, the University of Massachusetts, and the New Jersey Biological Control Laboratory released more of the wasps in their respective regions.

“It takes lot of people and lots of cooperation to pull (biological control) off,” said Richard Casagrande, URI professor of plant sciences.

“We’re seeing similar success with programs for purple loosestrife, cypress spurge, mile-a-minute weed, and perhaps, lily leaf beetle, but as you can see, it takes time…for complete control,” he said.