A tiny, exotic pest that is devastating forests of hemlock trees from the Carolinas to Maine has so far confounded scientists' efforts to check the destruction.
But forestry researchers at Oregon State University may have found a new weapon in the battle to save the giant evergreens in the eastern United States. They have identified two species of flies that are natural enemies of the hemlock woolly adelgid in the Pacific Northwest. The flies could be introduced to prey on the adelgid in the East after further testing.
Since the mid-1990s, the U.S. Forest Service has conducted mass releases in Appalachian states of several types of insects that eat adelgids, including more than 1.5 million Asian ladybird beetles. Despite the efforts, the infestation has continued spreading.
"We're losing just thousands of trees every year because of this pest," said Brad Onken, forest entomologist with the U.S. Forest Service in Morgantown, W.Va. "We don't have the natural enemy complex out here to keep it in check."
The eastern hemlock, known as the redwood of the East, grows to be a forest giant and can live up to 500 years. But tens of thousands of hemlocks in the eastern United States now bear the telltale signs of infestation -- cottony white adelgid egg sacs clinging to branches. The bare gray trunks of dead hemlocks are visible on mountainsides.
The adelgid, a native of Asia, was accidentally imported on nursery stock to the eastern United States in the 1950s. It is an aphid-like insect that sucks the juices from hemlock tree needles. Trees typically die within four to 10 years of infestation.
The hemlocks' loss threatens drastic changes to Appalachian forests, where the trees provide deep shade to keep mountain streams cool for trout and offer food and shelter for nearly 90 species of birds.
The adelgid has been present in Western states much longer, scientists say, and a larger number of natural predators prey on it, so it isn't causing the same problems. In addition, western hemlocks, which grow from northern California to southeastern Alaska, appear to have greater natural resistance. The western hemlock is related to the eastern hemlock, but it is a different species.
Two contenders
Darrell Ross, a scientist at Oregon State, said two species of Chamaemyiidae flies, a tiny silvery insect that feasts only on adelgids, offer good potential as natural predators to throw into the fight against the adelgids. The flies were identified through research funded by the U.S. Forest Service.
"They are as strong a candidate as some of the beetles that have been looked at and released," Ross said, noting that further testing will be needed before they would be released. "If things go well, it will pan out and contribute to efforts to control the hemlock woolly adelgid."
The researchers noted that flies within the same family have been used to control pests in Hawaii and Chile. The researchers' work appears in the April issue of Environmental Entomology, a peer-reviewed scientific journal.
Scientists are trying to find insects that prey exclusively on the adelgid to reduce the chances they will wreak unintended havoc if introduced. The search has taken scientists to the Pacific Northwest and to Japan and China, where the adelgid has lived much longer than in the eastern United States.
'Still searching'
"We're by all means still searching and looking," said Rusty Rhea, an entomologist with the U.S. Forest Service Forest Health Protection unit in Asheville. "We're looking at these flies and trying to evaluate what kind of role they might have."
The flies, which are so obscure they don't have a common name, were among a number of insects that Glenn Kohler, a graduate student at Oregon State, found feasting on adelgids during a 21-month survey of 116 infested western hemlocks in Washington and Oregon. The flies were among a number of predators found at infestations in the Pacific Northwest, indicating a diverse and abundant natural control system.
Studies of predators associated with infestations in Connecticut, North Carolina and Virginia found at least 10 species that commonly feed on adelgids. But the studies concluded the numbers of predators were too low to have a significant impact.
A beetle of promise
The most promising natural predator introduced to date has been a beetle from British Columbia, nicknamed the Lari beetle, Onken said. More than 50,000 Lari beetles have been released from Georgia to Massachusetts, including wide releases in the Great Smoky Mountains along North Carolina's western border. Scientists are seeing evidence that it is establishing itself.
Robert Jetton, hemlock conservation leader for CAMCORE, an international tree-conservation program at N.C. State University, said the identification of the flies is potentially significant. The flies comes from the same region as the Lari beetle, so it is part of the complex of predators controlling the adelgid in the Pacific Northwest.
"The other reason it's significant is no one of these biological controls is going to work on its own," Jetton said. "They all have their greatest impact at different times of year and in different climates. The more effective predators we can find and release, the better chance that biological control is going to work as a long-term solution."The News & Observer
