They arrive in packing crates, hidden in the wood slats and shavings that cushion manufactured goods from the other side of the earth.

Sometimes they travel around the country in cordwood, delivered conveniently at suburban patios and campgrounds, where plenty of tasty new trees stand by, ready to play host.

They are the invasive insects that attack—and destroy—trees. Some are recent immigrants from parts unknown, while others are long-time natives of North America. Whatever their origins, they've become much more prolific and widespread in just the last few years.

What's spurring their growth? "It could be changing weather patterns... and warming climate conditions," reports Chukka Andersen, staff arborist for the nonprofit Tree Care Industry Association, based in Manchester, New Hampshire.

Landscape-chewing pests have always been with us, she points out, but they're usually kept in check by natural means. "With 'invasive' pests, we don't have those natural, or sometimes even chemical, controls [available] in the United States. Even with chemicals," she adds, "that's a last-ditch effort... [These pesticides] are pretty potent, and affect other insects and the birds that eat those insects. Chemical treatments can harm the rest of the environment."

Among the current crop of critters being observed more in New England is the woolly adelgid, which attacks the eastern or the Canadian hemlock. Moving up from the south, it is being seen along the local coastlines, such as Massachusetts' North Shore. "This one is hard to treat," notes Andersen, "It leaves a waxy cocoon under the needles, starting at the bottom branches. The cocoon material is sticky, and can be spread by birds. To halt infestation, pesticides must be applied [within a week or two] immediately after hatching. This treatment is very tricky ...done one tree at a time."

Another pest, the emerald ash borer, has been devastating stands of green and white ash trees through the Midwest, and it has found its way recently to the Northeast. Ironically, Andersen states, "These ash trees were planted to replace the elm trees that succumbed, many years ago, to Dutch elm disease. It illustrates how we need to diversify when replacing species [that have been decimated]."

Hardwoods such as our region's beloved maples have a growing threat in the Asian longhorned beetle. "This bug prefers maple, but moves on to beech and birch," warns Andersen. "It's a big bug, at over an inch long, and chews sizable holes" in its host tree. "Plus, it doesn't seem to have any enemies, such as birds eating it."

Even conifers are at risk. "We're seeing the Sirex woodwasp coming in," Andersen says. "This is a European version of a native wasp, and it attacks pines as well as spruce and fir trees. It bores into the trunk and deposits a fungus that serves as a nest for the wasp eggs. There are huge pine plantations in the southeast that could get wiped out by this pest.

"The problem with all these pests," she adds, "is that chemical controls are being applied to keep them in check, but they are taking off faster than we can keep up."

What's the real loss?

From a landscaping viewpoint in a condominium complex, why should anyone care if a few trees die off in the common areas? Andersen likens mature trees to assets with real value. "You can't just buy and replace a big tree," she says. They are worth thousands of dollars, and as aesthetic specimens, trees anchor any landscape design as well as provide shade and wind protection and temperature modulation. "You want to take care of your assets," she points out. "Trees can't be easily replaced, like a lawn."

Signs of infestation include "a general die-back, starting at the crown," except the woolly adelgid which starts at the tree's base. Visible symptoms may be "brown, droopy leaves as well as little holes in the trunk. In pines, you'll see streams of pitch [running down the trunk] and needle loss." However, by the time anyone sees symptoms of pest invasion, "the bugs have been there a long time... they've reproduced and moved on." It may be too late to save individual trees.

For that reason, Andersen advises prevention, or at least early intervention. "With the first signs of pests [such as] die-back in the crown... that's when to call in a tree specialist" [and] maybe catch it in time. "Insects invade when trees are stressed," she contends, "Healthy trees are much less likely to get attacked."

To keep trees healthy, she advises, "Keep them watered during low rain periods, with a slow soak for an hour... once a week." That soak should cover the tree's "drip line," which is the ground area beneath the tree onto which its leaves drip rainfall. It's important to keep lawn outside the drip line, she adds, since "grass out-competes tree roots for water." She recommends mulching beneath a tree, but being careful not to pile it up around the trunk or exposed roots.

"Always clear out broken branches and dead wood ... these are magnets for pests ... Use a fertilizer that's specified [for the particular tree] and prune properly to encourage new growth."

She points out that tree damage may be due to something besides insects. "Drought and weather extremes have effects on trees, Andersen says. "Anytime you do any construction, such as walls, sidewalks or patios, around trees, it can reflect on the tree's health... how the tree responds."

But when trees seem to be attacked by pests, she notes, "It may be possible to save individual trees, depending on the original health of the tree. First, determine if damage may be due to other causes.. If it's a bug, is it an invasive species? You must identify the insect and seek professional help."

Andersen advises gathering quotes, and assessments, from several tree specialists, "and make sure they have the right insurance... and training." Information is also available on her organization's website: www.treecareindustry.org.

If it's not too late, and you've saved a tree, she says, "Go back to the four steps for prevention—fertilize, prune, irrigate and mulch—to keep your trees healthy." They are the linchpins of landscaping and often one of a property's most visible assets.