Forests face big challenges from small bugs

Cindy Ross

03/03/2009

The walnut trees along Blue Marsh Lake in Berks County, Pennsylvania were so enshrouded with insect webs last fall that the shoreline looked like a scene from a sci-fi movie.

When I questioned Chief Ranger John Cave, he consoled me. "Fall web worms are not a big deal in terms of forest health and I've yet to see trees die as a result of it," he said, adding that damage is insignificant because the trees have completed most of their photosynthesis for the year.

The similar, creepy-looking, insect tents that crowd the crooks of trees in the spring are spun by eastern tent caterpillars. The larvae venture out in the cool mornings and evenings to feast on leaves, and then hide in their tent during the day. The defoliated trees normally grow new leaves and suffer only minor growth loss.

Both eastern tent caterpillars and fall webworms are native insects. Their periodic outbreaks appear worse than they are, partly because their host trees tend to be edge and hedgerow species that grow along roads, so the webs are easily noticed.

These webby outbreaks seemed worse than normal this past year, and made me curious about what other pests might be damaging our forests.

What is scariest to many foresters is the unknown - insects moving north as global warming shifts the boundary lines of their populations and produces wide fluctuations in precipitation, and exotic insects being imported into native ecosystems from abroad.

"We could be facing a whole different sweep of critters," explains John Fiedor, Pennsylvania assistant state forester of field operations.

Some exotic pests are already established. Many people are aware of the grave plight of our eastern hemlock as it battles the hemlock woolly adelgid, a tiny insect native to Asia that is devouring this beautiful and important tree. Foresters are fighting the adelgid and collecting hemlock seed to protect the species.

Theemerald ash borer, now found in Pennsylvania, Maryland, and Virginia, is another example. It was introduced from Asia to Detroit in 2002. The larvae consume a tree's cambium layer, girdle the tree, and kill it within 2 years. The U.S. Department of Agriculture is so intent on destroying this beetle that it has adopted the radical practice of destroying every ash tree within a half mile of an infested tree. The borer has killed 25 million ash trees so far and threatens to kill most of the ash in North America.

Ash trees are common in our woods, are a valuable timber tree, and widely used to restore streamside buffers. Ash is also widely sold by nurseries as a street tree and for home planting. Although the borer is past the point of eradication, programs to halt campers from moving firewood and to destroy infested trees help prevent its spread.

The Asian longhorn beetle attacks maples. This critter arrived in the wooden frame of shipping crates, like many other wood-infesting insects. International trade agreements require heat treatment and fumigation of shipping crates, but only five percent are inspected.

"It's a nightmare situation," said Fiedor. "We've been bringing in invasive species since we colonized America, but free trade has caused an explosion. At least we're more aware of it."

Another devastating pest in our forests is the gypsy moth, an insect brought to Medford, Massachusetts in 1869. It isnow so prolific that it has become a part of the fabric of our forests. The caterpillars feed heavily on oaks and defoliate them. If defoliated often, the oaks die. A naturally occurring fungus disease caused the gypsy moth population to decline, and an extensive spray program provides some control.

"Oaks replaced the chestnut for wildlife food," said Dr. Donald Eggen, forest health manager with the Pennsylvania Bureau of Forestry. "There's nothing left after the oak that produces that kind of food."

Other threats to our forests include invasive plants, acid rain, high deer populations, and the segmentation of forests from roads, development, and logging.

"When we lose acreage," says Chris Asaro, forest health specialist for the Virginia Department of Forestry, "we create smaller parcels, with more edges, as opposed to large interior forests. The edges are the places invasive species attack. Once you open up the canopy, it allows shade tolerant, invasive plants like kudzu and multi-flora rose, to proliferate. Half of the green you see in the understory are invasive species."

"We have problems and threats," says Dr, Eggen, "But our forests are resilient and always changing."

It's easy to be pessimistic, yet we do have great forests in the Chesapeake watershed, and more of them than we did at the turn of the century, and that is cause for hope.