Northeastern Forests in Peril

Pests and disease can cause catastrophic damage to trees. They have forever altered forests of the northeastern United States. This StoryMap is my attempt to raise awareness around two invasive insects that are currently ravaging vital tree species. I want people to understand the threats to our forests and how to recognize the signs. But before I introduce these bugs, I should mention the American chestnut and an event that has been called "the greatest ecological disaster to strike the world’s forests in all of history" (The American Chestnut Foundation).

For thousands of years, the American chestnut dominated eastern forests with a population of nearly four billion trees. In 1904, a fungus native to Asian chestnuts was discovered in the New York Zoological Garden (Penn State). The fungal blight spread rapidly to native species and within fifty years, nearly every American Chestnut in the country had been killed. Today, chestnut saplings can live for about five years in the wild before the blight splits them apart (see image to the left). Almost no mature chestnuts still exist in our forests.

The history of the American chestnut is a tragedy, but there is no time to mourn its loss. Invasive threats continue to plague our native trees, and it is now two miniscule insects that threaten to wipe out more species.

Emerald ash borer (EAB) are a tiny green beetle native to northeastern Asia. Believed to have been accidentally introduced to the United States burrowed in wooden shipping crates, EAB was first discovered after severe ash tree mortality in Michigan in 2002 (U.S. Department of Agriculture, 2016).

As the map below shows, EAB spread rapidly throughout the eastern U.S. since its introduction. Today, in a land without natural predators and where all sixteen native ash species are without resistance, the beetles are wreaking havoc.

Infestation by EAB is a death sentence for an ash. The female beetle lays eggs within the bark of a healthy tree. Larvae hatch and begin to feed on the cambium (inner bark), burrowing in zigzags within the tree. These tunnels sever the tree's ability to transfer water and nutrients and eventually lead to its death (U.S. Department of Agriculture, 2016). After months of feeding, the larvae grow into metallic green beetles and exit the tree, leaving D-shaped holes in their wake.

These exit holes, along with the tracks left in the wood, are telltale signs that an ash tree is infested with EAB. Other symptoms can include foliage dieback, "blonding" (see photo below), and sprouts from the base of the tree.

Unfortunately, these signs are usually only visible when an ash has become heavily infested (U.S. Department of Agriculture, 2016). Early detection of EAB can be difficult and an infestation isn't often detected until it is too late to save the tree.

The future for the North American ash is bleak; tens of millions of tree have already been killed. And as the map above shows, that devastation will only grow worse. However, that won't stop us from fighting back.

There are several control methods being tested to combat EAB . Chemical controls include insecticide injections or spraying of ash trees to help protect them against future infestations and kill any EAB already present. The insecticides are designed to kill beetles and eggs but not hurt the tree.

There are also biological control methods. Researchers have identified several species of parasitic wasps that feed on EAB larvae and eggs. One of these wasp species has already been successfully released and scientists plan to release more after conducting further research. The hope is that the wasps will reproduce in the United States and reduce EAB numbers over time (U.S. Department of Agriculture).

The hemlock woolly adelgid (HWA) is a minuscule insect currently devastating two species of hemlock in eastern forests. Closely related to aphids, this creature is so small that it is barely visible to the naked eye.

Native to East Asia and the Pacific Northwest, HWA was discovered in a park in Richmond, Virginia in 1951. The invaders were slow to get going, but once they entered natural forests thirty years later, they ran rampant through the vulnerable population of eastern hemlocks, killing thousands (U.S. Department of Agriculture, 2020).

HWA are so small that detection can be difficult. The most obvious sign of an infestation are the presence of egg sacs along the branch of a hemlock. These egg masses are small white masses, and fluffy like cotton balls.

After hatching, adelgids begin feeding on the sap of the hemlock. The insect pierces tissue vital to the hemlock's nutrition dispersal and possibly injects a toxin which researchers believe may be the primary cause of death for the tree (U.S. Department of Agriculture, 2020). Other signs to look for in an infested hemlock is graying needles and branch mortality.

Tens of thousands of hemlocks have already been killed by HWA. But, like with emerald ash borer, natural resource professionals across the northeast are fighting back.

Unlike EAB, however, the hemlock woolly adelgid has no known parasitic species anywhere in the world (U.S. Department of Agriculture, 2020). For biological controls, researchers have instead focused on releasing predators that have been known to feed on HWA, including a native species of ladybug beetle.

Chemical controls have proven to be moderately effective for treating individual trees in non-sensitive areas. Insecticides or horticultural oils can be used to help prevent further infestation (U.S. Department of Agriculture, 2020). Unfortunately, there are still very few effective methods for protecting hemlocks growing in natural forests, but researchers are working hard to find host-resistant trees and new methods of control.

The loss of the American chestnut changed eastern forests forever. Today, with the rapid decline of ashes and hemlocks, we are seeing our forests change once again. Forests do adapt, but the entire ecosystem suffers. Hemlocks and ashes are crucial species in a northeastern forest. Many animal species rely on them for food and shelter, and the holes dead trees leave behind in the canopy are often filled by invasive plants. It's crucial to remain educated and aware of threats to our trees, to understand that forests are a sensitive ecosystem, and to realize how easily a tiny insect or fungus that does not belong here can wipe out an entire species.