The branches of the massive eastern hemlock loom more than 30 meters above us, but instead of craning our necks to look up Tennessee state botanist Todd Crabtree draws our attention closer to the ground and the thumbprint-sized dot of yellow paint near the base of the tree. This little dab of color is the only indication that this tree, unlike many of its kind throughout the country, has a chance at survival.

Crabtree is leading a group of members of the Society of Environmental Journalists through the Fiery Gizzard trail in Marion County. We're in Chattanooga for the organization's annual conference and Crabtree is one of several guides we will have that day through the state's natural biodiversity. The Cumberland Plateau region, which stretches through Tennessee and other southeastern states, is a biodiversity hotspot with a greater concentration of species than in just about any other part of the U.S.

But that biodiversity faces increasing threats. Hemlock trees in particular —both the eastern (Tsuga canadensis) and Carolina (T. caroliniana) species — have been hard hit by an invasive bug called the Hemlock woolly adelgid (Adelges tsugae). These aphid-like insects first turned up in the U.S. from their native Asia in 1951. Since then they have spread from Virginia into 16 states, increasing their range about 15 miles a year. Wherever the bugs go, hemlocks die. In some places entire forests have been devastated.

The adelgid is a tiny, almost invisible insect — just 1.5 millimeters long — but it has a massive impact. In both its nymph and adult stages the bug sucks the life out of the hemlock trees, which have no natural resistance to the insects. Much in the same way that mosquitoes draw blood from animals, the adelgids draw out the trees' sap and feed on the stored starches it contains. Without that critical energy source, the trees suffer, stop growing and eventually succumb. (It's possible the adelgids also inject a toxin that hurts the trees while improving the insects' feeding.) Once a region is infested with adelgids trees begin to die in as little as four years.

Hemlock woolly adelgids do not appear to have arrived in Fiery Gizzard yet but they're close. They were first observed in Marion County in 2012 and exist in three of the four surrounding Tennessee counties (pdf). Crabtree tells us he is sure that the insects will make their way to the trail sooner rather than later.

That's where the little yellow dot comes in. There aren't many ways to control the spread of adelgids or to protect the trees, but one relatively successful method is the pesticide Imidacloprid. The chemical is either injected into a tree or in this case the surrounding soil, after which it slowly travels through the entire plant, a process that will take about two years for a tree of this size and protect it for up to three years after that. In theory, that will provide the tree with at least some resistance to the adelgid and may even save it. The dab of paint is an indication that this particular tree has been treated. It will need another Imidacloprid injection — a rather laborious process — in a few years.

The Imidacloprid treatment won't work for all hemlocks. Crabtree points out another tree that's growing in the middle of a nearby stream. The pesticide can't be used in this case, as it could travel down the water and have unintended consequences on other trees or insect species. If the adelgids reach that tree, it will almost certainly die.

We still don't know the full impact of the loss of these hemlock species. They provide habitat and shade for dozens if not hundreds of species and protect rivers and land from erosion. As the hemlocks die off, other trees which may not provide the same ecosystem service could move into the regions the big trees once inhabited. So far at least 50 percent of the U.S. eastern and Carolina hemlock trees have been infected. That number will only increase over time. Meanwhile, some species which depend on hemlocks are already disappearing.

All is not lost, however. Many organizations have teamed up to protect hemlocks on public land or to teach private landowners how to preserve trees on their properties (sometimes that's as easy as spraying them off with a hose – assuming your hose can reach the trees). In a few regions an imported Asian beetle has been released to control the adelgids, although that could also have other consequences if the beetles spread or eat anything besides the invasive adelgids. Scientists are also exploring adding DNA from other hemlock species that can resist the adelgid to the eastern or Carolina species, although Crabtree says that would take years to have an effect and could also have its own unexpected side effects as new genes enter an ecosystem.

Although most eastern and Carolina hemlocks may eventually die out, Crabtree tells us that some trees may continue to survive in pockets of land which the adelgids cannot reach. But even then, those pockets will be a shallow memory of trees that once filled entire forests and provided critical roles for their ecosystems and the other species that depend on them.

Photos: Eastern hemlock at Fiery Gizzard Trail © John R. Platt. Hemlock wooly adelgids during their visible egg stage by Kathie Hodge via Flickr (used under Creative Commons license). Hemlock forest with dead trees by Todd Pierson via Flickr (used under Creative Commons license)