The Tiny Yet Deadly Invasive Species That's Destroying Our National Parks

While climate change and pollution are top concerns regarding the environment, there are other (possibly sneakier) threats that damage the ecosystem, too. Several invasive animal species have destroyed ecosystems, such as feral swine, Burmese pythons, and zebra mussels. Throughout the national parks, another invasive species is wreaking havoc on the flora: the hemlock woolly adelgid (Adelges tsugae).

The hemlock woolly adelgid (HWA) is a small insect similar to an aphid. After hatching from eggs, the nymphs (also known as crawlers) are only about 0.2 millimeters in diameter (less than 0.008 inches). You can identify their presence by the round, white cottony clumps that they produce — an ovisac that can measure 1.5 millimeters to 6.35 millimeters. These sacs are most visible between November and July at the bases of the needles on hemlock tree branches — hence the name. When the crawlers grow into adults, their oval-shaped bodies are only about 1 to 1.5 millimeters long and vary from brown to red in color.

Interestingly, the generation of HWAs that hatch in summer enter dormancy and come out of that to reach adulthood in winter and lay eggs in the ovisacs they produced while progressing through their four stages of life. The next generation that hatches in spring only takes three months to reach adulthood before some of them grow wings. The winged insects fly off in search of tigertail spruce trees, but die when they find none. Meanwhile, the non-winged insects lay eggs at the base of the needles on hemlock trees. There, HWAs suck away the trees' nutrients as they grow and multiply, and they're leaving a trail of devastation across the eastern United States' national parks.

The HWA's diet is the reason these insects are such a threat to national parks. With piercing proboscises (a tubular mouthpart), they enter the tissues of the hemlock trees they grow on and feed on the nutrients stored there — mainly the starch reserves in the sap. Eastern (Canadian) and Carolina hemlock trees are the most vulnerable — dying as soon as four years after the infestation — because these insects aren't native to eastern North America.

Originating from Asia, the first description of the HWA in North America dates back to 1924. However, they cause little damage to the hemlock trees in the U.S. Pacific Northwest. One possibility for this is because those hemlock populations have built up a tolerance in the 100 years since the HWA first arrived. Another possibility is that these areas have native predator insects to the HWA — a balance that the eastern United States is lacking.

Researchers have discovered that the HWA found in eastern North America came from southern Japan, and the first sighting in the eastern United States was near Richmond, Virginia, around 1951. While the insects can't go far on their own, they disperse around forests via birds, other animals, people, and wind. After being identified in Shenandoah National Park in the 1980s, HWAs had already caused significant and widespread devastation by the early 2000s. The insects have been found all along the East Coast — from northeast Georgia to Canada — affecting about half of the entire eastern hemlock range.

When a species of any kind is at risk of destruction or extinction, organizations jump in to do what they can to mitigate and preserve those species. Scientists have fought back against invasive species many times in the past, such as with the European grapevine moth in California. And, management strategies are underway to protect the hemlock trees from the woolly adelgids.

One of these strategies is insecticide treatments, including soil and stem injections, soil tablets, and sprays on the foliage and trunks. While these are effective, they require a lot of manpower and are expensive to implement, making pesticides an unsuitable long-term solution. Biological control — such as introducing predatory insects — is a more viable, long-term option when it comes to low financial and labor investment. Some predatory beetles and silver flies have been released in eastern North America with promising results. However, more research is needed to determine if introducing predatory insects where the HWAs are rampant in the east will be ideal for widespread control.

Implementing cultural practices is another long-term tactic that scientists have already started using to mitigate the destruction of hemlock trees in the east. These include reducing environmental stress so that the trees can better tolerate HWAs, regenerating sites where trees have already died, and breeding hemlocks so that they're more resistant.