By Rhonda Morrison
As the 21st century has taken hold, more and more trees in our forests are becoming extinct due to increased transport of invasive species and particularly, non-native insects that feed on healthy trees. In the US alone, there are 126 invasive species and 13 forest pests killing trees at an alarming rate in our forests. In WNC this is happening to two of our most treasured trees, the Carolina Hemlock and Eastern Hemlock. These once mighty and beautiful trees are dying by the thousands due to aphid like bug called the Hemlock Wholly Adelgid brought here from Japan on nursery stock in the early 1950’s. In a healthy ecosystem, the natural checks and balances in nature keeps predators like the Hemlock Wholly Adelgid (hereafter referred to as HWA) in check. However, as Bud Mayfield, Research Entomologist of the Forest Service at The Southern Research Center states, “when trees have not co-evolved with the pests, there is no natural defense.” Hence the healthy ecosystems we once relied upon are now disappearing. The Chestnut blight and the widespread death of Ash Trees, both also decimated by foreign parasites brought from Asia, are examples of the slippery slope of gradual extinction. If this trend continues, our lovely and valuable Hemlock trees will be next.
Can we save the Hemlocks? Will the forests of the Southern Appalachians and the eastern US now be robbed of yet another important species? Unfortunately, the answer is a complicated one dependent on forest professionals, scientists, conservationists, and volunteers working tirelessly to save the Hemlocks from what looks like certain extinction from the HWA. Presently the HWA is now in 18 eastern states from Maine to Georgia and covers half the range of native Hemlocks spreading 10 miles a year to the west and north. Sunnier, hillier, and colder areas have less Hemlock mortality.
Carolina and Eastern Hemlock are foundation or keystone trees that maintain a large quality of life around the streams and rivers in WNC (and all through the Southern Appalachians) supporting food for 120 species of vertebrates and almost as many birds as well providing shade for trout to prosper in cold water streams. A loss of Hemlock means less trout, period. Andy Dolloff, Research Fishery Biologist from Virginia Tech says “on a scale from 1-10 for supported life around Hemlock stands, Hemlocks are a solid 10 because so much life depends on their existence.” The HWA was discovered in North Carolina a decade ago, but have been present in other parts of the Southern Appalachians for much longer and if Hemlocks keep dying at the rate they have been the last few decades, the entire ecosystem will change before our eyes and a way of life in the local forests will be gone forever. Already forests such as The Joyce Kilmer Forest near Richardson, NC have chosen protect hikers from falling Hemlocks by placing explosives on the trees and detonating them leaving discolored jagged trunks in an effort to produce a “windswept” natural storm look. The massive die-off of Hemlocks in our local forests do not look much better. Regardless, dying Hemlocks all over our local forests are sobering and depressing to Hemlock lovers and the creatures that depend on them.
What is the Hemlock Wholly Adelgid, what does it do, and why is it so deadly to WNC Hemlocks? The HWA is an aphidlike insect (a very tiny bug) only 1.5 mm at adulthood and can be identified by the copious amounts of white wax emitted from the female that looks like tiny cotton balls (egg sacs) affixed to the base of the needles on Hemlocks where they feed. Literally this blob of protoplasm inserts and pierces the needles of the mighty Hemlock by sucking and feeding on the actual carbohydrates of the tree-all the nutrients in the xylem ray cells. Kurt Johnsen, Project Leader and Plant Physiologist from The Experimental Forest in Asheville, states that the HWA “stops the carbohydrates from transferring from the leaves to the rest of the tree and starves it to death.” The HWA also reduces water transport to the Hemlock. No needles, no Hemlocks. Needle death and loss of water is fatal for the Hemlocks.
According to Mayfield, the most deadly characteristics of this bug are that “it doesn’t need a mate to reproduce, produces two generations of eggs, and isn’t compatible to Hemlock hosts because it hasn’t co-evolved with the species naturally.” Since there are two generations of eggs that produces millions upon millions of tiny bugs who exist locally to kill Hemlock trees, even the healthiest Hemlock doesn’t stand a chance against these parasites because they have no natural defense.
Scientists and conservationists in the area are fighting the Hemlock Woolly Adelgid with both chemical and biological controls. Beginning in the early 1990’s insecticides called neonicotinoids began to be used applied by soil drench and soil and trunk injection. Horticultural oils are used as well in more protected areas or when one tree is affected. But insecticides are expensive, toxic, and not practical in large forests with Hemlocks. Because of the danger of insecticides, often only a small grove of Hemlocks in a protected area can be treated at a time. Most insecticides take from 1-3 months to be effective. Many scientists say that the environmental impact of treating the trees with insecticide and the environmental impact of doing nothing are both troublesome.
The second lines of defense are biological controls, most specifically predator beetles that feed on HWA. Biological controls can either be used with insecticides (a one two punch of both has proven to be effective), or independently. Two Laricobius beetles not native to the eastern US have been most effective. The Laricobius Nigrinus, discovered in 1997 (dubbed by some as “little Lari”), is native to the Pacific Northwest and feeds very effectively on the HWA. The second type of predator beetle is the Laricobius Osakensis, native to Japan and China and incidentally where the HWA came form, which was discovered in 2005 and has only had limited release since 2012. Area scientists have noted success using the predator beetles. Bud Mayfield reports “50% of the adelgid population was controlled using Laricobius Nigrinus in a forest study in GA for the winter months only. The Laricobius bugs are released onto the branches where they seek out the HWA and kill them. In addition, the newer introduction of Silver Flies has been promising. Mayfield notes that silver flies “feed on the summer generation of HWA and show promise as a way to treat the second generation of the HWA.”
The obvious problem is obtaining the beetles in large enough numbers to kill the millions of HWA. The problem with both predator beetles is obtaining clearance for their use from the Dept. Of Agriculture and then raising them to breed in an insectarium for mass release. This process is doubly hard when predator beetles must be imported from Japan. Scott Salom, professor of Forest Entomology at Virginia Tech notes “It is a major problem to get beetles over here to fight the HWA. First it is very expensive, then it take 2-3 years founding a colony to grow in large enough amounts to matter, and then the fitness of the colony must be tested.” That said, Bud Mayfield proposes that the “predator populations should increase slowly over time.” But what do we do about massive Hemlock death while we wait?
Jason Rodreiug of the US Forest Service in Asheville says, “HWA was discovered in NC 10 years ago. The Hemlock that have not succumbed to the HWA are in more isolated groves, perhaps in sunnier, often colder climes and in higher elevations.” One example of healthy Hemlocks are in Southern Highlands Reserve in Lake Toxaway, NC. where 500 healthy Hemlock trees still prosper. Reserve director Kelly Holdbrooks believes the reason they have so many healthy Hemlocks is because they are both very well cared for and because they have been lucky. For example a rainwater pond and irrigation systems are utilized at the reserve and horticultural oils are used as well. These trees are both isolated and protected in a way the forest is not.
Nothing can replace the Hemlocks and the hearty Rhododendron now growing en masse around dead Hemlock stands have proven to be a disappointing substitute for all the positive effects of a healthy Hemlock. There has been talk of planting more white pine (because they are super hardy) in dead Hemlock beds to provide shade-but that wood decomposes too quickly and would not give streams and forest floor consistent supplies of large wood the Hemlocks do. Also there have been attempts to restore dead Hemlocks with seedlings in experimental plots, which have been done on the Cold Mountain Game Lands and Dupont State Forest. In addition, efforts have been made to grow native Hemlocks in areas not infested with the HWA in places as far away as Brazil and as close as Arkansas. The idea would be to bring these healthy trees back and replant them once the HWA is under control. Besides Rhododendron, former Hemlock ecosystems are often replaced by birches, maples, and oaks. That said, the Hemlock ecosystem is in danger of being permanently lost.
Bud Mayfield thinks we are “gaining ground” against the HWA, but not fast enough. It will take time for the predator beetle population to accumulate fast enough for biological control and insecticides remain only viable for isolated areas. If more is not done and soon, the mighty Hemlock will quietly fade from our forests forever.