Invasive species have the ability to alter a wide array of natural processes which ultimately modify the way ecosystems function. The hemlock woolly adelgid (HWA) is doing just that. The HWA is an invasive insect introduced to America from East Asia in 1951 and has ever since been transforming the forest floor on the East coast. In the southern part of its range, this parasite is most ravenous, and quickly infests even the strongest eastern (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana). Ecosystems are dynamic congregations of species, and simply fill in niches that are open. Though, the fact holds that things are changing much faster with introduced species.
Unlike most closely related adelgids, HWA feed on starches stored in the hemlock’s tissue, not the tree’s sap. HWA do this by inserting their mouthparts into the base of the hemlock’s needles to feed on the stored nutrients located in the parenchyma cells. The tell-tale sign that there is indeed a HWA infestation are white ovisacs (tightly bundled cluster of eggs) located on the needle underside of low hanging branches. In the northern region of their distribution, the longer winters slow down HWA feeding but trees still succumb to the combination of this insect and other parasites/pathogens in 4-10 years. However, in the south, the demise of hemlocks occurs much faster, killing mature individuals in 3-6 years. This is a common trend with invasive species
Ganoderma tsugae parasitizing a weakened eastern hemlock
The destruction of our native hemlocks is as widespread as it is ecosystem altering for a series of reasons. The HWA escaped the natural predators they co-evolved with in their native regions. They occur at such great density on the east coast because most of our native fauna has not ‘learned’ to eat them. Learning a new food item can take centuries, time these trees simply don’t have. Also, the trees themselves have not evolved any defenses against an insect attack like this. In this human era, species that would never come in contact, clash and because they’re so evolutionarily removed, they have no means of defending or competing with the invaders. Since we caused this problem, we feel the need to help mitigate these ecosystem level changes.
With new genomic tools, we can locate and insert genes that code for HWA defenses. This however would be so expensive and time consuming it hasn’t even been seriously considered. Instead, we have opted to bring a few predators over from Asia that specialize in feeding on HWA. In 1992 a beetle from Japan (Sasajiscymnus tsugae) was found that exclusively fed on HWA. Since then hundreds of thousands of these beetles have been introduced as a form of biological control. You might think that introducing another species is crazy-it is. To combat economically and ecologically disruptive species quickly and affordably, we must sometimes risk unknown factors associated with the newly introduced species. Some say we should leave things alone and let nature take its course.
Young Ganoderma tsugae decomposing a dead hemlock
Last summer, I witnessed for myself nature taking its course in the Southern Appalachia. I saw the forest floor in a transitional state, changing from ecosystems heavily populated by eastern hemlocks, to something else. Dead hemlocks both standing and fallen, littered the land. The parasitic/saprobic hemlock varnish self-fungus (Ganoderma tsugae) was one of the most bountiful fruiting mushrooms seen all summer. This fungus is taking advantage of the weakened trees and helps HWA finish the job. This white rot fungus along with other opportunistic saprobes quickly decomposes these hemlocks so within 5-8 years there’s little debris is found as the tree's carbon becomes reabsorbed into the surrounding ecosystem. The shade tolerant hemlocks are quickly being replaced by an evergreen shrub called Rhododendron maximum. This American rhododendron casts much more shade than the hemlock produced so work is being carried out now to understand how this interacts with the other flora and fauna living from the forest floor.