This article appears in the Winter 2024 issue
These are difficult times for our forests. Non-native pests and pathogens are causing great harm and, in some cases, nearly eradicating entire tree species. Emerald ash borer, beech bark disease, beech leaf disease, hemlock wooly adelgid, and spongy moth (formerly known as gypsy moth) are radically changing forest composition, leaving many dead and dying trees. The question is what do these changes mean for our yards and woodlands, and what does the future for our forests look like?
The array of pests and pathogens target and affect their host tree species in distinctive ways. Natural defenses to these foreign invaders have not evolved, and so the severity of outbreaks is usually high. Two examples are the almost complete loss of American elm and American chestnut in the early 1900s, both major woodland components throughout their range including New England.
Efforts to counter non-native pests and pathogens center around biological controls and genetic alteration. Success to date is limited, and the future for all the targeted host species—oak, ash, beech, and hemlock—is uncertain.
Here are some of the current members of the rogue’s gallery of pests and pathogens impacting forestlands in the lower Connecticut River watershed that will also likely impact species throughout the watershed.
Spongy Moths.
The spongy moth (Lymantria diaspora) invaded the oak/hardwood forests of the eastern seaboard after its introduction from Asia to the northeast early in the 20th century. The caterpillars defoliate trees, especially oaks, restricting the trees’ ability to photosynthesize. Severe outbreaks occur every twenty to thirty years. The last outbreak in southeastern Connecticut took place between 2014 and 2017. It was broken when a pathogenic fungus (Entomaphaga maimaiga) killed the caterpillars. This fungus, first released in the early 1900s as a way to control gypsy moths, was believed ineffective, but a damp spring in 2017 was conducive to the fungus’s rapid spread and the end of the outbreak, but not before severe oak mortality.
The emerald ash borer (Agrilus planipennis) is a recent Connecticut arrival. The female of this small, green beetle lays her eggs on the bark of ash trees. After hatching, the larvae move into the bark where they feed on the inner bark and cambium layer. As those parts of the tree are responsible for the transport of nutrients, the tree starves over a period of two to three years and dies. As there is no effective control, there is concern that the species could be extirpated from the region.
Peeled bark from emerald ash borer.
Beech bark scale disease occurs with the interaction between the beech scale insect, Cryptococcus fagisuga, and the fungus, Neonectria spp., which attacks the cambium layer blocking nutrient transport and giving the once-smooth bark a patchy, scaled look. Although trees survive limited outbreaks, the disease results in significant mortality and defects to trees.
Beech leaf disease showed up only three years ago making long-term projections as to species survival difficult. It is caused by a nematode worm that creates dark interveinal bands and leaf deformation. It appears to affect trees of all ages, and in combination with beech bark scale disease, is killing many of our beech trees.
Beech bark disease.
The woolly adelgid (Adelges tsugae), which attacks eastern hemlock, is an aphid-like sucking insect introduced from Japan and first seen in New York in the late 1960s. At first, infested and untreated hemlocks were expected to die within four years, however even heavily infested areas in southern Connecticut are experiencing some improvement in that not all trees are killed outright. A ride through Devils Hopyard State Park in East Haddam, Connecticut, for example, shows areas with complete mortality as compared to other areas where infested trees appear to be making a comeback. One hope for arresting the pest lies in biological controls using predators, such as ladybeetles, and parasites brought in from their native Japan. Field studies being conducted at the Connecticut Agricultural Experiment Station are in their early stages, and any widespread success is at least a few years away.