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Controlling Adelgid and Scale in Our Forests

Plant Science Day 2002 Short Talk

Dr. Mark S. McClure
Chief Scientist, Valley Laboratory, Windsor
The Connecticut Agricultural Experiment Station
153 Cook Hill Road, P.O. Box 248
Windsor, CT 06095

Phone: (860) 683-4979 
Fax: (860) 683-4987 

Three species of adelgids and scale insects which are native to Japan are serious pests of eastern or Canada hemlock, Tsuga canadensis, and Carolina hemlock, Tsuga caroliniana, in the eastern United States. Eastern hemlock (shown here) is an important tree species in our forests and ornamental landscapes. Hemlocks can grow to more than 100 feet tall and can live for several hundred years. Hemlock comprises from about 11 to 55% of the timber in New England forests and is also commonly planted around homes for hedges and screens or as a specimen tree. Although it is not considered a particularly valuable timber species, it is becoming an increasingly important source of pulp and logs for international markets. Hemlock is of great ecological value. It is uniquely thrives in ravines, on steep mountain slopes and along lakes and streams. Hemlock stabilizes the soil, purifies ground water, moderates the temperature of nearby waterways, and provides a moist protective habitat for numerous plant and animal species.

In 1951, the hemlock woolly adelgid, Adelges tsugae, the egg masses of which are shown here, was observed in the eastern United States for the first time in an arboretum near Richmond, Virginia. It had probably been introduced accidentally from Japan on infested hemlock plants. In the three decades that followed, the adelgid spread westward into the Blue Ridge and then southward into the Appalachian Mountains, and also northward reaching Connecticut in 1985. By 1999 the adelgid had spread throughout Connecticut and could be found in all of our 169 towns.

Hemlock woolly adelgid is a small piercing and sucking insect, which prefers to feed at the base of the needles on the youngest twigs of hemlock. It drains fluids from the plant cells and probably also injects a toxic saliva during feeding. This causes hemlock foliage to discolor, desiccate and fall prematurely and infested branches die within a year or two. If adelgid populations remain uncontrolled, hemlocks usually die within 4 to 10 years depending upon the suitability of the growing site and other factors including weather. Unfortunately, no hemlocks have shown an ability to resist attack by hemlock woolly adelgid and entire hemlock forests have been destroyed such as this one in Litchfield County. None of the native predators, which inhabit our hemlock forests, have proven to be effective in controlling this exotic pest.

This map shows the 2001distribution of hemlock woolly adelgid (the brown and yellow shaded areas) relative to the natural growing area of hemlock (green shaded area). This year additional infestations have been reported in Tennessee, South Carolina and Georgia bringing the total number of states infested to 15 extending from Georgia to northern New England. Although hemlock woolly adelgid is sensitive to cold winter temperatures, I expect that it will adapt to increasingly harsh conditions as it spreads northward and eventually occupy most or hemlock’s natural range.

In 1992 I spent 2 months in Japan studying hemlock woolly adelgid in its homeland and searching for natural enemies. Among the predators that I discovered was this tiny ladybird beetle which was previously unknown to science. A Japanese colleague and I subsequently described and named this beetle Pseudoscymnus tsugae. Adult beetles are jet black and about the size of a poppy seed.

In 1994 my research team in Windsor began rearing and studying this ladybird beetle. We found that it can be reared continuously in the laboratory, that both larvae and adults of the beetle attack all stages of hemlock woolly adelgid, that its life cycle is very compatible with that of the adelgid, and that the beetle feeds only on adelgids. Based upon our intensive laboratory studies, the United States Department of Agriculture, Animal and Plant Health Inspection Service determined that releasing the beetle in the eastern United States was of no risk to the environment. Therefore, we released beetles for the first time in 1995 in a hemlock forest in Windsor. Data from this and subsequent releases in Connecticut, New Jersey, and Virginia revealed that ladybird beetles established a breeding population with as few as 2,500 adults released, that they spread from release sites, and that they can reduce hemlock woolly adelgid numbers on release trees compared with non-release trees by 47% - 87% in a single season. Based upon our encouraging results, the US Forest Service launched a cooperative study with us and the New Jersey Department of Agriculture to mass rear beetles for distribution throughout the eastern United States. During the past 5 years, more than 1 million beetles have been released at over 100 sites in 15 infested states.

My research team is responsible for rearing the beetles needed for releases in Connecticut and for evaluating the effectiveness of these predators in controlling adelgid numbers. This map shows the location of our release sites in Connecticut. We have released nearly 170,000 adult beetles in 21 hemlock forests representing all 8 counties. At many of these sites, we have been successful in establishing breeding populations of ladybird beetles and hemlocks have recovered following initial attack by hemlock woolly adelgid. Our early release sites in southern Connecticut included some areas that had been infested with hemlock woolly adelgid for several years and where trees were already damaged at the time of release. Unfortunately, beetles at some of these sites had insufficient time to build up their numbers before hemlocks were irreversibly damaged. In recent years we have concentrated our beetle releases in central and northern Connecticut where hemlocks are newly and only sparsely infested with adelgids, so that beetles would have the opportunity to increase their numbers before trees were irreversibly damaged.

The success of our beetle release efforts in Connecticut during the past five years, has been greatly influenced by a number of interacting factors, several of which are listed here:

1. Hemlock is a shallow rooted tree species, which often grows in thin, well-drained soils on steep slopes. This makes it especially drought sensitive. A prolonged, severe drought, which climaxed in the spring and summer of 1999 significantly compromised hemlock health at several of our poorer hemlock growing sites, despite the presence of beetles. This suggests that future beetle releases be done at sites, which provide good growing conditions for hemlock.

2. Hemlock woolly adelgid is sensitive to temperature below about -5 degrees F. A 2-week cold period in late January of 2000 killed from 85% to 100% of the adelgids throughout northern Connecticut and other areas of the Northeast. Beetles survived this cold period very well. As a result, adelgid populations declined sharply and remained low at release sites during the spring and summer of 2000.

3. Hemlock thrives in cool, moist conditions. The spring and summer of 2000, although not to good for vacations at the beech, was just what the doctor ordered for hemlocks that had been under attack by hemlock woolly adelgid. Hemlocks at our release sites and elsewhere flourished during 2000.

4. The winter of 2000 - 2001 featured average winter temperature and average adelgid mortality of about 60%. Adelgid densities generally remained low during 2001 at our beetle release sites and hemlocks produced abundant new growth.

5. The past winter of 2001 - 2002 featured very mild temperatures and very low adelgid mortality of less than 20%. This resulted in a slight resurgence in adelgid abundance by the spring of 2002.

6. Hemlocks produced abundant new growth in 2001 and 2002 and showed remarkable recovery. However, because hemlock woolly adelgid thrives on new hemlock growth, the stage is set for adelgid recovery as well. However, beetles are firmly in place and hopefully will be up to the task.

Another much larger multicolored ladybird beetle from Japan, Harmonia axyridis, shown here is helping to control hemlock woolly adelgid throughout the eastern United States. This beetle is a voracious predator of aphids, scale insects and adelgids and can develop on hemlock woolly adelgid. It is especially abundant on adelgid-infested hemlocks in the spring before its preferred aphid prey becomes abundant on other tree species. Unfortunately, this beetle has become a nuisance by congregating on houses in the fall seeking out shelters in which to spend the winter.

Two species of scale insects that were accidentally introduced into New York City from Japan in the early 1900’s are also serious pests of hemlock. The circular hemlock scale shown here is Nuculaspis tsugae; the elongate hemlock scale shown here is Fiorinia externa. Both scales feed on the undersides of the youngest hemlock needles and feed on the cell fluids through piercing and sucking mouthparts. If infestations of these scales remain uncontrolled, hemlock trees often die in about 10 years. Unfortunately, the ladybird beetles, which attack hemlock woolly adelgid, do not attack hemlock scales. Populations of these scales have been increasing at several of our release sites.

This small parasitic wasp, Aspidiotiphagus citrinus attacks both hemlock scales in Japan and in the United States. It consistently kills about 98% of the scales in Japan and is an excellent biological control agent there. However in the northeastern United States, this same parasitoid is ineffective because its life cycle is not synchronized with its scale hosts. Consequently, parasitism in Connecticut fluctuates between about 5% and 60% from year to year. While any level of scale mortality is helpful, the parasitic wasp is clearly not a solution to the hemlock scale problem at this time.

This native ladybird beetle, Chilocorus stigma, which is called the twice-stabbed ladybird because of its 2 red spots, also attacks both hemlock scales, but is also inconsistent in its efforts. Twice stabbed beetles can be abundant during some years and scarce during others on hemlocks that are heavily scale infested. I am now conducting studies to determine what factors limit the success of the parasitic wasp and twice-stabbed beetle and to evaluate their potential use in an integrated approach for managing hemlock scale populations.

A newly discovered and yet to be identified fungus that I observed at Myanis River Gorge Preserve in Bedford New York last fall may also eventually play an important role in the biological control of hemlock scale. In this poster the fungus appears as a black crusty substance on the surface of scales. By contrast, the brown individuals are healthy scales. I sampled scale-infested hemlocks along a 2 ½ mile transect through the gorge and found that this fungus, an ascomycete probably in the genus Hypocrella was attacking both hemlock species in significant numbers. Up to 12% of F. externa and 43% of N. tsugae were infected and had been killed by this fungus.

Dr. Jim LaMondia of the Station’s Valley Laboratory in Windsor and I have cultured this fungus, successfully inoculated hemlock scales, and recovered the fungus from infected scales thereby demonstrating its potential use as a biological control agent. This same fungus was collected from hemlock scales this spring at two other sites in New York, but I have not yet found it in Connecticut. The effectiveness of this fungus in controlling hemlock scale populations will be the focus of my ongoing investigations.

Our hemlock forests are being threatened by adelgids and scales which belong in another part of the world. Perhaps the best hope for controlling them here is to use some of those same natural enemies which do the trick in Japan. With the discovery and introduction of P. tsugae to combat hemlock woolly adelgid and with the new fungal pathogen of hemlock scales, perhaps the future of hemlock in our forests is less bleak than it appeared a few years ago.