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Enterprise Team

TECHNOLOGY

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Biological Control of Hemlock Woolly Adelgid

in the Eastern United States

Mark McClure

BIOLOGICAL CONTROL

Forest Health Technology Enterprise Team Morgantown, West Virginia

FHTET-2000-08

January 2001

Forest

Service

United States

Department of

Agriculture
Federal Recycling Program

Printed on recycled paper

About the Cover

Photo montage shows (from top left, bottom left, and right): (1) white

cottony ovisacs, each containing up to 300 eggs – the most obvious sign

of infestation, (2) adult coccinellid (Pseudoscymnus tsugae) predator

feeding on ovisacs, and (3) HWA damaged trees. Photos are courtesy of

the author and Brad Onken of the Northeastern Area, State and Private

Forestry, Morgantown Field Office.

Acknowledgments

Thanks to Roberta Burzynski for editing the manuscript and Lennie Eav

for layout and design. The USDA Forest Service, Forest Health

Technology Enterprise Team, Morgantown, West Virginia provided

funding for printing this publication.

Information and Copies

For information concerning natural enemies of hemlock woolly adelgid,

please contact The Connecticut Agricultural Experiment Station at

(860) 683-4977, or for copies of this publication, contact

Dr. Richard Reardon at (304) 285-1566 (email: rreardon@fs.fed.us).

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Room 326-W, Whitten Building, 14th and Independence Avenue SW, Washington,

DC 20250-9410 or call 202-720-5964 (voice or TDD). USDA is an equal employment

opportunity provider and employer.
Biological Control of Hemlock Woolly Adelgid

in the Eastern United States

Mark McClure

The Connecticut Agricultural Experiment Station

Windsor, CT

USDA Forest Service

Forest Health Technology Enterprise Team

Morgantown, West Virginia
1

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States in the Eastern United States

Introduction

The hemlock woolly adelgid, Adelges tsugae Annand

(Homoptera: Adelgidae), is a small insect that feeds on

stored nutrients from young twigs of hemlocks. The resulting

loss of needles and new shoots can seriously impair tree health.

Populations of hemlock woolly adelgid (HWA) on ornamental

hemlocks can be controlled by applying chemical insecticides. In the

forest, however, the best hope of controlling adelgid populations is by

biological control using natural enemies (parasitoids, predators,

pathogens). At this time, there are no known parasitoids of HWA

while native predators and pathogens are not capable of maintaining

HWA populations at non-damaging levels. As an alternative, scientists

are placing emphasis on rearing and releasing a complex of non-native

predators.

Distribution

HWA is native to

Asia, where it is a

harmless inhabitant of

several hemlock

(Tsuga) species. It is

present in Japan,

India, southwestern

China, and Taiwan.

HWA (Figure 1) was

first observed in

North America in the

1920’s on western

hemlock (T.

Figure 1. Hemlock woolly adelgid ovisacs

containing eggs on the underside of a hemlock

branch. branch.
2

heterophylla) and mountain hemlock (T. mertensiana) in Oregon

and British Columbia. HWA has now been found from northern

California to southeastern Alaska where it is generally harmless

and not considered a pest.

HWA was first observed in the eastern United States around 1950 in

Richmond, Virginia. Since then, it has spread into 12 states on the

eastern seaboard from North Carolina to New England, where it is a

serious pest of eastern hemlock (T. canadensis) (Figure 2). HWA

also attacks Carolina hemlock (T. caroliniana), a rare species that

occurs in isolated pockets in Virginia, North Carolina, South

Carolina, and Georgia. In the past 15 years, the main front of the

HWA infestation has been advancing at a pace of approximately 15

miles per year. However, from time to time isolated infestations have

been discovered well away from the main front.

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

Figure 2. Native range of eastern hemlock (in green) and range of Figure 2. Native range of eastern hemlock (in green) and range of

hemlock woolly adelgid (in red) as mapped in 2000. hemlock woolly adelgid (in red) as mapped in 2000.
3

HWA spreads mainly as eggs and crawlers (or mobile immatures of

the insect that hatch from the eggs), which are carried by wind,

birds, other forest animals, and people. Because HWA survives in

central Japan where winter temperatures drop below –350

C and

summer temperatures exceed 400C, it will probably continue to

spread and threaten eastern and Carolina hemlocks throughout

much of their natural ranges in North America where similar

climatic conditions exist.

Biology and Damage Biology and Damage

On hemlock, HWA is parthenogenetic, that is, all individuals are

females capable of reproducing. HWA populations build rapidly

because a single female lays up to 300 eggs. Two generations of

HWA are produced in an annual cycle.

Feeding by HWA on eastern

and Carolina hemlocks causes

the needles to desiccate (dry

up), turn grayish-green, and

drop from the tree – usually

within a few months (Figure 3).

Most buds are also killed, so

little if any new growth is

produced on infested branches.

Once a tree becomes heavily

infested, dieback of major

limbs usually occurs within

two years, and trees often die

within four years. Some infested hemlocks survive longer with only

a sparse amount of foliage at the very top of the crown. Severely

weakened hemlock trees are unsightly and have little chance for

recovery. Weakened trees often fall victim to other insects and

diseases, are easily broken and thrown by wind.

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

Figure 3. Hemlock branch shows heavy

infestation of hemlock woolly adelgid. infestation of hemlock woolly adelgid.
4

Natural Enemies Natural Enemies

In Japan a predatory beetle and a mite help control HWA on the two

native hemlocks, T. diversifolia and T. sieboldii, and even on highly

susceptible eastern hemlock. A coccinellid or lady beetle,

Pseudoscymnus tsugae, discovered by the author in Japan in 1992,

is the most common and effective predator of HWA in that country

(Figure 4a-b). P. tsugae was found in 24 of 74 infested forests and

ornamental plantings sampled, where mortality was 86%-99% of all

adelgids. Both the larvae and adults of the beetle feed on all growth

stages of HWA.

The oribatid mite (Diapterobates humeralis) was found in 40 of the

74 sites, where the mite almost completely destroyed (91%-98%) all

adelgid eggs. The mite eats the woolly material surrounding the eggs,

but not the eggs. Thus, without the protection of the woolly material,

the eggs are dislodged and fall to the forest floor where they either

dry up or are eaten by ants and spiders.

Focus of Biological Control Focus of Biological Control

In the United States, there are only a few native predators, such as

chrysopid and hemerobiid lacewings, syrphid flies, and cecidomyiid

flies, attacking HWA. This complex of native predators has not

controlled HWA populations. For this reason, efforts since 1992

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

Figure 4a-b. Adult (left) and larval (right) stages of the coccinellid

beetle Pseudoscymnus tsugae.
5

have focused primarily on the development and use of introduced

biological control agents as a means to lessen the destruction of HWA

in the eastern United States.

Initial efforts by the author’s research team at The Connecticut

Agricultural Experiment Station, based in Windsor, CT, concentrated

on P. tsugae and the mite, D. humeralis. This mite (D. humeralis) is

no longer considered a viable candidate for biological control for

several reasons, including its inherent low fecundity. P. tsugae,

however, is amenable to mass culturing. Three or more generations

can be reared each year in the laboratory on live adelgids under

controlled temperature conditions.

P. tsugae possesses other qualities that make it an excellent biological

control candidate, as follows:

 It is adapted to a wide range of climate conditions.

 It strongly prefers to feed on adelgids.

 It has at least two generations per year.

 Its life cycle is synchronized with that of HWA.

 It has a high searching efficiency and dispersal ability.

China has also been explored for natural enemies and several lady

beetles were discovered feeding on HWA. Three of these, all in the

genus Scymnus, have been imported and are being studied under

quarantine by Dr. Michael Montgomery’s research team at the USDA

Forest Service, Northeastern Research Station, in Hamden,

Connecticut. Scymnus sinuanodulus, S. camptodromus, and S.

ningshanensis have different life cycles from one another, but all three

species produce only one generation each year. All three scymnids

favor HWA over other prey and all prefer to feed on HWA eggs, but

also will eat other HWA stages. Although having a single generation

will make it difficult to mass culture these predators, they are viable

candidates for biological control (Figure 5).

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States
6

Laricobius nigrinus, a predatory beetle commonly associated with

HWA in western North America, has been imported to Virginia for

study by Drs. Scott Salom and Loke Kok and their research team at

the Virginia Polytechnic Institute in Blacksburg, VA. Although this

derodontid beetle also produces only one generation a year, it does

have attributes of a biological control candidate. For example, L.

nigrinus feeding stages are well

synchronized with HWA spring

generation egg and nymphal

stages, it prefers to feed on HWA

over other closely related

adelgids and other homopteran

insects, its larvae feed primarily

on eggs while adults feed on all

stages, and it completes

development only on HWA

(Figure 6).

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

Figure 5. Adult coccinellid beetles (genus Figure 5. Adult coccinellid beetles (genus Scymnus) Scymnus).

Figure 6. Adult derodontid beetle

Laricobius nigrinus.
7

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

Introducing Predators Introducing Predators

So far, P. tsugae is the only predator that has been released into the

field to control HWA. From 1995 to 2000 the author’s research team

has released more than 160,000 adults of P. tsugae in hemlock

forests in three states: 18 forests in Connecticut, 2 in Virginia and 1

in New Jersey. Availability of beetles and the nature of the

investigation determined the number of beetles released each year.

To determine local impacts of P. tsugae, 2,000 to 3,000 beetles

were released in 1995 and 1996 onto trees in 15-acre forests located

in Connecticut. In 1997, to facilitate the establishment of P. tsugae

in the northern and southern parts of the adelgid infestation, more

than 5,000 beetles were released in several larger forests in

Connecticut and Virginia. Most of these sites were later augmented

with additional beetles to bring the numbers released to at least

10,000 beetles. Dispersal and long-term impacts over a larger area

are being evaluated in studies of releases from 1998 to 2000 of

10,000 to 20,000 beetles in 50-acre forests in Connecticut, New

Jersey, and Virginia.

Results have been encouraging when HWA densities on hemlock

trees in release areas were compared with those in similar control

areas (beetles absent in the area) at least half a mile away. In 1996,

adelgid densities on monitored branches in release areas were

reduced by 47%-87% in just five months by a starting population of

only 2,400 to 3,600 adult beetles, indicating a remarkable shortterm impact of P. tsugae on HWA (Figure 7) (see next page).

P. tsugae has overwintered, established, reproduced, and spread at

these release sites. In forests where P. tsugae has been present for at

least three years and where at least 10,000 adults were initially

released, beetles are now abundant and can be collected from

infested trees. Dispersal of P. tsugae over more than half a mile was

documented at one of the release sites in Connecticut. At new sites
8

Biological Control of Hemlock Woolly Adelgid

in the Eastern United States

where 10,000 or more P. tsugae adults were released in spring

2000, larvae were observed a few weeks later on release and nearby

trees indicating successful reproduction and dispersal. Adult

beetles remain on trees year-round, indicating a strong preference

for HWA and a high degree of cold-hardiness.

The encouraging results from the cooperative USDA Forest Service

State and Private Forestry and The Connecticut Agricultural Experiment

Station biological control project spawned an additional cooperative

effort with the Phillip Alampi Beneficial Insect Laboratory in Trenton,

New Jersey to mass-rear P. tsugae. In 1997, the author donated a starter

colony of P. tsugae to the Alampi Laboratory, and State and Private

Forestry provided annual financial assistance to support mass

production of P. tsugae.

Figure 7. HWA populations on monitor A populations on monitored trees at P. tsugae r elease sites. elease sites.
9

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

From 1997 through 2000, the Alampi Laboratory produced more

than 350,000 beetles for release in New Jersey and eight other

eastern states (Maryland, Massachusetts, New York, North Carolina,

Pennsylvania, Rhode Island, Virginia and West Virginia). Changes

in HWA density and hemlock health as well as the establishment,

reproduction and spread of P. tsugae at release sites in 10 states in

the Northeast are being monitored and recorded annually. Early

results from this multistate release effort indicate that P. tsugae has

established and overwintered successfully in several sites.

Although it is too early to assess the impact of P. tsugae on HWA

populations and hemlock health at the new release sites, certain trends

are evident. A string of relatively mild winters from 1995 to 1999, which

enhanced the survival and growth of HWA populations, together with

the severe drought of 1999 in the eastern United States, significantly

reduced hemlock health and hampered the HWA biological control

effort. However, a two-week period with sub-zero (F) temperatures in

January 2000 reduced adelgid populations by more than 90% at the

northern edge of its range in Connecticut, New York, and New Jersey.

The cold weather and the unusually cool and rainy spring and summer

of 2000 throughout much of the infested area have allowed hemlocks

to recover and flourish, which may enhance the effectiveness of

P. tsugae.

Outlook Outlook

In just a few years, scientists have made good progress in finding and

studying potential biological control agents for HWA. P. tsugae is the

only non-native predator released into the environment, and results in

the field have been encouraging thus far.

Additional studies are needed, however, to substantiate that P.

tsugae will be at least part of the solution to the HWA problem, and

to justify the additional effort and cost required to rear enough
10

Biological Contr gical Control of Hemlock Woolly Adelgid

in the Eastern United States

beetles for release throughout the HWA-infested area. Managers

hope that the beetles will reproduce and spread on their own from

release sites to other infested areas. Scientists are developing ways

to optimize and reduce the cost of establishing this and other

predators throughout the HWA-infested area and to discover other

good biological control candidates.

It remains uncertain if any one species will be able to control HWA

populations, or if a complex of released predators will be needed.
Contacts

For further information concerning natural

enemies of hemlock woolly adelgid, please

contact the Connecticut Agricultural experiment

Station by phone at (860) 683-4977.

For paper copies, please contact Dr. Richard

Reardon, USDA Forest Service, Forest Health

Technology Enterprise Team, Morgantown, West

Virgnia, by phone at (304) 385-2566, or via email

at rreardon@fs.fed.us.

You may also print a hardcopy by simply clicking:

File>print.
USDA Forrest Service

Forest Health Technology Enterprise Team

180 Canfield St.

Morgantown, WV 26505-3101

FHTET-2000-08

January 2001