|
back
| home
University
of Rhode Island GreenShare Factsheets
Hemlock
Woolly Adelgid Adelges tsugae
The hemlock
woolly adelgid is a small aphid-like insect that feeds on several
species of hemlock (Tsuga spp.) in Asia, its homeland,
and in North America where it was introduced. To date, populations
of this pest in eastern North American forests have been unmanageable
(although new biological control programs offer promise). However,
hemlocks growing in nurseries and landscapes can be managed through
an integrated pest management (IPM) approach. This is important
because hemlock is a unique and versatile landscape species for
which there is no good substitute. An IPM approach for A.
tsugae on ornamental hemlocks includes carefully monitoring
for the presence of the adelgid, implementing various cultural
practices to enhance tree vigor and to discourage pest invasion,
using mechanical and chemical control measures as needed to reduce
adelgid populations, and promoting biological control by encouraging
the activity of natural enemies. A fully grown
adult of the hemlock woolly adelgid is only about the size of
a period on this printed page. However, this insect is easily
recognized during most of the year by the presence of a dry,
white woolly substance on the young twigs. This "wool" is associated
with all stages of the adelgid, but it is most abundant and conspicuous
during spring when egg masses are present. An egg mass resembles
the tip of a cotton swab, although somewhat smaller. Adelges
tsugae injures eastern and Carolina hemlock by sucking sap
and probably also by injecting a toxic saliva while feeding.
This causes the needles on infested branches to desiccate, turn
a grayish-green color, and then drop from the tree usually within
a few months. Most buds are also killed, so little new growth
is produced on infested branches. Dieback of major limbs usually
occurs within two years and progresses from the bottom of the
tree upwards, even though the infestation may be evenly distributed
throughout the tree. Trees often die within four years, but some
survive longer in a severely weakened condition with only a sparse
amount of foliage at the very top of the crown. These weakened
trees are unsightly and have little chance for recovery. They
often fall victim to wood-boring insects and diseases and are
readily broken and thrown by wind. Hemlock woolly
adelgid completes two generations per year on hemlock. During
March and April, adults of the overwintering generation lay 50
to 300 eggs each in a cottony mass on the young twigs. Nymphs
(called crawlers) hatch from these eggs in April and May. Within
a few days, they settle on the twigs near the base of the needles
where they insert their piercing and sucking mouthparts. There
they feed and remain throughout their development. This spring
generation matures by the middle of June. Some of the adults
produced at this time are winged individuals that are unable
to reproduce on hemlock. Other adults produced at this same time
are wingless and are able to reproduce. In the middle of June
these wingless adults lay 50 to 300 eggs each in a cottony mass
on the twigs. Crawlers hatch in early July and settle on the
new growth. They soon become dormant until the middle of October,
when feeding resumes. Nymphs feed and develop during the winter
and mature by spring. Adelges
tsugae has been spreading relatively rapidly in North America
even though its life stages are wingless and are firmly attached
to hemlock twigs by mouthparts for most of the year. Eggs and
crawlers, the only stages that are unattached, are abundant
from March through June when they are readily dispersed by
wind, birds, deer and other forest dwelling mammals, and humans
during logging and recreational activities. Moving of infested
nursery plants could also facilitate the spread of this pest. Because hemlock
woolly adelgid can damage trees so quickly, it is important to
detect infestations early and to implement a management program.
Frequent visual inspection is the most effective means of determining
whether or not a hemlock is infested. For most of the year the
dry, white "wool" produced by the adelgid on the twigs is quite
conspicuous in contrast to the dark green needles. It is particularly
noticeable on the undersides of the young twigs. A new infestation
could be difficult to detect during July through September when
adelgids are small dormant nymphs with very little "wool" associated
with them. However, periodic examination of hemlocks throughout
the year would minimize the chance of a new infestation being
overlooked. Further evidence of an adelgid infestation is the
thinning or grayish-green (not red or yellow) color of the needles
on some branches. Usually by the time these symptoms appear,
the tattered "wool" of a previous adelgid generation is also
present on the branches. A number
of cultural practices may be effective in reducing the risk of
hemlocks becoming infested by hemlock woolly adelgid. Because
birds, squirrels and deer are important dispersal agents, any
effort to discourage these animals from visiting hemlocks will
reduce the risk of those trees becoming infested. Care should
also be taken when moving plants, logs, firewood or bark chips
from infested areas onto an uninfested property, especially from
March through June when adelgid eggs and crawlers are abundant.
Cleaning vehicles, clothing, etc., after visiting forests, recreational
areas, parks or other properties with infested hemlocks is also
advisable during this period. Infestations of Adelges tsugae often
start on large hemlocks that intercept the prevailing wind or
that are especially attractive to birds and other wildlife. When
such a tree becomes heavily infested, it can serve as an effective "launch
pad" for adelgid eggs and crawlers. Selective removal of these
heavily infested reservoir trees from the immediate vicinity
will retard the establishment of new infestations. Trees that
are growing in poor sites or experiencing stress from drought
and other agents succumb to adelgid attack more quickly. Therefore,
maintaining good growing conditions can play an important role
in the survival of hemlock. Because hemlock is a shallow rooted
tree, it is particularly prone to stress when precipitation is
abnormally low. Therefore, during periods of drought, trees should
be watered as often as needed to ensure that they receive 1 inch
of water per week (including rainfall) over the area beneath
the dripline of the crown. Water should be applied slowly so
that the roots will be soaked thoroughly. Pruning may also be
of some value in improving the health of hemlock. Removing dead
and dying branches and limbs from hemlock will promote new growth
by allowing more light to reach the foliage, and will reduce
the likelihood of attack by other insect pests and diseases.
Although applying fertilizer may improve the growth and vigor
of uninfested trees, fertilizing infested hemlocks with nitrogen
also enhances adelgid survival and reproduction. As a result,
a fertilized hemlock becomes more heavily infested and more severely
injured than an unfertilized one. Eggs and crawlers
of hemlock woolly adelgid are readily dislodged from the young
hemlock twigs by wind and rain. Most of these dislodged individuals
are unable to find their way back onto the tree and die. Therefore,
intentionally dislodging eggs and crawlers by directing a strong
stream of water at infested branches periodically during April
through June may be of some value in an integrated approach to
managing hemlock adelgid populations. Clipping heavily-infested
twigs from branches will also reduce adelgid populations on a
tree. However, extensive clipping may have undesirable effects
on the appearance and health of the tree. Two Japanese
and two western North American hemlock species are much more
resistant to hemlock woolly adelgid than are their eastern North
American counterparts. Of these four resistant species, the western
hemlock (T. heterophylla) is most similar to eastern hemlock
in appearance, growth form, and utility. Although adelgids do
infest these resistant species, they seldom reach densities high
enough to cause injury. Therefore, planting resistant Japanese
and western hemlocks should reduce the impact of the adelgid
in the ornamental landscape. However, the long-term success of
these exotic hemlocks in the forests of eastern North America
has not been evaluated. There are
a number of insects that can be found feeding on the hemlock
woolly adelgid in eastern North America, but overall they have
not been able to keep up with adelgid reproduction. The Halloween
lady bug, Harmonia axyridis, a recent arrival in the Northeast,
is now found in considerable numbers on infested hemlocks, and
may be having a beneficial impact. More promising is the current
research on natural enemies from Japan and China. Of the several
new predatory beetles that have been discovered in Asia in the
past few years, the one that has been researched the most is Pseudoscymnus
tsugae. It has been released in several states and appears
capable of permanent establishment in the Northeast. These insects
are not yet available for homeowner distribution, but they offer
hope for the future, and may influence homeowners to maintain
their hemlock trees for a few more seasons while the potential
for biological control is evaluated. The use of
chemical pesticides is an essential component of any integrated
approach to managing populations of hemlock woolly adelgid. Even
though cultural control measures can significantly reduce adelgid
numbers on hemlock, infested trees are usually unable to survive
for more than a few years unless chemical pesticides are applied.
It is important to understand at the outset that hemlocks will
need to be protected from the woolly adelgid as often as necessary
until the danger has passed. This may be for a period of several
years. Therefore, the initial decision on whether or not to use
chemical control measures should consider the value of the trees
relative to the anticipated cost of protecting them over the
long term. It may be advisable to identify individual trees or
groups of trees that have special value or significance on the
property and to concentrate control efforts on those trees. This
may be more successful than the overly ambitious approach of
trying to save all the trees at first, only to lose them all
when the resources have been depleted a few years hence. Several pesticides
are registered for control of hemlock woolly adelgid. Some are
available for homeowner use, while others are available for commercial
use only by a licensed arborist. Because each of these pesticides
has a relatively short life in the environment, treating an uninfested
tree with pesticide offers no protection from invasion by hemlock
woolly adelgid. Therefore, hemlocks should be treated only when
an adelgid infestation is known to be present. Before applying
any pesticide, read the product label very carefully. It will
provide important information on safety, toxicity, and methods
and rates of application. Also be aware that regulations governing
the use of pesticides for controlling hemlock woolly adelgid
may differ between states and are subject to change. The most common
and effective method for controlling hemlock woolly adelgid on
ornamental hemlocks is to thoroughly drench infested trees with
horticultural oil, insecticidal soap or any one of numerous petrochemical
insecticides that are specifically labeled for this use including
diazinon, fluvalinate, imidacloprid and malathion. Horticultural
oil and insecticidal soap are used most often because they are
highly effective in killing adelgids, and yet they are relatively
safe to the applicator, to beneficial insects, and to the environment.
Unlike the petrochemical insecticides which kill insects indiscriminately
by contact or ingestion, the oil and soap selectively kill soft-bodied
insects, such as adelgids, by "suffocation" rather than by poisoning.
Fortunately, it is unnecessary to target a particular life stage
of the adelgid for control; all are equally susceptible. Therefore,
pesticide sprays can be applied at any time during the year,
weather permitting. Two thorough spray treatments each year are
necessary for most situations.
Adapted
from the University of Maryland Cooperative Extension and
Dr. Mark S McClure, Connecticut Agricultural Experiment
Station,
1999
Pesticides
are poisonous! Read and follow all safety precautions on labels.
Handle carefully and store in original containers out of reach
of children, pets or livestock. Dispose of empty containers
immediately, in a safe manner and place. Pesticides should never
be stored with foods or in areas where people eat.
When trade names are used for identification, no product endorsement
is implied, nor is discrimination intended against similar materials.
Be sure that the pesticide you intend to use is registered for
the state of use.
The user of this information assumes all risk for personal injury
or property damage.
For more
information, call the URI CE Gardening and Food Safety Hotline
at 1-800-448-1011 or (401)874-2929 from outside Rhode Island;
Monday-Thursday between 9 am and 2 pm.
University
of Rhode Island Cooperative Extension provides equal program
opportunities.
|