Scouting and Control of Common Pests of Fraser Fir
The balsam woolly adelgid (Adelges piceae), (BWA), is probably
the most damaging pest of Fraser fir, as well as the most difficult to
control. Scouting is a very cost efficient practice because it affords
early detection and replaces costly preventative sprays. Early detection
results in the best control.
The balsam woolly adelgid, like most Fraser fir
pests, is a very small, hard-to-see insect. Many growers have chosen to
spray on a regular basis to be "safe rather than sorry." However, a BWA
treatment can cost $300 to $500 per acre and can create future problems
with the spruce spider mite because of effects to the natural predators.
Scouting offers a cheaper alternative that allows the grower to treat only
when necessary.
The goal of scouting is to find the insect as soon
as possible after it comes into a block of trees. Though BWA spreads slowly,
it often goes undetected for a year or more as tree symptoms may not appear
for more than 6 months after trees are first infested. By treating trees
when the first infested trees show symptoms, the grower has a better chance
at field-wide control and will probably not have to cull any trees. If
the infestation is localized to only one portion of the block, spot treating
that area may also be an option.
Symptoms. Careful attention to tree symptoms
can clue a scout into early infestations of BWA. A crooked terminal or
top is usually the first symptom of BWA infestation and the easiest symptom
for the scout to find. Healthy tops should be straight after the new growth
hardens off in July. However, when a tree is infested with BWA, the top
can be anywhere from a slight angle from vertical to almost horizontal,
and is usually shorter than on a healthy tree. Crooked tops are often accompanied
by shorter, weaker, and/or drooping new growth over most of the tree.
Infested trees, when rocked back and forth, will appear stiff. This is the result of another symptom that only becomes apparent when the tree is cut. Trees with BWA will have growth rings with red, hard reaction wood. Fraser fir produces a single growth ring each year. Counting the affected growth rings can show how long the tree has had BWA.
If the tree has been infested for more than a year,
other symptoms become apparent, such as dead shoots or branches and swelling
around the shoot nodes (gouting). Drought can aggravate the expression
of BWA symptoms.
In some years, BWA-infested trees appear to have
better color than healthy trees in early summer. The BWA-infested trees
harden-off sooner than healthy trees. Because their over-all growth is
stunted, they only appear to have better color than the healthy trees which
are still actively growing and will have a lighter green foliage.
Pest description. All stages of the
BWA are small, requiring a hand lens to see well. The adults are covered
with a white, waxy covering that can be seen with the naked eye, but the
presence of the insect under the waxy covering must still be confirmed
with a hand lens.
The BWA has an unusual life cycle. In the United
States, all individuals are females. The adult females lay eggs that
are genetically identical to herself. These eggs hatch within a month to
produce the mobile crawler stage. This is the only stage which can move
on the tree or be blown to adjacent tree. Once the crawler finds a suitable
feeding site, the feeding tube is inserted into the bark of the tree and
the insect never moves from that spot. The crawler molts in place to the
nymph, and finally to the adult. There are two to three generations every
year. The BWA overwinters as the nymph.
Crawlers are cinnamon brown. Since they must find
a feeding site within 2 days or die, they are not frequently observed.
Crawlers can be present from April through October.
Nymphs are black with three rows of short, white,
waxy rods along either side of the body and down the middle, and multiple
rows of rods running at intervals across their bodies. The nymphs are not
readily seen with the naked eye. In the winter and early summer after the
first generation is produced when all the adelgids are in the nymphal stage,
it can be difficult to identify a low level of BWA infestation in Fraser
fir.
The adult adelgid is slightly larger, and more plump
than the nymph. She is also black though this is not readily apparent since
she is covered with much longer white rods that resemble wool. The woolly
covering can be pulled away from the insect. Though the adults do not move
from the feeding site, they still have legs, which can be seen to move
under a microscope.
Looking for balsam woolly adelgids
The eggs are laid in a clutch behind the females. Eggs are oblong
and cinnamon brown, like the crawlers.
Similar appearing problems. There are many
other problems that can mimic BWA. One of the most common are trees that
have been improperly sheared to have multiple tops. This will often make
the uppermost top appear weak and crooked. In addition, trees that are
dying from Phytophthora root rot will often have weak tops, poor
growth, and dead branches. Trees grown on extremely windy ridges, or with
poor fertility, will sometimes have stiff trunks and red, hard reaction
wood. Because there are many causes of similar symptoms, the scout must
always find the insect on the suspect tree to confirm the presence of BWA.
Christmas trees near large untreated Fraser fir
The eggs of the balsam twig aphid can be mistaken
for BWA nymphs. Both are black with short, white, waxy rods. However, the
white rods on the twig aphid eggs are scattered over the egg while in the
BWA nymphs, they line up into rows. With a microscope, legs can also be
seen on the BWA nymphs.
When to scout. An experienced scout can look
for BWA any time of year. However, the easiest time to look for this pest
is from July through October. Any later in the year, the BWA are all in
the nymphal stage, which has not produced the thick, white, waxy covering,
and is, therefore, harder to find with the naked eye. From bud break until
July, it is difficult to identify trees with crooked tops because the new
growth has not yet hardened off.
Scouting method. Use the rigid block scouting
method to walk through the field. It is necessary to see the tops of every
tree and to see a full side of most trees. Examine closely any tree with
a crooked top, stunted growth, or dead branches. Push on the trunk of these
trees at 4 to 5 feet off the ground to determine if the trunk is stiff.
Then look on the tree trunk and branches to find the white wool of the
adult female. The entire trunk should be examined with special attention
to the trunk and branch unions. If trees are scouted in the winter when
adults are not present, look for swollen buds or stems on suspect trees
and examine them with a hand lens for the presence of nymphs.
Determining that BWA is in fact the cause of tree
symptoms is the most important aspect of scouting. To confirm the presence
of the insect, remove any bark with white wool on it with a pocket knife
and examine it under the hand lens. If the presence if insects is confirmed
on that tree, flag it so that it can be found later.
Treatment threshold. If a single tree having
BWA is found, a chemical treatment must be applied sometime before bud
break the following year, unless all the trees in the block are to be harvested
that year. Spot treatment is possible if only one or a few trees are affected
in one portion of the field. Assume that live woolly has spread to symptomless
trees within 100 feet of any tree with confirmed symptoms. Therefore, spot
treatments are only possible if the one or two trees showing symptoms are
close together.
If BWA is discovered in a field of trees where the
majority will be harvested that winter, delaying the treatment until after
harvest is often a good idea. Selectively harvesting trees with BWA will
increase the successful control of the pest in the remaining trees.
Scouting frequency. Scout trees once or twice
a year from the time they are waist high until harvest. Scout smaller trees
if they are near large, untreated Fraser firs.
Effects of weather. Weather can affect symptom
expression. If the weather turns dry and trees are under stress, especially
in the fall, wilting, needle shed, and branch death may increase. Trees
with BWA may exhibit very few symptoms until they are put under stress.
However, the weather seldom reduces BWA reproduction or spread in western
North Carolina.
Control. Before treating, cut and remove
heavily infested trees from a block (trees with a crooked top for 2 or
more years). This will greatly improve the longterm chemical control of
BWA. Also, remove any large fir on the property that cannot be adequately
sprayed.
The time of year treatments are made will determine
which pesticides are available for control. Remember some pesticides that
are usually used in the summer, such as lindane and Asana, are deadly to
the predatory mites of the spruce spider mite.
Treat trees with a high-pressure sprayer (at least
200 pounds of pressure) and 200 to 800 gallons per acre. Spray every tree
from opposite directions, covering the entire surface of the tree and especially
targeting the trunk. Spray a few trees, and then examine them to make sure
the entire trunk has been wetted.
Spraying trees
New growth damaged
In larger trees (over 10 feet), BWA is very hard
to control. When BWA is found in this size trees, annual spraying is often
required to maintain control.
Scouting after control. Post-treatment scouting
to evaluate the effectiveness of a treatment has become a valuable tool.
Scout the previously flagged infested trees 2 to 4 weeks after they have
been treated. Remove bark pieces showing wool of the BWA from several of
the worst trees, especially from areas on the tree where the spray was
less likely to hit, such as under branches or at the base of the tree.
Examine the insects under a microscope, pricking them with a pin to see
if they are still alive. If they are alive, some leg movement will be seen,
and when pricked, purple liquid will come out of them. Scouting hot spots
again 1 year after treatment is also strongly advised.