| Forest pests, whether insects
or diseases, cause significant damage to our forest resources.
In Canada, annual losses from pests represent over 100 million
m3 of timber, which is equivalent to more than five times
the yearly production of timber in Quebec. From very early
on, researchers at the Laurentian Forestry Centre (LFC) of
the Canadian Forest Service (CFS) have been interested in
these pests. A number of our researchers are working to identify
and understand forest pests and develop effective measures
of controlling them. This article will provide an overview
of the work of four of our researchers: F. Wolfgang Quednau,
Robert Lavallée, Michel Cusson and Gaston Laflamme.
They are working on biological control, resistance to the
white pine weevil in trees and treatments for white pine blister
rust.
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| BIOLOGICAL
CONTROL |
| |
Biological control consists in using living organisms to
limit the proliferation or destructiveness of various pests.
Dr. F. Wolfgang Quednau of the CFS - LFC is a pioneer in the
use of biocontrols to fight forest pests. He has achieved
one of the greatest successes in this field in Quebec, the
use of a parasite to control the mountain ash sawfly. The
sawfly was accidentally introduced in North America in 1914
and, by the mid-1970s, damage from the larvae of this species
was so extensive that the pest threatened to wipe out the
entire mountain ash population in the Quebec City region.
The work done by Dr. Quednau allowed the pest to be controlled
so that populations were maintained at acceptable levels,
thus sparing our mountain ashes.
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| THE
STRUGGLE CONTINUES |
| |
Aphantorhaphopsis (Ceranthia)
samarensis |
Another exotic insect threatens Canadian forests: Lymantria
dispar, more commonly known as the gypsy moth. This pest
was found in Quebec by entomologists for the first time in 1924.
The larval form feeds mainly on oak, birch, aspen and poplar.
A research program on biological control carried out by the
International Institute of Biological Control in Switzerland
and the Canadian Forest Service resulted in the discovery by
Dr. Quednau of a parasite, Aphantorhaphopsis (Ceranthia)
samarensis, which preys on the gypsy moth. The larval stage
of this small fly develops inside the gypsy moth caterpillars.
After importing Aphantorhaphopsis into Canada, Quednau
had to develop a method of rearing it on a large scale to allow
its release at sites infested by gypsy moths.
Since 1989, Dr. Quednau and his colleagues have been working
to establish a colony of Aphantorhaphopsis at the Laurentian
Forestry Centre of the Canadian forest Service. This has required
conducting research and gathering information on the species’
biological requirements, allowing the team to develop an effective
method of rearing the parasite. It is a multistep process, and
each step has to be followed to the letter to obtain adequate
results. Just like livestock breeders, Dr. Quednau and his team
must look after their charges’ food, mating, cages and
hibernation and maintain suitable environmental conditions (temperature,
light and humidity) to ensure that their colony survives. Successful
rearing also entails supplying oaks seedlings and gypsy moth
larvae.
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| HIGH-QUALITY
FOOD |
| |
Just as breeders have to produce fodder for their stock, the team
had to grow oak seedlings for their moths. The process began
in the fall with the harvesting of around 20,000 acorns, the
equivalent of the moths’ food needs for a year. At the
same time, the rearing of the gypsy moths that would serve as
the hosts of Aphantorhaphopsis also began with the
collection of egg masses. The moths were then allowed to go
into diapause (hibernation). In February, Dr. Quednau’s
team hatched the eggs in order to obtain caterpillars that,
in April, would be ready to receive Aphantorhaphopsis
eggs.
The rearing of Aphantorhaphopsis itself began in
February, by ensuring that the pupae had suitable conditions
in which to emerge from their cocoons. After that came the
mating of the flies and the females’ 14-day gestation
period, which ended in March. At the end of the gestation
period, the gypsy moth caterpillars were put in the cages,
allowing the Aphantorhaphopsis to lay their eggs on the caterpillars.
At the end of April, the Aphantorhaphopsis emerged
from their hosts and pupated.
At this time, a second generation of Aphantorhaphopsis
was produced to increase the size of the colony. After emergence,
mating and gestation, the females began to lay eggs. At this
point, the population was separated into two groups, the first
made up of flies ready to lay eggs that would be released
in the field and the second, of parasitized caterpillars,
which were kept to obtain pupae for a new generation of Aphantorhaphopsis,
to be used the following year.
Since 1991, 200 to 600 female Aphantorhaphopsis
have been released in southeastern Ontario. Measures to monitor
this introduction have shown that the parasite has become
successfully established, but in very small numbers. Flies
reared by Dr. Quednau were also released in New Brunswick.
The experiment also showed that Aphantorhaphopsis
could be introduced fairly easily in sites infested with gypsy
moths and that it adapted well.
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| TREES
RESISTANT TO THE WHITE PINE WEEVIL? |
| |
Dommages causés par le charançons. |
The white pine weevil is considered to be one of the most
destructive pests of conifer plantations in North America.
In eastern Canada, it attacks mainly white pine and Norway
spruce, while in the West, its preferred hosts are the Sitka
spruce, white spruce and Engelmann spruce. Like many other
forest insect pests, the adult weevil causes little damage
to trees and it is the larval stage, which attacks the terminal
shoots, that is so destructive. Trees attacked repeatedly
show reduced height growth. This results not only in lost
volume, but also in a significant decrease in wood quality.
Due to the weevil’s depredations and the increase in
its populations, Norway spruce and white pine are almost never
used anymore for reforestation in Quebec. Sitka spruce has
suffered a similar fate in British Columbia. The weevil is
also widespread in the Maritimes, where it preys on at least
eight species of conifers.
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| A
PROMISING APPROACH |
| |
Various measures have been proposed to control the white
pine weevil. Planting quick-growing species to provide shade
for host species, removing and destroying infested terminal
shoots, increasing populations of the weevil’s natural
enemies, and insecticide applications have all been tested.
In most cases, however, these strategies have either been
impractical, only effective temporarily or too expensive to
use on a large scale. Although these measures must not be
completely neglected, there is growing interest in using resistant
trees species as the main component in an integrated pest
management program (integrated management implies that all
appropriate techniques and methods are used).
Larves de charançons |
Two LFC researchers, Dr. Robert Lavallée and Dr. Michel
Cusson, have been working on resistance in trees to the white
pine weevil. Although various resistance mechanisms have been
proposed, the only one that has been consistently associated
with resistance in trees is the production of one or more substances
by the tree that reduce egg production and development. Other
mechanisms, though promising, have been shown to be of highly
variable effectiveness, and the presence of specific characteristics
(such as a high density of resin ducts) has not been demonstrated
to be necessarily associated with resistance to weevil attacks.
The reduction of egg maturation is the only mechanism that has
been partially explained and directly related to resistance
to the weevil.
Resistance to the white pine weevil resulting from this mechanism
was observed for the first time in Sitka spruce in British
Columbia. Adult female weevils reared on resistant spruce
shoots laid very few, if any, eggs. Resistance appeared to
have no effect, however, on weevils’ foraging behaviour.
In addition, the physical characteristics of resistant and
nonresistant trees were identical. This suggests that resistant
trees contain one or more substances that slow egg development,
while susceptible trees have one or more substances that stimulate
egg maturation.
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| A
QUESTION OF HORMONES |
| |
In most insects, egg growth is controlled by the juvenile
hormone. Egg development is stimulated by the presence of
high concentrations of this hormone in the female’s
haemolymph (blood). The resistance observed in some trees
appears to result from substances released by the tree that
may decrease the production of juvenile hormones in female
weevils. It has even been observed that, when female weevils
reared on resistant trees were given synthetic juvenile hormones,
egg development resumed and they began to lay eggs again.
White pine weevil. |
There is still a great deal of work to be done in pinpointing
resistance mechanisms in trees. Once resistance mechanisms
have been identified, researchers may be able to discover
or replicate the phenomenon in other trees and genetically
transformed trees could be developed that react to the injuries
caused by the weevil and naturally produce substances that
reduce egg growth.
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| USING
SILVICULTURE TO FIGHT BLISTER RUST |
| |
Another pest, blister rust, also attacks white pine, causing
high rates of mortality in the species and therefore limiting
its use in plantations. The disease was introduced through
white pine seedlings imported from Germany to the United States
and was observed for the first time in Quebec in 1916 at Macdonald
College near Montreal. The disease has had such a severe impact
that it has discouraged foresters from using white pine in
reforestation, considerably hindering white pine regeneration
in Quebec.
Blister rust fructification. |
Despite the presence of the rust in much of the species’
range, white pine still persists in Quebec. It occurs, however,
in much smaller numbers than a century ago, due mainly to
logging and the difficulty white pine has in regenerating
itself. When white pine does manage to regenerate, the rust
usually kills off most of the regeneration. Planting white
pine could help to solve the problem but again the rust devastates
plantations. In some regions, mortality rates in white pines
may reach 90% in less than 20 years.
One of the means deployed to counter the disease is the eradication
of Ribes species (currants), since this plant plays
an important role in the transmission of the fungus (Ribes
acts as intermediate host of the disease, with the spores
of the rust maturing on the leaves). This technique is costly
and not very effective, however, since it is almost impossible
to eliminate all Ribes. The systematic removal of
low branches of the pines is another technique that has been
tested in western Canada. However, it has not had consistent
success when tested in the East.
Élagage de branches basses. |
A researcher at the CFS - LFC, Dr. Gaston Laflamme, has proposed
a silvicultural method to help reduce the incidence of the rust
in white pine plantations, based on his studies and tests in
the field. Work done by Laflamme and his team has shown that,
under certain conditions, pruning trees susceptible to the rust
considerably reduces the incidence of the disease. The age of
the plantation is an important factor in the success of the
method. The best results are obtained when plantations are between
8 and 12 years old (rather than 15 to 20 years old as proposed
in another method). In addition, work by Gaston Laflamme has
shown that the presence of Ribes near a plantation
accelerates the spread of the disease. Therefore, he recommends
combining pruning with the eradication of Ribes from
around the plantation wherever possible to obtain the best results. |
