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A DISEASE OF THE WHITE BIRCH.
BY JOHN IAIRSEN.
*
(Reprint from Third Report of the Michigan Academy of Science, 1901.)
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46 MICHIGAN ACADEMY OF SCIENCE.
A DISEASE OF THE WHITE BIRCH.
BY JOHN LARSEN.
The beetle Agrilus anxius, one of the Buprestidae, is causing great
ravages among our shade birches, both the white and also the cut-
leaved variety. •
A tree that has been attacked does not die the first season but seems
to live during several seasons, the beetle doing its work each season.
Infected trees very often have last year's leaves remaining upon
branches that have suffered severely. The death of the trees begins
at the top and proceeds downwards. In trees that are nearly dead
only a few of the lowest branches bear leaves tardily in the spring
only to wither soon. The burrowing of the larvae weakens the limbs
to such an extent that many of them break from their own weight.
Trees may be found where nearly all the limbs hang down broken.
The trees are usually first attacked towards the top where the limbs
are from three-fourths to one and one-half inches in diameter and
usually only in badly affected trees are the burrows found towards the
base in the trunk of the tree.
Aside from the appearance spoken of the presence of the beetle
is shown by narrow swollen ridges on the bark which often on the
smaller limbs run spirally for some distance. Cutting through these
a burrow is always found just beneath the surface. The burrows follow
no definite direction in the limbs but may be found running zigzag in
any direction, sometimes running upwards, then doubling to again run
downwards; sometimes near the bark, then turning inwards even run-
ning straight through the center of the limb to the opposite side. The
smaller the limb the straighter usually is the course upwards or down-
wards of the burrows. -
The burrows are sometimes found in twigs only one-fourth inch in
diameter where they run straight in the center. In the trunk they do
not penetrate deeply but twist around among and through each other
making it impossible to trace them for any distance as they often
form a complete network. In the trunk they do not form regular ridges
but irregular swellings; in the smaller limbs and twigs no external sign
is shown. The smallest limb upon which ridges were found was one-half
inch in diameter. No ridges are found except where the burrow has
cut the cambium. e º
During the early period of the investigation leading to this paper in
the early spring of 1899, “Bulletin No. 19, New Series,” of the Agri-
cultural Department at Washington came to hand. This furnished
the identity of the beetle and data of its distribution. The bulletin
speaks of the birches in Buffalo, N. Y., and in Detroit, Mich., as having
suffered very severely. Aside from this the observer has found most
of the trees here in Ann Arbor suffering more or less severely and
many of them dead. The same was found to be the case in various
LARSEN ON A DISEASE OF THE WHITE BIRCH. 47
parts of Chicago, Ill., particularly in Humboldt Park where many have
died and others were in a precarious condition. Upon inquiry it was
found that nursery stock had been brought from Rochester, N. Y., and
that some of these were infected. Some specimens were brought here
from Rochester for experimenting and planted upon the campus. Upon
one of these were found the ridges mentioned above.
The life history of the beetle as far as observed is this: The eggs
are deposited in crevices in the bark in the early summer. Insects con- .
fined in a glass jar were found to be depositing eggs on June 8, and
for a week or more afterwards. Pieces of fresh limbs were supplied,
but the insects did not deposit their eggs upon these, but moved about
feeling for crevices with their long prehensile ovipositor and having
found a place such as between the glass and the lower part of the
cork or under a piece of wood, from five to ten or more eggs were put
in one place. Copulation had gone on for some time before this. Great
activity was exhibited during the copulation and egg-laying. No obser-
vations were made of the development of the eggs. Fully formed
larvae were found early in the winter. They remain torpid until
Spring. - •
Ilarvae that were still in the winter stage were found until April 12.
Shortly after this the pupa stage began to appear and development
went on rapidly until May 1, when a few adult insects were taken com-
ing out from the burrows. At this date, however, most were still in
the pupa stage. Then came a spell of cold weather and not until
more than a week later did the insects come out in numbers, many be-
ing taken from the burrows as late as May 15.
That the weather, or rather the temperature, decides the time of
development is indicated by the above facts. And it was further demon-
strated by material kept in the laboratory. Parts of branches con-
taining larvae were taken into the laboratory in March and development
began immediately and an adult insect obtained in the middle of April.
The mode of escape of the beetles from the burrows and places in
which the larvae spend the winter is interesting. The larva burrows
out near the bark, then turning into the wood at a slight angle an en-
larged oblong chamber is formed in which the larva rests and under-
goes further development in the spring. The burrow has been carried
outwards in a rather sharp curve which brings its end to just beneath
the bark beyond the chamber. When the beetle is ready to escape it
makes its way to this and, eating through the bark, escapes. The
larva is often found doubled up in the chamber. Normally it lies with
its ventral side turned outwards and the beetles are found in this
position. Some were found reversed and could not escape. The escape
is rather laborious as the insect must squeeze its way to the bark
and then eating a small hole again squeeze its way through. Beetles
are found with the forward parts of their bodies protruding for hours
making long rests between efforts to free themselves. There seems to
be no rule governing the direction, whether upwards or downwards,
in which the head of the insect is turned during development.
The extent of the burrows is not an easy matter to determine, nor
precisely where they take their origin. They are compressed in shape
to suit the flattened shape of the larvae. In one place in a slight swell-
48 MICHIGAN ACADEMY OF SCIENCE.
ing on the bark were several small openings, less in diameter than a
pin. From these openings burrows were traced. The burrows are
at first very small and lie close under the bark and are filled with dark
granules, probably colored by tannin. As they become larger and
begin to run deeper, they are filled with a whitish dust shown by the
microscope to be parts and groups of wood cells.
As to extent one was followed through its winding course a dis-
tance of 1 foot 7 inches in a length of branch of 4 inches, now near
the bark, now deep down in the wood; now running upwards in the
branch, now running downwards. Neither the beginning nor the end
of this burrow was found. The branch was somewhat less than an inch
in diameter. As mentioned above in small branches the course be-
comes straight. One burrow was traced upwards in a branch of about a
half inch in diameter a distance of about eighteen inches, then doubling
upon itself ran downwards parallel to the upward course. When the
branch becomes a fourth of an inch or so in diameter no doubling is
possible and chambers are often found hollowed out in such twigs. The
wood for some distance around a burrow is often found discolored and
decaying thus weakening the branch and producing the breaking men-
tioned above. This breaking often takes place at a chamber, expos-
ing the larvae to cold and to birds. Woodpeckers are in the habit of
burrowing down to the chambers and extracting the larvae.
When the beetles leave the trees they leave a hole which in outline
has one straight side and one curved. The back of the insect in com-
ing out is turned towards the curved side. Either side may be upwards
or downwards on the tree, but always one or the other. These holes
may be very numerous on one tree. Eleven were counted, in one in-
stance, on a circular space of two and one-half inches in diameter.
Although the beetles may be coming out in numbers, few are found
upon the tree. They seem to crawl about for a short time and then
taking flight leave the tree. - - -
Methods of destruction were thought of but are hard to apply, on
account of this habit of leaving the tree, and besides it was found
that they do not feed upon the leaves of the birch. This was demon-
strated by keeping a number in a large glass jar in the laboratory and
supplying them with fresh leaves. When only birch leaves were sup-
plied they fed but very sparingly. Some elm leaves were then put in
with the birch and they fed greedily upon these. This led to further
experiment and various sorts of leaves were used. They fed upon
almost any leaf of a soft texture. But their favorite food was willow,
poplar, and aspen leaves with preference strongly marked in the order
glven.
It seems from this that the beetles upon leaving the birch feed on
other trees until the time for reproduction.
Peculiar growths in the tree itself are caused by the burrows, or
rather by some stimulus furnished by the burrows or by the larvae. It
was stated that wherever the burrows were near the cambium corre-
sponding ridges were to be found on the outer surface of the bark. This
is due to a growth of sclerotic cells which is formed as an arch over the
burrow on the side towards the bark. When the burrow is entirely
below the cambium no such growth occurs. It appears that the cam-
TARSEN ON A DISEASE OF THE WHITE BIRCH. 49
bium must be injured before this growth takes place. Between the
burrow and the sclerotic tissue a secondary cambium is formed which
gives off cork cells towards the burrow tending to fill it up. In some
cases this cambium could be connected with the cork cambium of the
tree and may take its origin from this. But it was not demonstrated
that this was always the case. It may to some extent take its origin
from the parenchyma. In any case it seems to give off cells both to
form the corky layer mentioned and also in the opposite direction to form
the sclerotic tissue. -
The cambium of the tree after a time bridges over the burrow and
its covering of sclerotic tissue, forming an arch and performing its
normal function covers the whole with wood, and while preserving the
external ridges buries the burrow deeper and deeper. Burrows, buried
in this way, were found with three annual rings above them showing
that the trees would overcome the injuries were it not for renewed
attacks by numerous beetles. The forming of sclerotic tissue and sec-
ondary cambium is probably a means to this end. .
That this is true and that the growth mentioned is due to mechanical
injury, at least to a large extent, was demonstrated by driving nails
through the cambium, leaving wounds which showed the same growth
of sclerotic tissue as in the case of the burrows.
In many cases the holes left by the escape of beetles were filled up
by the cork cambium growing down and filling them with cork, even
to beyond the chambers. Holes left by the falling of dead twigs are
filled in the same way. That the tissues mentioned as cork were really
such was shown by testing with strong sulphuric acid.
Agrilus anxius has been found upon the willows and it may be that
the birch is a new host. In that event, it is not so strange that they
should thrive; it may take some time before they are followed by
their natural enemies.
7
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