Digger Spade, MG Plant Detective
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| Serbian Spruce |
The client came in with a picture of his Serbian Spruce (Picea
omorika). The tree looked about 18 feet
tall and instead of having a single leader, it had three or so - but they were all
dead. We asked him when he noticed this, and he said last year and again this year.
The rest of the tree seemed in robust good health. It seemed that last year the new growth died
and this year the several shoots competing for leadership had suffered the same
fate.
The damage pattern didn’t seem that of a disease since it was
limited to new growth. Nor did it seem
likely to have an environmental
cause. That leaves insect damage as a
likely factor. Quite a few insects have
a taste for new growth that is rich in nutrients, but two or three particularly like various
parts of spruce. A quick search
identified the most likely culprit, the white pine weevil (Pissodes
strobi). In this part of the world it used to be known as the spruce weevil, but it turned out to be the same species that had been named earlier as the white pine weevil. As in most things precedence counts.
But the client wants to know what he can do about it. His spruce is turning into a bush and doesn't
look good with a dead top. The weevil both flies and climbs to lay its
eggs at the base of the new bud on the leader (or leaders) just as they begin
to open. The larvae enter the stem and munch their way down through the cambium
of the previous year. So during the
growing season the new bud may produce a spurt of growth, but dies and the side
shoots from two years earlier begin to challenge for leadership.
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| White pine weevil damage |
Intercepting the weevil in a timely way is tricky. Getting insecticide onto the tree so that the weevil is killed presents difficulties, and once the larvae are in the cambium layer the only way to get at them is with a systemic insecticide. There is only one pyrethrin based insecticide (non-systemic) registered in Washington for homeowner use on the pine weevil. It lasts only a day or so, which would require frequent applications to the top of the tree as the new terminal bud(s) are beginning to expand. Alternatively, the climbing weevils can be trapped with sticky bands on the tree trunk.
There are also several systemic insecticides available to licensed operators, so a certified arborist may be be the client's best resource. Some of these are used in soil treatments and
some as injections, again timing them so that larvae are killed before
substantial damage has occurred.
But even successful treatment will not produce a stately spruce tree unless a single leader can be created and trained vertically. That may mean aesthetics will determine the next steps. If a bushy or distorted tree isn’t acceptable, then ground-level pruning will be in order and a suitable non-spruce replacement installed.
Not all investigations have a happy ending.