Effect of. Douglas-fir Fertilization
on the Cooley Spruce Gall Aphid
by R.G. Mitchell and R. E. Miller Pacific Northwest Forest and Range Experiment Station USDA Forest Service P ortland, Oregon Most growers know that applying
nitrogen fertilizer to young Douglas-fir
can improve tree color and growth. But
what does nitrogen fertilization do to
insect populations that feed on these
trees? Reports from Germany show
that fertilizing conifers can reduce
populations of chewing insects (such
as caterpillars and weevils), but
ominously note that numbers of
sucking insects (such as aphid and
scales)
are
sometimes
increased
(Schwenke 1961, Merker 1962).
Because the Cooley spruce gall
aphid (Adeles c Jeyi (Gillette)) is
such a common sucking insect on
Douglas-fir, a study was begun to
investigate the response of the aphid
to fertilization of 7-year-old trees
planted near Olympia, Washington.
The study was a cooperative one
between the Pacific Northwest Forest
and Range Experiment Station of the
U.S. Forest Service and the Union Oil
Company of California.
Nitrogen
(N)
was . applied
to
individual trees either as dry urea prill
or as concentrated solutions of urea or
ammonium nitrate. Application rates
were SO and 200 lb N/acre. Application
was on April 15, about 1 week before
bud burst. During the next 2 years, we
measured aphid population levels,
number of eggs laid, and survival and
establishment on both fertilized and
unfertilized control trees.
In the first year, egg production on
fertilized trees was 11 to 42 percent
higher than on unfertilized trees, with
the greatest increase on heavily
fertilized trees (200 lb N/acre). Winter
survival on heavily fertilized trees also
increased-about 20 percent more
than the control. In the second year,
egg production on fertilized trees
dropped back to normal, but winter
survival was still up about 10 percent.
In both years, however, a counter­
action also occurred-aphids did not
FEBRUARY, 1976
establish on fertilized trees as readily
as on unfertilized trees.
The result of these opposing factors
was that 2 years after fertilizing, the
aphid population was about the same
on fertilized trees as on unfertilized
trees. Because different results might
be obtained on other soils or with trees
of other genetic backgrounds, it is
risky to generalize from a single test.
But it is clear that, while aphid
populations are affected by fertiliza­
tion, this does not necessarily lead to
increased number of
aphids on
Douglas-fir.
In fact, our results show that a
Christmas tree grower might even
profit from a light fertilization (SO lb
N/acre) in the spring of the harvest
year. Light fertilization in this study
was associated with about SO-percent
reduction in the aphid population
colonizing new growth in the year of
treatment. And it is the generation
infesting new growth which causes
yellow spots and twisted needles. In
situations where the aphid populations
surviving the winter. are large'•(200 or
more aphids per 1,000 needles),
·
fertilizing could improve marketability
not only by reducing aphid numbers
and damage, but also by improving
color and length of needles.
Study results were the same with all
three formulations of nitrogen tested;
but ammonium nitrate solution should
probably be avoided - at the SO-Ib-N
dosage, it caused foliage burning and
some needle loss. At the 200-lb-N
dosage, concentrated ammonium ni­
trate solution caused some top and
tree kill. Urea prill provided the best
improvement of needle color and
length in both the first and second
growing seasons after treatment.
Readers wishing more information
on the insect phase of this study are
referred to the article cited below by
Mitchell and Paul (1974).
Reference Cited
Merker, E. 1962. Augenblicklicher
Stand der Uentersuchungen ueber
die schaedigende Wirkungsweise
von Duengestoffen auf Waldschaed­
linge. Allg. Forst u. Jagdztg. 133:
81-83.
Mitchell, R.G., and H.G. Paul. 1974.
F{e[d fertilization of Douglas-fir and
its
effect
on Adelges cooleyi
populations. Environ. Entomol. 3:
501-504.
Schwenke, W. 1961. Waldduengung
und
lnsektenvermehrung.
Proc.
13th Congr.· Int. Union For. Res.
Organ. Part 2, vol. 1, 13 p.
Reproduced with permission from the
AMERICAN CHRISTMAS TREE JOURNAL,
Volume XX,
Number 1,
February,
by the FOREST SERVICE,
ment of Agriculture,
U.S.
1976,
Depart­
for official use.
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