Most spruce trees are damaged by one of two species
of gall forming adelgids (aphids); the Eastern spruce gall adelgid, A.
abietis, and the Cooley spruce gall adelgid, A. cooleyi. These soft-bodied
insects feed by sucking plant juices. They are small (1/16 inch) and not
readily seen, but their feeding on young plant tissue causes the formation
of obvious cone-like growths called galls, which stunt and eventually
kill the twigs. These insects are not a serious pest under forest conditions
because they injure only a small portion of the foliage. At times, they
disfigure ornamentals, and continued infestations may weaken trees and
make them susceptible to attack by other insect pests and disease-producing
organisms.
Damage
Feeding by the adelgids produces a characteristic cone-like gall. The
gall’s shape, size and position on the twig aid in identifying the
species involved. Galls interfere with the natural formation of twigs
and cause curling, stunting and the eventual death of new growth. Heavy
infestations give the trees a ragged appearance and destroy their beauty.
Continuous infestations can seriously devitalize trees and render them
more susceptible to attack by other organisms. Some trees have a natural
resistance or immunity to these adelgids, and in spite of the presence
of the insects, the galls never completely develop or may not form at
all.
When galls are first produced, they are green, soft and made up of many
individual cells that are inhabited by the developing adelgids. The galls
mature by midsummer when the cells open and the adelgids emerge. The empty
galls soon dry, become brittle and turn a light to dark brown.
Eastern spruce gall adelgid feeding causes the formation of pineapple-like
galls about 1/2 to 1 inch long at the base of twigs. Twig growth is normal
when the gall is still green. The twig dies after the gall matures.
The Cooley spruce gall adelgid produces elongated, often-curved galls
about 1 to 3 inches long that usually involve the entire twig.
Eastern Spruce Gall Adelgid, Adelges abietis L.
The Eastern spruce is an introduced species that commonly infests Norway
spruce and is also found on white, red, black and Englemann spruce. Only
females of this adelgid are known to occur. The adelgids overwinter as
immature insects (nymphs) on twigs near the terminal buds and resume feeding
in the spring. The adelgids mature about early May and produce a conspicuous,
waxy cover under which they lay their eggs. The eggs hatch in about a
week, and the young insects begin to feed on the buds that had been fed
upon by the over wintering forms. Continued feeding induces abnormal growth
of plant cells and progressive enlargement of the bases of infected needles.
Eventually, the enlarging needles coalesce and form the characteristic
pineapple-shaped gall within which the insect lives and grows. Galls protect
the adelgid from most insecticide sprays. Both adelgids and galls mature
in midsummer. The galls split and the mature adelgids emerge, develop
wings, fly to the needles of the same or some other spruce and lay eggs
at the base of the new growth. The eggs soon hatch and the immature forms
that emerge will feed until the onset of cold weather, overwinter at the
needle bases and then resume their life cycle the following spring.
Cooley Spruce Gall Adelgid, Adelges cooleyi
The Cooley spruce gall adelgid is a native insect most commonly infesting
blue spruce but also found on Sitka and Englemann spruce as well as Douglas
fir.
Only females of this species are known. Their life cycle is similar to
that of the Eastern spruce gall adelgid with a few minor exceptions. The
feeding that triggers or stimulates gall formation apparently occurs at
a later time than that of the Eastern spruce gall adelgid and, hence,
the difference in the timing of spring treatments for the control of each
species.
The Cooley spruce gall adelgid will frequently utilize Douglas fir, if
present, as an alternate host. Adults may migrate to Douglas fir in late
summer and lay eggs. These eggs hatch and the immature forms pass the
winter there. In the spring they resume feedings, mature and lay eggs.
These hatch and develop into adults that may remain on the Douglas fir
or fly back to spruce to lay eggs for the next over-wintering generation.
Galls are not produced on Douglas fir. Injury appears as a yellowing and
kinking of the affected needles. In severe infestations, partial to complete
needle fall may occur on injured terminals.
Adelgid Control
Eastern Spruce Gall Adelgid (Spruce)
Hand pick galls before August and destroy. Use horticultural oil for dormant
spray in mid-April to early May just before buds begin to break. Use horticultural
oil or insecticidal soap in mid- to late September when insects emerge
from galls and the majority of galls are green.
Several grades of horticultural oil are available. Some are strictly limited
to dormant use; others can be applied to actively growing plants in spring
and summer. Summer treatment can be used on most trees, but check the
label to be sure. Never spray when trees are under stress (e.g. moisture
stress) or when temperatures are expected to be 80 degrees F or above.
Under adverse spraying conditions, such as high heat and relative humidity,
sensitive species may suffer from phytotoxicity. Spruce and Douglas fir
may show sensitivity to horticultural oils. Glaucus evergreens (those
with a bluish bloom) may temporarily lose their blue color if oil is applied,
and the natural bloom may not return for one to two years.
Cooley Spruce Gall Adelgid (Spruce)
Hand pick green galls when first noted and destroy. Use horticultural
oil in mid- to late April just before buds begin to break for dormant
spray. Use horticultural oil or insecticidal soap in late July to mid-August
after galls have opened but before adelgids move to alternate hosts.
Cooley Spruce Gall Adelgid (Douglas fir)
Use horticultural oil or insecticidal soap. Spray in early May and again
in late July to early August.
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
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