Insects - Pests of Ornamental Trees, Shrubs and Flowers - page 88

Adelges cooleyi (Gillette) ( 181-182 )
Douglas fir adelges
A common pest of conifers, alternating between spruce
( Picea ) and Douglas fir ( Pseudotsuga menziesii ).
Originally restricted to western North America but
introduced into Europe in the 1930s, where it is now
widely distributed.

mature into pseudo-fundatrices. They then deposit pale
yellow eggs, usually in batches of about 50, partly
covering them with white waxen threads. Nymphs
hatching from these eggs are the gall-inducing, gall-
inhabiting forms. They feed at the base of the needles,
causing a localized swelling which eventually develops
into the familiar pineapple-like gall. Each gall contains
numerous chambers within which groups of the pale
pinkish-orange nymphs develop. This species exhibits
only a limited migration, so that heavy populations tend
to build up locally on affected trees.

DESCRIPTION
Alate female (gallicola on spruce): 1.7-2.5 mm long;
reddish brown to purplish black. Alate female [ex:
Douglas fir]: 1.2-1.7 mm long; reddish brown to

purplish black. DAMAGE

The galls cause considerable distortion of the shoots
but, unlike those formed by Adelges viridis (see p. 98),
they usually fail to encircle the shoot and do not,
therefore, stop further growth. However, galling on
trees no more than a few years old may be extensive,
rendering young spruces grown as Christmas trees
unmarketable.

LIFE HISTORY
On Douglas fir, eggs deposited in the early spring by the
overwintering forms hatch at bud-burst. The insects
then develop on the needles, amongst conspicuous
masses of white, waxen 'wool'. Both winged and
wingless forms occur. The former depart in June for
spruce trees, where sexual forms are reared. These
aphids eventually give rise to overwintering morphs
which finally mature in the spring and produce a gall-
forming generation. The latter initiate characteristically
elongate galls on the shoots, each gall maturing in the
summer. Winged forms (gallicolae) then migrate back
to Douglas fir, from late summer onwards, where
wingless females eventually settle down to overwinter
on the underside of the needles.

181

DAMAGE
Spruce: the shoot galls cause a twisting of the new
growth. Douglas fir: foliage is yellow-mottled and tree
growth is retarded; the foliage also becomes heavily
encrusted with masses of white, fluffy wax and
discoloured by sooty moulds which develop on the
copious quantities of honeydew excreted by the
adelgids.

181 Douglas fir adelges ( Adelges cooleyi ) damage to foliage
of Pseudotsuga .

182

182 Douglas fir adelges ( Adelges cooleyi ) on Pseudotsuga .