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Pest Status
Chapter 46
Background and Evaluation of Control Attempts
Adelges piceae (Ratz.), Balsam
Woolly Adelgid (Homoptera: Adelgidae)
H.O. SCHOOLEY, l.W.E. HARRIS and B. PENDREL
The balsam woolly adelgid, AdeJges piceae (Ratz.), is a destructive pest of firs, Abies
spp. In recent years, however, populations have been low and dispersal has been
limited. The distribution of the adelgid remains about the same as in 1968 (Clark el al.
1971). In eastern Canada, expansion of the infestation has been largely into those small
areas that had been bypassed as the initial infestation developed (Schooley 1980). The
adelgid is now present throughout most of the fir forests of the Maritime Provinces,
Newfoundland, and the eastern tip of the Gaspe Peninsula. A total area of about 103600
kml is affected. In western Canada there have been only minor increases in the size of
the infestation, which now covers about 10 360 kml in southwestern British Columbia.
The infestations in Canada extend southward into the United States on both the east and
west coasts (Mitchell el al. 1970). Climate seems to be the main factor limiting further
spread of this insect in Canada (Greenbank 1970).
The balsam woolly adelgid attacks all species of firs in Canada but some are more
resistant to injury than others. In eastern Canada, mortality of balsam fir, Abies balsamea
(L.) Mill., has been common. In western Canada, subalpine fir, A. Jasiocarpa (Hook.)
Nutt., growing at low elevations, is the most vulnerable species, but it rarely occurs
within the present infestation boundaries (Harris 1968). Pacific silver or amabilis fir, A.
amabilis (Doug!.) Forb., is the second most vulnerable western host and considerable
mortality of this species has occurred. Grand fir, A. grandis (Doug!.) Lind!', is also
deformed and killed by the adelgid, primarily on southern Vancouver Island. The
adelgid is also threatening to destroy Fraser fir, A. fraseri (Pursh.) Poir., in the few
remaining spruce-fir stands of the southern Appalachian Mountains of eastern United
States (Johnson 1980).
Some of the causes of variation in the response to adelgid infestation among Abies
host species have been identified. Puritch (1973), for example, has shown that the onset
and intensity of adelgid attack caused premature heartwood formation and a resulting
abnormal water stress. Thus, ability to withstand this stress among fir species corresponds
to their susceptibility to adelgid damage. It has also been observed that adelgid attack
has little effect on European species because a layer of dead bark tissue develops during
the wound healing process and protects the trees from further attack for several years
(Oechssler 1962). This healing process is delayed or absent in balsam fir that is under
continuous adelgid attack.
Clark et al. (1971) stated that there was little scope for additional study in the control of
this adelgid by introduced predators. Field trials in eastern Canada tested all the
apparently suitable insects over a 35-year period and failed to find a predator or predator
complex that would control the adelgid. Only eight species of introduced predators have
become established, at least temporarily, but none has been effective (Table 55) in
reducing damage to economically tolerable levels. Consequently, it was recommended
that the importation and release of predators be discontinued. This recommendation has
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