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Feeding by A. gloverana results in mats of severed needles combined with silk and frass, which dries to produce a brick-red appearance on fresh shoots of infested trees in late summer. The effect is most pronounced on the distal portions of branches in the upper crown. Severe defoliation produces this appearance over the entire crown. As these damaged structures are weathered, the crown becomes thin or bare, especially in the upper half. Upper crowns, which are completely defoliated, may die and be evident as dead tops for several years before either breaking off or being overtaken by a new, dominant shoot.
Trials using various formulations of the bacterial insecticide, Bacillus thuringiensis, have produced variable results but have mostly been encouraging (Otvos et al., 2002). Conclusive demonstration of the effectiveness of insecticide use in the field has been obscured by high levels of natural mortality in outbreak populations and the difficult terrain typical of the insect's habitat. Increased public opposition to the use of insecticides in forests has imposed an additional constraint on operational programmes.
Severe defoliation of western hemlock by A. gloverana is most frequent and extensive in Pacific coastal forests in the northern part of the range, specifically north Vancouver Island (Shepherd and Gray, 2001) and Queen Charlotte Islands, Canada (Garbutt, 1992), and south-eastern Alaska, USA (Mask, 1992). Outbreaks are less severe in the interior of British Columbia, Canada (Koot, 1991).
The mortality of trees appears to be infrequent, especially in older forest stands, although Hard (1974) estimated a loss of one-third of the net live volume in a mature western hemlock (Tsuga heterophylla) stand. Downing (1957) reported that nearly 20% of the trees were killed in another mature stand in Alaska, USA. Top-kill is common in severely defoliated mature stands. There are few measures of loss in radial increment resulting from defoliation. Wood and Garbutt (1990) suggested a minimum of 30% increment reduction for 5 years in mature western hemlock based on preliminary measurements. Concern over these losses in old stands of western hemlock, has diminished with the availability of the age classes for harvest.
Prebble and Graham (1945a) anticipated that the most serious economic effect of defoliation by A. gloverana would result from the high susceptibility of and consequent losses in, the extensive, homogeneous stands of regenerating western hemlock that dominate post-harvest forests in the Pacific northwest. There is increasing evidence to suggest that this concern was valid. Mortality is more common in juvenile stands than mature stands, and approached 70% in one study (Wood and Garbutt, 1990). However, in general, losses are most evident as stripped tops. There are several reports where more than 50% of the trees in a juvenile stand had dead tops as a result of defoliation by A. gloverana (see annotations in Mask, 1992; Otvos and Fajrajsl, 1997). Several years' growth may be affected and the recovery may be slow (McCambridge, 1956; Hard, 1974). The most severe top-kill in these juvenile stands occurs in the tallest trees so losses are greatest for the most valued trees (McCambridge, 1956). When defoliation is very severe, high-value Sitka spruce may also be impacted (Hard, 1974).