. Hazard-rating systems in forest insect pest management : symposium proceedings, Athens, Georgia, July 31-August 1, 1980. Forest insects Biological control United States Congresses; Forest insects United States Analysis; Forest insects United States Management. haps by genetic selection of Douglas-fir trees containing the resistant mechanism that is universal in live larch. Susceptibility of Attacked Trees We recognize three categories of sus- ceptibility of trees in infested groups: susceptible, intermediate, and resistant. Susceptible trees are readily killed. Their egg galleries are free o
. Hazard-rating systems in forest insect pest management : symposium proceedings, Athens, Georgia, July 31-August 1, 1980. Forest insects Biological control United States Congresses; Forest insects United States Analysis; Forest insects United States Management. haps by genetic selection of Douglas-fir trees containing the resistant mechanism that is universal in live larch. Susceptibility of Attacked Trees We recognize three categories of sus- ceptibility of trees in infested groups: susceptible, intermediate, and resistant. Susceptible trees are readily killed. Their egg galleries are free of resin, and their foliage discolors within approxi mately 1 year. Such trees yield DFB prog- eny at the highest rates. Intermediate trees eventually die, apparently from in- oculation with fungi carried by beetles (Harrington et al. 1981, Rumbold 1936). Foliage of some trees of this category discolors more slowly, sometimes in the second year following attack. Resistant trees survive; their galleries are impreg- nated with resin and eventually are cov- ered over by callous growth (Belluschi et al. 1965) (fig. 3). Table 1 compares the characteristics of 739 attacked trees in all three cate- gories, in western Montana. The trees averaged 188 years in age. We obtained similar data from somewhat younger trees (average 111 years) in northern Idaho (Furniss et al. 1979). An average of 29 percent of those trees were attacked un- successfully, whereas 22 percent of the Montana trees survived attack. Thus, in unmanaged stands, beetles attack groups of nearly mature or older trees. Generally attacks are more dense and successful in trees that are larger, more dominant, and more productive of resin (fig. 4 A and B). Less dominant trees are smaller and tend to be poor pro- ducers of resin (fig. 4 C). We theorize that good resin producers may be inherent- ly more resistant (higher threshold of successful attack density) but may be killed at a higher rate because they at- Table
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