In 1957, spruce beetles, Dendroctonus rufipennis developed into outbreak numbers in logging slash at a north—central Colorado site, entered living spruce trees, but remaining epidemic for only two years. Reduced beetle fecundity was the first indication of outbreak decline; this was caused by nematodes and unknown agents. Significant summer mortality agents were pitch, intra— and interspecific competition for food, predation by woodpeckers and flies, and parasitism by wasps. Desiccation of both food and beetle larvae, enhanced by woodpecker feeding activity, contributed significantly to outbreak decline. Winter mortality was attributed mainly to woodpeckers, although temperature of —29°C caused additional losses. The effects of the outbreak on the spruce were considerable. Large diameter trees in small patches were killed, and species composition was altered in favor of subalpine fir and lodgepole pine, but mean tree diameter was not significantly reduced.

Beetle-killed trees in the Front Range of Colorado were observed for their rate and direction of falling. No trees fell within the 2 years following infestation. Thereafter, trees generally fell at the rate of 3-5?40 per year unless winds exceeded 75 mph. Most trees fell to the east and broke off between ground level and 2 feet above ground. Keywords: Dendroctonus ponderosae Hopki ns, Pinus ponderosa Management ImpIicationsForest managers need not be concerned with the falling of beetle-killed ponderosa pine until more than 3 years after infestation. Thereafter, strong winds will blow down many trees when winds exceed 75 mph.

Mountain pine beetles (Dendroctonus ponderosae Hopkins) in outbreak numbers do not kill whole forests. Many ponderosa pine (Pinus ponderosa var. scopulorum EngeJm.) trees of all diameters, as well as relatively intact patches and stands of trees, survive. A few scattered stands will be almost completely killed. If overstocked stands of relatively large tree diameters are allowed to develop over widespread areas, control programs aimed only at treating infested trees are unsound. Trees must also be thinned to some acceptable stocking level. Silvicultural manipulation to favor nonhost trees, such as Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) over ponderosa pine in order to "bug proof stands, seems of little benefit in an outbreak unless one is willing to convert most of the stand to nonhost trees. Dwarf-mistletoe-infected (Arceuthobium vaginatum subsp. cryptopodum (Engelm.) Hawksw. and Wiens) ponderosa pines are more susceptible to mountain pine beetle attacks than are healthy trees. Consequently, forest management practices aimed at reducing stand risk to bark beetles by thinning stands should simultaneously remove dwarf-mistletoe-infected trees.

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From 1965 to 1978, mountain pine beetles killed 25% of the pines taller than 4.5 feet in a study area in north-central Colorado.Average basal area was reduced from 92 to 58 square feet per acre. Mortality increased with tree diameter up to about 9 inches d.b.h. Larger trees appeared to be killed at random. Mortality was directly related to number of trees per acre and presence of dwarf mistletoe, but not to site index, elevation, or percent Douglas-fir in the stand.Many ponderosa pine (Pinus ponderosa var. scopulorum EngeJm.) trees of all diameters, as well as relatively intact patches and stands of trees, survive. A few scattered stands will be almost completely killed. If overstocked stands of relatively large tree diameters are allowed to develop over widespread areas, control programs aimed only at treating infested trees are unsound. Trees must also be thinned to some acceptable stocking level.Silvicultural manipulation to favor nonhost trees, such as Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) over ponderosa pine in order to "bug proof stands, seems of little benefit in an outbreak unless one is willing to convert most of the stand to nonhost trees.Dwarf-mistletoe-infected (Arceuthobium vaginatum subsp. cryptopodum (Engelm.) Hawksw. and Wiens) ponderosa pines are more susceptible to mountain pine beetle attacks than are healthy trees. Consequently, forest management practices aimed at reducing stand risk to bark beetles by thinning stands should simultaneously remove dwarf-mistletoe-infected trees.

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