We describe the biology and life stages of the birch bark beetle, Dryocoetes betulae Hopkins, and report its second known occurrence in Idaho. One annual generation was observed with broods overwintering as larvae and sexually immature adults. The species is polygamous with a ratio of 1.8 females per male. Two females (rarely three) joined a male after he entered the bark, and each female created a 3- to 4-cm-long egg gallery with short lateral spurs. Eggs were laid in niches along each side of the main gallery. Larvae have three instars. No hymenopterous parasitoid was found; however, two apparent predators, Rhizophagus dimidiatus Mannerheim, and a clerid, Thanasimus undatulus (Say), were present in galleries. Two mites, Histiostoma sp. and Proctolaelaps n. sp., also occurred in galleries, and a nematode of the Order Rhabditida occurred in the midgut of larvae and adult D. betulae. An ambrosia beetle, Trypodendron betulae Swaine, also infested the basal stems apart from D. betulae. Stems of infested trees were infected with a root rot fungus, Armillaria ostoyae (Romagnesi) Herink. Several generations of beetles infested the basal portion of stems of either decadent or recently dead paper birch. This behavior preserves a scarce host resource and is enhanced by a relatively low fecundity and ability to establish new galleries without flight dispersal.
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