Controlled Mating, Karyology, Morphology, and Sex-Ratio in the Dendroctonus ponderosae Complex12

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Abstract
Laboratory mating experiments and observations on developmental rate, karyology, and morphology confirm the recent synonymy of Dendroctonus monticolae Hopkins with D. ponderosae Hopkins, but establish the species integrity of D. jeffreyi Hopkins, also previously considered a synonym of ponderosae. D. monticolae from British Columbia and 8 California localities were interfertile and morphologically similar. D. ponderosae from Colorado were interfertile with monticolae from California, but their mean pronotal widths, 2.44 mm and 2.10 mm, respectively, and indices of pronotal punctation, 5.43 and 6.12, respectively, were very significantly different. D. monticolae and its hybrid with ponderosae showed the karyotypic formula 11AA + Neo-XY. D. jeffreyi is larger (pronotal width = 2.63 mm) and its pronotum less densely punctured (index = 4.19) than monticolae and ponderosae. The karyotype of jeffreyi is also 11AA + Neo-XY, but the Neo-X is usually constricted and 1.4 times longer than the Neo-Y while the Neo-X of monticolae is rarely constricted and only slightly larger than the Neo-Y. In addition, a tiny supernumerary chromosome was present in some jeffreyi. In reciprocal pairings of monticolae and jeffreyi, none of the 846 eggs laid by 21 monticolae females hatched, but 8 larvae and 1 sterile female resulted from 362 eggs laid by 10 jeffreyi females. Five pairings of jeffreyi and pondcrosae produced no brood. Several jeffreyi (control) pairings showed a marked reduction in egg hatchability and only female progeny were produced. A male-lethal factor attacking the embryos is suggested, but pedigree data are insufficient to determine whether chromosomal or cytoplasmic factors are involved.