Pineus pinifoliae (Fitch) belongs to the Adelginae, a group characterized by unusually complex life-cycles. The typical species have at least five distinct forms, one bi-sexual and the others parthenogenetic. They alternate between two coniferous hosts, one of which is always a species of spruce (Picea). Galls are formed on spruce by a modification of the growth of the new shoot.

The life-history of P. pinifoliae is only partially known. Patch has reported on observations in Maine which showed that the gall-making form flew from “black spruce” to the needles of white pine and that its offspring settled on the new shoots. She also described a morphologically similar winged form which developed on white-pine shoots and which she believed to be the return migrants. Annand made similar observations in Oregon and gave careful descriptions of three forms: the fundatrix, the gallicola migrans, and the exulis.

Species of Balcluthini show great similarity in external appearance and many exhibit much individual variation. Examination of the male genitalia is the only certain means of identification. Baker (1896, Can. Ent. 28: 35-42) attempted to separate the species on external characters, and Davidson and DeLong (1935, Proc. Ent. Soc. Wash. 37: 97-112) revised the North American species and figured the male genitalia. Little has been published on the species occurring in Canada. Five species from Canada are represented in the Canadian National Collection. One of them is previously undescribed.

Aside from M. acerbus (Jordan), which is apparently a breeding parasite of Tamias striatus (L.), a chipmunk, the holarctic genus Megabothris Jordan 1933 appears to be truly associated with the Microtinae, some of the species showing special affinities for particular species of these mice, others infesting several species, or even genera, within a geographical region.

The nearctic Megabothris asio (Baker) favours the common meadow vole, Microtus pennsylvanicus (Ord), and occurs throughout most of the range of that mouse. The writer recognizes two valid subspecies, M. asio asio, which is restricted to eastern North America, and M. asio megacolpus (Jordan), of western North America. The nominate race ranges from Labrador to Delaware. M. asio megacolpus (=M. asio orectus Jord.) ranges from western Quebec, western Ontario, and Michigan westward to the Coast Range, and from California north at least to the 60th parallel.

Among the attributes of the parasitic Hymenoptera that contribute to the ability of certain species to maintain host populations at low densities is the power to restrict egg deposition to sites suitable for the continued development of the offspring, and to regulate, regardless of host density, the number of eggs deposited per host (Flanders 1947). High searching capacity is, in part, a function of female longevity correlated with the conservation of reproductive material. It is self evident that time used in wasteful oviposition means less time for searching and consequently less effective control of the host.