1982] 
Torchio & Tepedino — Osmia Bees 
235 
proportion of one- and two-year forms within sites for each species 
(Fig. 1) suggests that the relative fitness of these forms is determined 
by environmental conditions. What these conditions are and how 
they interact with the genotypes to maintain a balanced polymor- 
phism (if indeed it is balanced) remains to be studied. 
An interesting ramification of varying selective pressures upon 
one- and two-year forms is the indirect effect upon the sex ratio of 
the population. Elsewhere, we (Tepedino and Torchio 1982a) have 
suggested that data from a long-term field study of O. lignaria pro- 
pinqua Cresson (a univoltine species) supports Fisher’s (1958) the- 
ory of an equilibrium sex ratio. In the three species studied here, 
however, it appears that any approach toward equilibrium sex ratio 
values is dependent upon constraints imposed by selection for parsi- 
voltinism. For example, since there is an association between the 
two-year form and the female sex, an increase in the relative fitness 
of two-year forms in any year could divert the population away 
from equilibrium and towards a female bias in subsequent years. 
The potential for such diversion should depend on the genetic sys- 
tem responsible for the polymorphism. However, the absence of any 
consistent tendency for population sex ratios of these species to 
move towards equilibrium (Fig. 1) suggests that this may be a real 
phenomenon. 
Summary 
Offspring from nests constructed in wooden domiciles by three 
non-social species of Osmia bees at two sites in northern Utah dis- 
played differences in the time required to complete development to 
the adult stage. Some members of each age cohort emerged in the 
following year, but a substantial proportion required two years to 
complete development. We propose the term “parsivoltine” to de- 
scribe such emergence patterns. 
There were differences in the proportion of one-year forms, both 
between years, within sites and between sites, within years for each 
species. The factors influencing these changes are unclear at present. 
The distribution of one-year and two-year individuals within nests 
suggests that environmental factors alone do not act on either the 
female parent or on her offspring to determine the developmental 
fate of the offspring. Many nests contained both one- and two-year 
