1982] 
Torchio & Tepedino — Osmia Bees 
233 
A second point of interest is that two-year forms of all three 
species undergo two periods of diapause (once as post-defacating 
larvae during the first winter, and again as adults over the second 
winter) whereas one-year forms diapause only as adults. For most 
other insect species, diapause is stage specific and occurs only once 
in the life cycle (Beck 1980); there are a few reports of non- 
hymenopterous insect species that enter diapause in more than one 
stage (e.g., Harvey 1967, Lounibos and Bradshaw 1975). The physi- 
ological mechanisms which enable species to undergo two discrete 
periods of diapause are unknown (Chippendale 1977, Waldbauer 
1978). 
A third unusual result of this study is the relatively large propor- 
tion of individuals in each age cohort which were two-year forms 
(Fig. 1). When data for live offspring were combined for all site- 
years by species, 41% of all O. montana, 57% of all O. calif ornica 
and 13% of all O. iridis required two years to complete develop- 
ment. In contrast, the percentage of individuals requiring prolonged 
periods to complete emergence in most other species with parsivol- 
tine emergence patterns is low (Powell 1974, Waldbauer 1978, Sha- 
piro 1979, Tauber and Tauber 1981). 
A potential explanation for the high proportion of two-year 
forms among these species has been provided by Cohen (1966, 
1968). In his treatment of optimal reproductive strategies, Cohen 
noted that when weather and/ or resources exhibit large year to year 
fluctuations and, as a result, the year to year variance in reproduc- 
tive success is also large, it would be adaptive for organisms to 
produce offspring types that differed in the time required to reach 
maturity. By this means, the effects of years unfavorable to repro- 
duction would not fall upon all members of an age cohort (See also 
Powell 1974, Hedrick et a/. 1976, Waldbauer 1978, Shapiro 1979, 
Real 1980, Tauber and Tauber 1981). Cohen (1966, 1968) also 
hypothesized that variance in reproductive success should be posi- 
tively associated with the proportion of offspring that require an 
extra year (or more) to complete development; and that, as the 
viability of two-year forms decreased relative to one-year forms, the 
proportion of two-year forms in the population should also 
decrease. Thus, Cohen’s theoretical results suggest that the high 
proportion of two-year forms in these Osmia species may be due to 
