1982] 
Torchio & Tepedino — Osmia Bees 
227 
tion. Although all three species nest in pre-existing holes in dead 
wood, in natural situations it may sometimes be possible for emer- 
gent one-year adults to gain egress without destroying their two- 
year siblings. For example, if the nest is in a rotting log emergent 
forms may be able to chew around nestmates. Thus, the estimates 
given here should be regarded as maximums. 
The results of these comparisons (Table 3) demonstrate that a 
large portion of two-year forms was doomed in each site-year (range 
40.5-61.3%). This mortality would be in addition to any losses due 
to enemies or developmental arrest. It is also interesting to note that 
in each site-year a significantly lower proportion of two-year 
females than males would be destroyed by their siblings (X 2 tests, P 
< 0.001 all cases). This is because O. montana, like most bees that 
construct nests in pre-existing holes (including O. californica, O. 
iridis ), deposit female eggs in cells in the lower reaches of the nest 
and males in outer cells (Krombein 1967). Thus, the probability that 
a two-year male larva will be destroyed by an emerging sibling adult 
is greater than for a two-year female. 
Table 3. “Doomed” offspring by sex from mixed nests of Osmia montana and 
O. californica. 
Males 
Females 
Totals 
Species, 
Site, Year 
N 
% 
Doomed 
N 
% 
Doomed 
N 
% 
Doomed 
O. montana 
Faust 1979 
179 
84.9 
165 
35.8 
344 
61.3 
1980 
158 
69.6 
269 
23.4 
427 
40.5 
Totals 
337 
77.7 
434 
28.1 
771 
49.8 
Mendon 1979 
81 
81.5 
95 
35.8 
176 
56.8 
1980 
64 
64.1 
74 
24.3 
138 
42.8 
Totals 
145 
73.8 
169 
30.8 
314 
50.6 
O. californica 
Faust 1979 
5 
40.0 
2 
50.0 
7 
42.9 
1980 
207 
69.1 
69 
18.8 
276 
56.5 
Totals 
212 
68.4 
71 
19.7 
283 
56.2 
Mendon 1979 
79 
46.8 
40 
17.5 
119 
37.0 
1980 
49 
75.5 
47 
19.1 
96 
47.9 
Totals 
128 
57.8 
87 
17.8 
215 
41.9 
