226 
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[Vol. 89 
Table 2. Transitions within mixed nests between one- and two-year forms for 
Osmia montana and californica. Transition from inner one-year forms to outer two 
year forms = 1-2 yr and similarly for 2-1 yr. Double switch signifies the transitions 
1-2-1 yr or 2-1-2 yr. 
Osmia montana 
Osmia californica 
1-2 yr 
2-1 yr 
Double 
switch 
1-2 yr 
2-1 yr 
Double 
switch 
Faust 
1979 
22 
16 
47 
0 
1 
1 
1980 
6 
31 
63 
5 
4 
52 
Totals 
28 
47 
no 
5 
5 
53 
Mendon 
1979 
11 
17 
32 
5 
21 
12 
1980 
3 
12 
17 
3 
9 
22 
Totals 
14 
29 
49 
8 
30 
34 
For example, in a study of marked O. lignaria only 5 of 1 1 1 nests 
(same type as used here) made in a greenhouse contained offspring 
produced by more than one female (Tepedino and Torchio 1982b). 
Distribution of one- and two-year forms in mixed nests is sum- 
marized in Table 2. Nests with double switches were more numerous 
than those in the other two categories combined in all site years; 
transitions from two-year forms in inner cells to one-year forms in 
outer cells were about twice as common as the reverse situation. 
Thus, factors such as photo- and thermoperiods, which act on the 
maternal genotype to induce diapause in the offspring of other spe- 
cies of Hymenoptera (Parker and Tepedino 1982), do not seem to 
influence the determination of one- or two-year forms in O. 
montana. 
The interspersion of one- and two-year forms in mixed nests 
creates the potential for fratricide. Observations of trap-nests both 
in field and laboratory demonstrated that one-year forms destroy 
any two-year larval siblings positioned above them in the nest when 
they emerge (Torchio, unpub.). We therefore examined the data for 
mixed nests to determine the number of surviving and “doomed” 
two-year offspring by sex. The category doomed was assigned to 
any two-year form with a one-year form between it and the inner 
limit of the nest. All two-year forms without one-year forms posi- 
tioned below them were classified as surviving. Our estimates of the 
percent doomed two-year offspring should be regarded with cau- 
