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Psyche 
[Vol. 89 
between and within individual nests? 4) Are inter-individual differ- 
ences in the time required to complete development due to environ- 
mental factors or to a genetic polymorphism, or both? 
The three Osmia species are restricted to the western U.S. where 
they are sympatric and at least partially synchronic (late spring- 
early summer). Each nests gregariously in pre-existing holes, usually 
in wood. The biologies of O. montana and O. californica are sum- 
marized by Rust (1974); the biology of O. iridis is currently under 
study (Torchio, unpub.). Briefly, nests of each species are composed 
of a linear series of cells. Each cell is provided with pollen, nectar 
and an egg; cells are separated by partitions constructed of macer- 
ated leaf material (O. montana, O. iridis ) or mud mixed with macer- 
ated leaf material ( O . californica ), and nests are plugged with one or 
more partitions. Osmia montana and O. californica are oligoleges of 
the Compositae; O. iridis is restricted to a non-composite host plant. 
Methods 
Nests of these Osmia species were obtained from trap blocks 
placed at two field locations (Torchio 1976). Trap blocks of sugar 
pine contained 49 drilled holes to accommodate paper soda straws 
measuring 14.5 cm long and 7 mm inside diameter. One hundred 
nest blocks were placed at each of two study sites during both study 
years (1979-1980). 
The Faust trapping site was located 42 km south of Logan, Cache 
Co., Utah at 1800 m elevation. This location is on a hillside with a 
SW exposure and is covered by large stands of mature aspen ( Popu - 
lus tremuloides Michx.) trees surrounded by open, grassland mead- 
ows. The Mendon site is located 24 km west of Logan at 1500 m 
elevation on a hillside having a SE exposure. Solid stands of maple 
(Acer glabrum Torr.) or aspen trees surrounded by open meadows 
were characteristic of the area. 
During both study years, nest blocks were attached individually 
to standing trees during mid-May prior to Osmia flight and returned 
to the laboratory in early July where they remained at room illumi- 
nation and temperature. All nests were dissected in early September; 
individual cocoons were opened to determine sex ratios of adults 
(one-year forms) and position of larvae (two-year forms) in nests. 
Larvae of two-year forms were weighed on an electrobalance (0.1 
