PARSIVOLTINISM IN THREE SPECIES OF OSMIA BEES* 
By P. F. Torchio and V. J. Tepedino 
Bee Biology & Systematics Laboratory, 
Agricultural Research Service, USDA, 
Utah State University, UMC 53, 
Logan, Utah 84322 
Individuals of most insect species follow a relatively inflexible 
tempo of immature development and adult emergence that includes 
a single period of diapause in one generation per year at a specific 
stage in the life cycle. A few species depart from this pattern in that a 
small proportion of individuals of an age cohort require an addi- 
tional year or more to complete development to the adult stage 
(Waldbauer 1978, Beck 1980). Among bees, for example, there are 
brief reports of delayed emergence for several species (Davidson 
1896, MacSwain 1958, Krombein 1967, Torchio 1975, Parker 1980, 
Rust 1980) but none of these studies provides quantitative evidence 
to demonstrate that delayed emergence is an integral part of the life 
cycle. 
In this study we supply quantitative evidence to document pat- 
terns of delayed emergence in three species of megachilid bees 
( Osmia montana Cresson, O. californica Cresson, O. iridis Cocke- 
rell and Titus). Individuals of these species complete development in 
either one or two years, i.e., the emergence pattern of each age 
cohort is bimodal. Waldbauer (1978) used the term “type c” to 
describe bimodal ahd polymodal emergence patterns in which the 
peaks of emergence of an age cohort occur in different years. Here 
we introduce the more descriptive term, “parsivoltine”, to refer to 
this phenomenon. “Parsi” is adapted from the Latin pars for part or 
partial; -voltine, from the Italian volta for time or cycle is used in its 
usual entomological sense, as generations (cycles) per year. 
Our study addresses the following questions: 1) Does the propor- 
tion of one- and two-year individuals in a cohort differ between the 
two years of study and/or between the two sampling sites? 2) Is 
there an association between sex and time required to complete 
development? 3) How are one- and two-year forms distributed 
* Manuscript received by the editor September 9, 1982 
221 
