1979] Shapiro — Phenology of P. napi microstriata 
9 
most years 75-80% of potential hosts are censused at Gates, but 
only 50% at Lang. 
There are two major modeling approaches to insect phenology. 
One, the physiological-time or degree-day approach, began with 
Shelford’s (1927) study of the codling moth, Laspeyresiapomonella 
(L.) (Tortricidae). This is still the best-quantitated species (Riedl, 
Croft and Howitt 1976; Riedl and Croft, 1978). Here the emphasis is 
on the development of predictive algorithms from empirical data. 
The other approach proceeds from Darwinian principles and is 
exemplified by the treatment of Levins (1969) or Cohen (1970) who 
derive strategies from survival and reproduction parameters. The 
two approaches are complementary: responses to real meteorologi¬ 
cal events are the proximate consequences of selection for a genetic 
blueprint of development. The fusion of the two approaches into a 
comprehensive theory of insect phenology will require data on how 
seasonality contributes to resource utilization, competition, and 
survival. 
Acknowledgements 
Field assistance in 1977 was provided by Susan J. Katz. This 
research was funded in part by grant D-804 from the Committee on 
Research, U.C. Davis. 
Literature Cited 
Bradshaw, W.E. 1974. Phenology and seasonal modeling in insects, in H. Lieth, 
ed.. Phenology and Seasonality Modeling. Springer, New York, pp 
127-138. 
Cohen, D. 1970. A theoretical model for the optimal timing of diapause. Amer. 
Nat. 104: 389-400. 
Ehrlich, P. el al. 1972. Weather and. the “regulation” of subalpine populations. 
Ecology 53: 243-247. 
Emmel, T. C. and J.F. Emmel. 1973. The Butterflies of Southern California. Los 
Angeles County Museum of Natural History. 
Figgins, W. E. 1971. Climate of Sacramento, California. NOAA Tech. Mem. 
NWS-WR65. 63 pp. 
Giesel, J. T. 1976. Reproductive strategies as adaptations to life in temporally 
heterogeneous environments. Ann. Rev. Ecol. Syst. 7: 57-79, 
Harcourt, D. G. 1966. Major factors in survival of the immature stages of Pieris 
rapae L. Can. Ent. 98: 653-662. 
Levins, R. 1969. Dormancy as an adaptive strategy. Svmp. Soc. Exptl. Bio! 23- 
1 - 10 . 
