1979] Shapiro — Phenology of P. napi microstriata 
1 
populations are pooled. There is a difference between genetic “obli¬ 
gate” diapausers, which are refractory to photoperiod, and faculta¬ 
tive diapausers which have been reared on inducing regimes, as well 
as a large spread of developmental times in each group. How this 
variability is expressed afield is unclear. Except for some pupae 
which do not develop at all in the first year, all individuals probably 
come out of diapause in January and subsequent development is 
temperature-dependent. The accumulated chilling in continuous 
cold storage is much greater than the wild pupae experience in 
fluctuating temperatures, so the “chilling requirement” is really a 
more complex chilling-time interaction. Except for pupae which 
carry over to a second or subsequent year, the effect of intrapopula- 
tional variance may be largely masked in cold, wet springs and 
maximally expressed in warm, dry ones. 
The carryover pupae provide the only reasonable explanation for 
the large 1978 population at Gates after the 1977 disaster. The 
numbers, however, suggest that a large number of carryover pupae 
may have been involved. This in turn suggests that something about 
the 1976/77 season cued a carryover response in a larger-than-usual 
fraction of the population. The temperature pattern is conveniently 
similar to 1977/78; the striking difference is rainfall. 
Carryover pupae occur in other Pierids, including Anthocaris 
sara and A. cethura (Felder & Felder), A. lanceolata Lucas, Euchloe 
hyantis (Edw.), E. ausonides Lucas, and Pirn's sisymbrii Bdv. in 
western North America. They also occur in Papilio rudkini Com¬ 
stock (Papilionidae), a desert swallowtail noted for its aseasonality, 
correlation of flights with heavy rains, and diapause pupae which 
may carry over for 6 years or more (Emmel and Emmel, 1973 and 
pers. comm.). There is strong circumstantial evidence for this spe¬ 
cies that a physiological response to water initiates post-diapause 
development—the insect behaves like a desert annual plant with a 
water-soluble seed-germination inhibitor. Of the Pierid species 
listed above, some occur in xeric and some in more mesic habitats, 
but not enough data are available to say whether the frequency of 
carryover pupae is correlated with rainfall uncertainty in the 
habitat. 
In any case, P. n. microstriata is a surprising insect to have the 
carryover response, given that it is almost certainly of Arcto- 
Tertiary mesic origin (Shapiro, 1975, 1977). Years of 25 cm or less of 
rain have occurred 14 times at Sacramento since 1849 and 14 times 
