1979] Shapiro — Phenology of P. napi microstriata 
3 
The California drought began with the failure of the 1975 autumn 
rains. The 1975 flight of P. n. microstriata was rather poor, and 
both adult and immature numbers were down. Spring 1976 started 
warm (January +1.1°C compared to Vacaville means), turned 
cooler in April, then very warm (+1.9°) in May. The flight of P. n. 
microstriata was the largest ever observed at Gates and lasted for 
three months, although only one generation was involved. The sex- 
ratio was normal (about 1.7:1) and over 200 eggs and 50 larvae were 
seen. The host plants, Barbarea verna (Mill.) Asch. and Dentaria 
californica Nutt, (both Cruciferae) were early, of somewhat better 
than average luxuriance, and senesced early with the onset of hot 
weather in May-June. There was nothing meteorologically unusual 
about the months when 1975 eggs and larvae were developing, 
which would have promoted unusual survival. The very large adult 
population in 1976 could be due to diminished mortality due to 
predators, parasites, and disease or to direct meteorological effects 
on the dormant pupae. 
Winter 1976/77 was the second grossly deficient rainfall season. 
January-March 1977 were statistically unremarkable for tempera¬ 
ture. The adult population of P. n. microstriata was almost non¬ 
existent, despite the great burst of reproduction the previous year. 
Four males were seen—one each on four days—and at least one 
Table 1. Populations of Pieris napi microstriata at study sites. 
Year 
Inclusive dates of flight season 
N (rounded up to 
nearest 5 from census) 
1972 
1973 
1974 
1975 
1976 
1977 
1978 
(a) Gates Canyon, Solano County, California 
iii.28 (only visit) 
iii.23—iv. 18 
iii. 13 v.19 
iv.9-v.2 
ii. 21 —v.2 
iii.10-iv.28 
iii. 14-V.7 
30 
50 
25 
75+ 
5 
30 
5 
and (2nd brood) V.20-V.28 
(b) Lang Crossing, Nevada County, California 
1974 
1975 
1976 
1977 
1978 
v.5-vi.2 
v.l8-vi.9 
not studied 
iv. 17-V.13 
v.3-vi,9 
50 
20 
20 
30 
