1979] Shapiro — Phenology of P. napi microstriata 
5 
Table 2. Precipitation at nearest weather stations to study sites. 
Vacaville Blue Canyon Airport 
Water year (July l- June 30) rainfall, cm total precip., cm snowfall, cm 
1971-72 
26.19 
133.48 
676.15 
1972-73 
88.67 
180.28 
743.46 
1973-74 
66.35 
244.43 
769.62 
1974-75 
55.73 
175.06 
743.20 
1975-76 
23.47 
88.27 
302.26 
1976-77 
23.32 
68.53 
225.55 
1977-78 
95.78 
223.19 
751.84 
of the egg-laying had been completed. Cold rains then began and 
turned to snow: on 5 May, 15 cm of snow fell; by 8 May this had 
increased to 33 cm. There were daily rain showers thereafter until 18 
May. On 13 May, with north-facing slopes snow-covered, as many 
plants as were exposed were censused. 43 eggs and 2 small larvae 
were found on Barbarea. On 21 May the same plants, along with 
others and several Arabis glabra (L.) Bernh., were again censused 
but not one egg or larva was found. Larval feeding damage was 
limited to the pinholes inflicted by first-instar larvae. Insofar as 
could be determined, the entire 1977 reproductive output was lost. 
Other than a soft-winged female found 13 May, no more adults were 
seen. 
Harcourt (1966) found that rain was a major mortality factor for 
eggs and larvae of Pieris rapae (L.) on cabbage in Canada. 
Winter 1977/78 saw a return of heavy snowfall, but there was also 
so much warm rain that snow packs were not good, and Lang was 
snow-free 12 April: 10 species, not including P. n. microstriata, were 
flying. 32 cm of snow fell 15 April, with cold rain 24-25 and 28 
April. On 3 May one male was seen. Snow fell again 23-24 May. On 
29 May and 2 June P. n. microstriata were common, with eggs on 
the latter date on (in descending order of preference), Arabis, Nas¬ 
turtium, and Barbarea. The last adult seen was 9 June; on 15 June 3 
mature larvae were still found on Arabis. 
Discussion 
There is little in the literature on catastrophic extinctions of natu¬ 
ral butterfly populations (Ehrlich et a/., 1972). Both populations of 
P. n. microstriata under study flirted with extinction in 1977, for 
