164 
Psyche 
[Vol. 91 
In contrast, the Pierid Colias eury theme Bdv., which overwinters as 
a quiescent (non-diapausing) larva, flew continuously during the 
drought years of 1975-76 and 1976-77 and nearly so in 1983-84. 
Collections of mature larvae of both P. zelicaon and P. rapae made 
in late October and November over 12 yr have consistently given 
from 85-100% diapause pupae. Larvae of P. zelicaon are normally 
absent from all sites by the third week of November. Rapae larvae 
are occasionally found on garden cabbages and weedy Crucifers 
into early January. A fifth-instar larva was collected on a weed on 
29 January 1979. 
The winter of 1982-83 produced 200% or more of normal rainfall 
over most of northern and central California. Nearly 20 cm of rain 
fell in March 1983 at our study sites, and the weather remained 
showery and unsettled into May. The spring flights of most butter- 
flies were late and poor; for many species the densities observed 
were the lowest in 12 yr. Both P. rapae and P. zelicaon were severely 
depressed at most sites; P. rapae was actually rare in spring, and the 
Willow Slough population of P. zelicaon, north of Davis, went 
extinct overwinter. The summer was unusually cool, cloudy, and 
moist. By autumn the populations of both species were near normal 
levels and at the Suisun Marsh P. zelicaon was commoner than 
usual at the end of the season. The latest flight recorded at Suisun 
for this species is 18 November (1973), a “false brood.” Although the 
species shut down nearly a month earlier in 1983, the autumn brood 
oviposited three to four weeks later than average. Similarly, at 
Davis, the last flight date of P. rapae was unexceptional but the 
population densities at the end of the flight were unusually high. For 
both species, these circumstances translated into an unprecedented 
number of larvae through mid-winter. 
In the Suisun Marsh, fourth- and fifth-instar larvae of P. zelicaon 
were common on Sweet Fennel, Foeniculum vulgare Mill. (Umbelli- 
ferae) on levees; collections were made on 28 December (4 L 5 , 1 L 4 , 
1 L 3 ) and 16 January (2 L 5 ). In addition, one L 3 was found on the 
same plant at Gates Canyon in the Vaca Hills, Inner Coast Range, 
near Vacaville, Solano County, 1 January. 
In the Davis area, a total of 67 larvae of P. rapae were collected 
from cultivated and weedy Cruciferae and Tropaeolum between 29 
December and 15 January. Although L!-L 3 were still numerous 
early in the period, only larger larvae (L 4 -L 5 ) were taken. Numbers 
were so high that Cruciferous vegetables were seriously damaged in 
many gardens at this time. 
