1974] 
Shapiro — Pieris napi Populations 
363 
During 1974 a number of Sierran and Coast Range populations 
of P. napi were sampled for photoperiodism studies, resulting in the 
incidental accumulation of data on host selection. The locations of 
these populations are shown in figure 1. The pattern of host selection 
proved to be of considerable interest, as reported below. 
San Andreas Reservoir , San Mateo County. — This is a coastal, 
facultatively bivoltine population from a gully (elevation about 200 
feet) subject to summer fog. On 10 April 1974, 58 ova of P. napi 
were found by searching 86 plants of Barbarea verna (Mill.) Asch., 
and three females were observed ovipositing on this plant. No ova 
were found on 25 plants of Dentaria californica Nutt, growing 
nearby in the same canyon. Ova on B. verna were placed on leaves 
(mostly on lower surfaces), stems, and pedicels. Most of the Bar- 
barea plants had only cauline leaves and were in the early stages of 
flowering. Dentaria were in the advanced stages of flowering and 
had some green siliques. 
Mix and Gates Canyons , Solano County. — This is a strictly uni- 
voltine population from summer-arid canyons of the east slope of the 
Vaca Mountains, central Inner Coast Ranges, open to the Sacra- 
mento Valley (elevations 500 to 2000 feet). On 4 April 1974, 
39 ova of P. napi and 21 of Anthocaris sara Reakirt were found on 
66 plants of Barbarea verna; the sara ova were mostly in the in- 
florescence and on the upper surfaces of leaves, while the napi ova 
were scattered as described for San Mateo County, but seldom on 
the upper surfaces of leaves. No Pierid ova of any kind were found 
on 40 Dentaria californica near the Barbarea. On 19 May 1974, 
14 larvae of P. napi were collected on B. verna. They were feeding 
on leaves, and the green siliques had not been damaged although 
most of the plants were nearly or quite defoliated; only one napi 
larva was found feeding on siliques, and this was on a plant whose 
leaves and petioles had been entirely consumed. Seven larvae of 
A. sara were found on five plants of Sisymbrium officinale (L.) 
Scop, (hedge mustard) and two on B. verna. All of these were 
feeding on green siliques only, on plants whose leaves were undam- 
aged. Four of 20 Dentaria examined had been defoliated, appar- 
ently by Pierid larvae, but no larvae were collected. 
Near Washington, Nevada County. — Located in the South Yuba 
River canyon at about 2600 feet on the west slope of the Sierra, this 
is also a univoltine population. On 3 May 1974, 78 ova of P. napi 
were collected by the author and S. R. Sims from about two dozen 
immature Arabis glabra (L.) Bernh. (tower mustard). The ova 
