A. M. Shapiro.
Host-plant Utilization by Pieris napi Populations in California (Lepidoptera: Pieridae).
Psyche 81:361-366, 1974.

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PSYCHE
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Vol. 81 September-December, 1974 No. 3-4 HOST-PLANT UTILIZATION BY PIERJS NAPI
POPULATIONS IN CALIFORNIA
(LEPIDOPTERA : PIERIDAE)
BY ARTHUR M. SHAPIRO~
Department of Zoology, University of California, Davis, Calif. 95616
Oligophagous or polyphagous insects frequently exhibit ecologi- cally or geographically complex patterns of host-plant utilization. Such patterns have recently been documented for the butterflies Colias alexandra Edwards (Pieridae) (Ellis, 1974) and Euphydryas editha Boisduval (Nymphalidae) (White and Singer, 1974). This paper reports a similar situation among California populations of the Gray-Veined White, Pieris napi Linnaeus (Pieridae) and notes its potential significance in interspecific competition. Pieris napi is a circumpolar species; in its extensive boreal and temperate range it has been recorded on a great variety df plants of the family Cruciferae. In California it is widely distributed in foothill and lower montane (Yellow Pine-Incense Cedar-Douglas Fir) environments in the Coast Ranges south to San Luis Obispo County and in the Sierra Nevada at least as far south as Yosemite, but its host preferences have been very poorly documented. In their survey of Yosemite butterflies, Garth and Tilden (1963) listed in an Appendix "some plants on which Yosemite butterflies feed as larvae." Unfortunately these records, which are mostly not at- tributed, do not appear to be limited to Yosemite populations. Garth and Tilden list four Crucifer genera as hosts of P. napi: Barbarea (winter cress, yellow rocket), Brassica (mustard), Raphanus (rad- ish), and Dentaria (milkmaids, crinkleroot, toothwort). Of these, all but Raphanus are known hosts of P. napi in the northeastern United States. Tilden (1965) recorded P. napi in the "San Fran- cisco Bay area" on Dentaria only.
'This research was financed in part by grant D-804 from the Committee on Research of the Academic Senate, U.C. Davis. Manuscript received by the editor August 16, 1974.



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Psyche
[September-December
Figure 1.
Locations of California Pieris napi populations.



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19741 Sheiro - 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 I. 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 df 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.
6
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 sum 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 oficinule (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



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364 Psyche [September-December
had been laid on both upper and lower surfaces of rosette leaves, lower surfaces of the (appressed) cauline leaves (often near the point of attachment, as was also true on Barbarea verna), and on stems. One larva (3rd instar) was found, and two ovipositing females were observed. On 22 May, 16 additional ova and 5 larvae were found at the same spot on the same plants, which were now mature and beginning to set fruit. Abundant evidence of larval feeding was found on the rosettes and lower cauline leaves, but the flowers and siliques were undamaged. Fourteen plants of flowering Barbarea orthoceras Ledeb. were found in a grassy meadow 0.7 mile up the canyon, within the area where adult P. napi had been seen, but no ova, larvae, or feeding damage could be located on them. Lang Crossing, Nevada County. -This population is at about 4500 feet in the South Yuba River canyon, at or near the upper altidudinal limit of Sierran P. napi; it is also univoltine. On 22 May 19'74, 3 ova of P. napi were found in a stand o'f 30 immature Arabis glabra and none on 20 flowering Barbarea orthoceras growing immediately adjacent to them. On 2 June the same stand was again searched, producing I I ova of P. napi, 20 of P. rapae Linnaeus, 3 of A. Sara and 2 of Anthocaris lanceolata Lucas from A. glabra, and 8 of A. Sara from B. orthoceras. An additional 34 ova of P. napi were collected from A. glabra elsewhere in the vicinity. Four larvae of P. napi were 'found feeding on rosette leaves of A. glabra, and two of A. Sara on inflorescences and siliques of B. orthoceras. On 26 June four mature A. Sara and two immature A. lanceolata larvae were found on siliques of A. glabra and one mature A. Sara larva on siliques of B. orthoceras. Many A. glabra ~lants showed feeding damage to the rosettes, but only seven P. rapae larvae were found; probably most napi had already pupated. Pieris rapae, which is multivoltine, was flying in abundance at Lang Crossing on 19 July. Despite the large number of Pierid species and individuals, the visible impact of feeding on the Crucifers at Lang Crossing was quite small. In particular, Arabis glabra plants o'ften produced several hundred siliques on leafy, two- to four-foot stems. There are ten species of obligate Crucifer-feeding Pierids in California, occurring in various combinations at different localities. Some of these are spring-univoltine, a few are spring-bivoltine, and some are multivoltine. How they partition the available Crucifers among themselves may shed valuable light on the broad problems



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19741 Shapiro - Pieris napi Populations 365 of niche differentiation, competitive exclusion, and species packing. Emmel ( I 974) reported three species o'f inflorescence feeders (Pieris protodice Boisduval and LeConte, A. Sara and A. lanceolata) on Arabis glabra in Riverside County, 2 June 1973. Shapiro (1974) described five- and six-species Pierid assemblages in two high Sierran localities and concluded that competition is reduced by behavioral mechanisms (habitat selection) .
Pieris napi occurs with up to six other Crucifer-feeding Pierids at the four localities described above. Based on adult collections, the Crucifer-feeding Pierid faunas of the four localities are (inflorescence feeders are marked "I"; others primarily leaf feeders) : San Andreas Reservoir: P. napi, P. rapae, A. Sara (I), Euchloe ausonides Lucas (I).
Mix and Gates Canyons: P. napi P. rapae, P. sisymbrii Boisduval (I?), A. Sara (I), E. ausonides (I).
Washington: P. napi, P. rapae, A. Sara (I), A. lance~olata (I), E. ausonides (I).
Lang Crossing: P. n@i, P. rapae, P. sisymbrii (I?), A. Sara (I), A. lanceolata (I), E. ausonides (I), E. hyantis Edwards (I). Not all of these breed in the same microhabitats. At Lang Cross- ing, 'for example, P. sisymbrii and E. hyantis are found only in exposed rocky situations and appear to breed only on Streptanthus, and are thus not in competition with the woodland species. In examining the fauna of particular plants in particular habitats, the division of Pierids into a leaf-feeding and an inflorescence/silique feeding guild seems paramount. At Mix and Gates Canyons, where Barbarea is obviously in "short supply" and frequently completely defoliated, at least one species from each guild (P. napi, A. sara) can occur on this plant. At Lang Crossing the combined visible impact of four species - two of each guild -on Arabis glabra is so small that it is tempting to speculate that the populations are regulated by other 'factors below the level at which interspecific competition would be significant.
Despite the potential for interspecific competition, at each of the study areas one plant received the bulk of the attention from Pierids while another appeared largely or wholly unutilized. It remains to
be seen whether this reflects nutritional or toxicological unsuitability of certain 'Crucifer species. (In unpublished laboratory studies, Shapiro and F. Slansky (pers. comm.) have found variation in the suitability of native and weedy Crucifers as hosts of Pieris rapae



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366 Psyche [September-December
and other species.) It is also possible that host selection by ovi- positing females is closely tied to host plant phenology, and that flowering condition of the plants, and possible correlated changes in mustard oil concentrations, may determine the patterns observed. ELLIS, S. L.
1974. Field observations on Golias alexandra Edwands (Pieridae). J. Lepid. SOC. 28: 114-125.
EMMEL, J. F.
1974. in R. L. Langston. Zone 1. Season Summary, Lepid. Soc. News, 25 June, p. 3.
GARTH, J. S. AND J. W. TILDEN
1963. Yosemite Butterflies. J. Res. Lepid. 2: 1-96. SHAPIRO, A. M.
1974. Ecological and behavioral aspects of coexistence in six Crucifer- feeding Pierid butterflies in the central Sierra Nevada. Amer. Midi. Naturalist, in press.
TILDEN, J. W.
1965.
Butterflies of the San Francisco Bay Region. Berkeley and Los Angeles: University of California Press. 88 pp. WHITE, R. R. AND M. C. SINGER
1974. Geographical distribution of host plant choice in Euphydryas ed'ttha (Nymphalidae). J. Lepid. SOC. 28 : 103-106.



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