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Journal of the Lepidopterists' Society 48(2), 1994, 171-172
IMPLICATIONS OF INTRODUCED GARLIC MUSTARD
(ALLIARIA PETIOLATA) IN THE HABITAT OF
PIERIS VIRGINIENSIS (PIERIDAE)
Additional key words: host expansion, conservation.
Pieris virginiensis (W. H. Edwards) (Pieridae) is an uncommon and local univoltine member of the P. napi (L.) group from eastern North America, confined to forest un-derstory habitats where its primary hosts, Dentaria spp. (Brassicaceae), have relatively dense populations (Opler & Krisek 1984). Most other populations of the napi group are multivoltine, as are other Pieris species (brassicae [L.], rapae [L.], manni Mayer, etc.). The other univoltine napt-group members are confined to habitats with short growing seasons, and typically hybridize in ecotones with multivoltine races; (Petersen 1963; my unpubl. data). Allozyme data confirm that virginiensis is very close to other North American napi-group taxa (Geiger & Shapiro 1992). Univoltinism in P. virginiensis is presumed to be a derived adaptation to early senescence of Dentaria because summer generations would not have adequate host resources to complete larval growth (Shapiro 1971).
Garlic mustard, Alliaria petiolata (M.B.) (Brassicaceae), is an aggressive forest-edge and understory plant that was introduced into northeastern North America from Europe, and has become widespread in the midwestern U.S. since around 1950; there is interest in control of its spread (Willis 1992). Alliaria germinates in the spring and summer and forms a large, overwintering rosette, which then bolts in the early spring and blooms from late March to July. Plants with edible foliage can thus be found throughout the year.
At the Cedar Bluffs Nature Preserve in Monroe Co., Indiana, Dentaria laciniata Willd. is native and patchily distributed, whereas Alliaria petiolata is more widespread on the site. On 1 May 1993, Dentaria had already set seed, but the Alliaria were still in bloom. At approximately 1400 h, I observed a female P. virginiensis while she laid 6 eggs over a 10 minute period. Host-searching behavior was typical for Pieris: she flew slowly and frequently contacted leaves and flowers of a variety of plant species, but flew up immediately after contact with non-cruciferous leaves. When she contacted an Alliaria, sometimes by diving at the flower (suggesting visual cues were involved), she began searching and alighting on the lower leaves, ~l-2 dm above the ground. In most cases, she would lay a single egg on the underside of a leaf at this height:, but several plants were rejected. After oviposition, she always flew at least several meters before searching for the next host. She did not contact any Dentaria during this time. After collecting this female as a voucher (retained in my personal collection), I searched Dentaria, and occasional Arabis (Brassicaceae) plants for 10 minutes, and did not find any eggs or larvae. The female subsequently laid eggs on Alliaria in captivity; these eggs were reared into the second instar with moderate mortality before fieldwork overseas terminated the study.
There are three possible outcomes of the use of Alliaria as an oviposition site by P. virginiensis, and they depend on the palatability of the plant to the larvae. If the plant is toxic, then there are two possible outcomes: the virginiensis populations will decline because females waste eggs on Alliaria; and/or there will be selection on females to avoid Alliaria together. However, if Alliaria is palatable, even to some larvae in the population, then virginiensis will probably evolve a broader host range because Alliaria is a more widespread and reliable resource than Dentaria.
Isolated populations of virginiensis are likely to be in danger of extinction if Alliaria is toxic to the larvae. Pieris virginiensis maintains relatively small and localized populations, and does not often fly outside the forest understory. Alliaria is likely to be much more apparent than Dentaria in virginiensis habitat, both because Alliaria reaches high abundance over larger areas than Dentaria, and because Alliaria have larger plants with a longer flowering season. Female virginiensis searching for crucifers are likely to encounter and oviposit on Alliaria more often than Dentaria, and perhaps be prone to fly
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out of Dentaria patches while continuing to discover Alliaria plants. The resulting decline is likely to lead to extinctions of already small virginiensis populations before the ability to discriminate can evolve. Even if the ability to discriminate; does evolve locally, it is unlikely to spread rapidly and "rescue" other populations because of the philopatric tendencies of virginiensis.
On the other hand, it would be a welcome development if P. virginiensis were to expand its host range to include Alliaria. It would give P. virginiensis a better foothold where it does live, and perhaps allow it to expand its range of suitable habitats. In Europe, Alliaria petiolata is a common host of Pieris napi (Petersen 1963, Bowden 1971; my unpubl. data), but in North America, P. napi oleracea Harris has reportedly been unsuccessful on Alliaria (Bowden 1971), although I have frequently found P. rapae to use it (unpubl. data). Incipient host-range expansion is also of interest to population and community ecologists: because Alliaria foliage can be found in abundance throughout the year, natural selection would favor the return of Pieris virginiensis to its ancestral, multivoltine life cycle.
Population studies following the historical progress of the interactions between Alliaria and P. virginiensis, regardless of the outcome, would be quite relevant to our understanding of how host selection and host use evolves in natural populations. For example, Rhagoletis pomonella (Diptera: Tephritidae) is presumed to have switched from hawthorn to apple in New York State, whereupon the apple-feeding race spread over the northeastern U.S. (Bush 1968). Even though this case is among the best studied, the evidence for this scenario is weak because historical data are scant, and the switch may as easily have occurred more or less simultaneously in several parts of the species' range. Courant et al. (1994) independently report interactions between Alliaria and P. napi oleracea and Alliaria and P. virginiensis in New England, with similar observations to mine from Indiana; notably, New England populations have been in the presence of Alliaria longer than midwestern populations and may have had an evolutionary "head start." Strong selection by Alliaria is probably occurring over much of the range of virginiensis, and simultaneous observations of the interactions in geographically separated populations are needed to help understand processes involved in the evolutionary response to this selection.
This note has benefitted from discussions with Francie Chew and Art Shapiro, and two anonymous reviewers' comments. Francie was kind enough to send me her unpublished manuscript. Thanks are due to Mark Dybdahl and Lynda Delph for accommodations in Bloomington, and to Lynda for identifying the Dentaria.
Literature Cited
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25:6-12. Bush, G. 1968. Sympatric host race formation and speciation in frugivorous flies of the
genus Rhagoletis (Diptera, Tephritidae). Evolution 23:237-251. Courant, A. V., A. E. Holbrook, E. D. Van der Reijden & 'F. S. Chew. 1994. Native
pierine butterfly (Pieridae) adapting to naturalized crucii:er? J. Lepid. Soc. 48:168-
170. Geiger, H. & A. M. Shapiro. 1992. Genetics, systematics and evolution of holarctic
Pieris napi species group populations (Lepidoptera: Pieridae). Z. Zool. Syst. Evol.-
forsch. 30:100-122. Petersen, B. 1963. Breakdown of differentiation between Pieris napi L. and Pieris
bryoniae Osch. and its causes. Zool. Bidr. Uppsala 35:1-262. Opler, P. A. & G. O. Krisek. 1984. Butterflies east of the plains. Johns Hopkins Univ.
Press, Baltimore. Shapiro, A. M. 1971. Occurrence of a latent polyphenism in Pieris virginiensis (Lepidoptera: Pieridae). Entomol. News 82:13-16. Willis, J. 1992. Garlic mustard is hard to control. Trillium 1992:8-11.
Adam Porter, Department of Biological Sciences, Bowling Green State University, Bowling Green, Ohio 43403-0212, USA.
Received for publication 8 May 1993; revised and accepted 20 November 1993.
Date of Issue (Vol. 48, No. 2): 17 May 1994