The Papilio Glaucus Group"'

1958 The Lepidopterists' News 103 LARVAL FOOD PLANT SPECIFICITY IN BUTTERFLIES OF THE PAPILIO GLAUCUS GROUP"' by LINCOLN P. BROWER The Pap'ilio glaucus group contains three species (P. eurymedon Bois duval, P. mu/ticaudatus Kirby, and P. rutulus Boisduval) which occur to gether over extens:ve areas of the western United States, and a single almost entirely alJopatric one (P. glaucus Linne) which is widely distributed in northern and eastern North America. In addition to these four, the group includes P. alexiares H opffer which occurs only in the mountainous regions of eastern .Mexico. On the basis of larval, pupal, and male genital characters, it was shown that the three western butterflies are morphologically closer to each other than anyone is to P. gLaucus, and that P. pilumnus Boisduval be longs to the P. troi/us Linne group (Brower, in press, a). In order to understand the pattern of speciation in such a group of closely related animals, it is necessary to know the similarities and differences in their ecological requirements, one of the most important of these being food. The purpose of this paper is to present a critical review of the published records of the larval food plants of the butterflies of the Paf,ilio glaucus group to which will be added new data gathered by my research activities in the west ern United States during 1954-1956 and obtained through the cooperation of members of the Lrpidopterists' Society. Dur to the extensive sympatry of the three western butterflies and the similarity of their larv~, which have never been figured together in a single plate until the present publication (see figure), considerable error has un doubtedly been introduced into the literature by authors who did not rear eggs and/or larv~ found in the wild to the adult stage for positive identifica tion. Moreover, the only larval figure of P. eurymedolZ, first published by EDWARDS (1884) and later repu bl ished by COMSTOCK (1926, 1927), was based on a drawing which inaccurately represented the eye-spot pattern on the thorax of the fifth instar larva. A second source of error arises from the fact that it has only recently been established (Brower., in press, a) that the distribution of P. glaucus overlaps the northern extremity of the range of the three western species in southern British Columbia. A third is that worn females of P. glaZlcZls, P. rutulus, and P. eurymedolZ are almost indistinguish able in flight, which means that oviposition records themselves are subject to error. Fourth, and finally, several later authors have cited earlier ones who were undoubtedly mistaken for the above reasons. Because of these possibilities for confusion, all published records for each of the western species have been divided into three categories and tabulated (Tables 1-3). The first category summarizes the data given by authors who documented their records. Authors 111 the second category gave food plant records with insufficient or no support- *Submitted to the faculty of the graduate school of Yale University as partial ful fillment of the requirements for the degree of Doctor of Philosophy, May, 1957. Other portions of the dissertation are being published elsewhere. 104 BROWER: Papilio foodplants Vo1.12: Ilos.3-4 ing data. Those of the third definitely or apparently cited some or all records from previous publications. Such a stringent breakdown for P. glaucus is not necessary because most of the records are from eastern North America where confusion with the western species is impossible. Therefore, only a short discussion of the larval food plants of P. glaucus is included. No food plants have ever been recorded for P. rzlexiares. 1. Larval Foodplants of P. glaucus P. glaucus has been reported feeding on more species of plants in a greater number of families than any other member of the P. glaucus group, but there appears to be a tendency for different preferences in different locali ties. For example, EDWARDS (1885) reported that neither he in Coalburgh, West Virginia, nor J. AKHuRsT in Brooklyn, New York, had ever found larv::e on Salix spp. Previously EDWARDS (1884) had reported that he had attempted to force P. glazu'us larv::e to eat willow without success, but in 1886 he published several records which indicated that in northern New England, Salix is a common foodplant of this swallowtail. Following SCUDDER (1889), CLARK and CLARK (1951) stated that Fraxinus spp., Magnolia, and Lirio dendron tulipifera Linne are favorite food plants in the southeastern part of the range of P. glaucus, but in the northeast the main food plants are Prunus, Betula, and Populus. Data very kindly given to me by Drs. K G. MUNROE and T. N. FREEMAN which were gathered by the Canadian Insect Survey •T em • .1 Last (5th) ins tar larvre of Papilio multicaudatus (top), P. rutulus (center), and P. eurymedon (bottom). The larvre of P. multicaudatus and P. eurymedon have attained th eir maximum size while that of P. rutulus has not. From Kodachrome by L. P. Brower, Boulder County, Colorado, August, 1955. For comparison with P. glaucus, see Clarke & Sheppard, Lepid. News 11: 201-205; the larva of P. eurymedon figured here is the same as in their paper. 1958 The Lepido plerists' News 105 from 1947 to 1955 in southern Canada indicated the following larval fre quecics on plants from Ontario to Alberta: Betula J\apyrifera Marsh, 52; Populus tremuloides Michx., 43; Salix spp., 22 ; M alus pumila Mill, 14; Alnus spp., 12; Sorbus americana Marsh, 10; Prunus spp., 10; Betula lutea lVIichx., 6; Fraxinus americana Linne, S; Populus tacamahacca Mill, 3; A cer spp., 3; Fraxinus pennsylvanica var. lanceolata (Borkh,,) Sarg., 1. P. gLaucus is thus a polyphagous species eating plants in the families Rosaceee, Oleaceee, Salicaceee, Corylaceee, Magnoliaceee, Betulaceee, and Acer aceee, as well as others (see Scudder, 1889). 2. Larval Foodplants of P. rutulus I t can be seen in Table 1 that plants in the families Salicaceee, Plata Ilaceee, and Corylaceee are the only documented food plants of P. rutulus. In spite of the fact that several authors state that plants of the family Rosaceee serve as food of P. rutulus, the records with data do not support this view. MEAD (1878) found that although a captive female oviposited on Prunus sp. when confined in a bag about a branch of this plant, the larvee died after their first molt. He fu rther reported that six other females under similar conditions would not oviposit at all. EDWARDS (1884) reported that W. G. WRIGHT was likewise unable to obtain eggs by confining females over apple or cherry, although they did lay on Salix lasiolepis Benth. and S. lasiandra Benth. ]VIOl·eover, as mentioned below, EDWARDS' first instar larvee chose willow over apple, cherry, and Tulip Tree when presented with leaves of these four plants. Further evidence that Prunus is not a food plant of P. rutu Ius was indicated from the results of collecting larvee during 1955 in Boulder County, Colorado, when an extensive search of Prunlls virginiana Linne pro duced numerous larvee of P. Irlulticalldatus. but none of P. rutulus. (Weber's handbook. 1953, was used to determine all Colorado plants.) During the same summer, Dr. P. M. SHEPPARD and I observed a female laying one egg on Populus angustifolia James (N arrowleaf Cotton wood) along Left Hand Creek at 6,950 fe et altitude. The larva which hatched from this egg was reared to the adult stage on P. tremuloides (Quaking Aspen). This latter species was searched assiduously along the south-facing slopes of Left Hand Canyon, but no larvee were found on it. I t seems probable that the large cottonwoods along the creek are the princi pal food of P. rutulus in this area, but due to the large ,ize and inaccessibility of these trees it was not possible to confirm this view even though individuals which appeared to be ovipositing females were often seen fluttering about them. (That they were ovipositing is supported by our discovery of two eggs on cottonwoods along Big Cottonwood Creek in Salt Lake County, Utah during .T uly 1956; the resultant larvee were positively determined as P. rU lulus). Dr. C. L. R EMINGTON informed me that he in 1955 had found several larvee on the aspens at the University of Colorado Science Lodge which is not far from Left Hand Canyon, but at a higher altitude (9,500 feet). The absence of larvee on the aspens of the south facing slopes of this canyon and their apparent limitation to the cottonwoods along the creek may be related 106 BROWER: Papilio foodplants Vo1.12: nos.3-4 to the presence of larvx of P. ellryrnedon and P. Inliltiwudatus on these slopes. It is possible that natural selection has favored this microgeographical separa tion of P. rutu/us from the other two due to some adverse larval interaction resulting from their occurrence together on the same slopes. (See, for ex ample, Brower, 1958). In Boulder, Colorado, I obtained two larvx from Weeping Willow in September 1956. Several experiments on the suitability of L. tulipifera (Tulip Tree) as food for P. rutulus have been carried out which are particularly important due to the fact that this plant is one of the principle foods of P. glaucus in the southern part of its range. In a letter, F. P. SALA stated that wild P. rutulus females had oviposited for two successive years on an imported tree he had planted in his yard at Burbank, California, but the larvx would not eat the leaves. This confirmed the report of EDWARDS (1884, 1885) who in West Virginia tried to feed the leaves to larvx hatched from eggs sent to him by W.

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