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1958

The Lepidopterists' Nevus

103

LARVAL FOODPLANT SPECIFICITY IN BUTTERFLIES OF THE PAPILIO GLAUCUS GROUP*

by Lincoln P. Brower

The Papilio glancus group contains three species (P. eurymedon Bois-duval, P. multicaudatus Kirby, and P. rutulus Boisduval) which occur together over extensive areas of the western United States, and a single almost entirely allopatric one (P. glaucus Linne) which is widely distributed in northern and eastern North America. In addition to these four, the group includes P. alexiares HopfTer 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 any one is to P. glaucus, and that P. pilumnus Boisduval belongs to the P. troilus 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 foodplants of the butterflies of the Papilio glaucus group to which will be added new data gathered by my research activities in the western United States during 1954-1956 and obtained through the cooperation of members of the Lepidopterists' Society.

Due to the extensive sympatry of the three western butterflies and the similarity of their larvae, which have never been figured together in a single plate until the present publication (see figure), considerable error has undoubtedly been introduced into the literature by authors who did not rear eggs and/or larvae found in the wild to the adult stage for positive identification. Moreover, the only larval figure of P. eurymedon, first published by Edwards (1884) and later republished 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. glaucus, P. rutulus, and P. eurymedon are almost indistinguishable 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 in the second category gave foodplant records with insufficient or no support-

*Submitted to the faculty of the graduate school of Yale University as partial fulfillment of the requirements for the degree of Doctor of Philosophy, May, 1957. Other portions of the dissertation are being published elsewhere.

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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 foodplants of P. glaucus is included. No foodplants have ever been recorded for P. alexiares.

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 localities. For example, Edwards (1885) reported that neither he in Coalburgh, West Virginia, nor J. Akhurst in Brooklyn, New York, had ever found larvae on Salix spp. Previously Edwards (1884) had reported that he had attempted to force P. glaucus larvae 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 foodplants in the southeastern part of the range of P. glaucus, but in the northeast the main foodplants are Prunus, Betula, and Populus. Data very kindly given to me by Drs. E. G. Munroe and T. N. Freeman which were gathered by the Canadian Insect Survey

T

Sfe.          t&S I cm

Last (5th) instar larvae of Papilio multicaudatus (top), P. rutulus (center), and P. eurymedon (bottom). The larvae of P. multicaudatus and P. eurymedon have attained their 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.

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from 1947 to 1955 in southern Canada indicated the following larval fre-quecies on plants from Ontario to Alberta: Betula papyrifera Marsh, 52; Populus tremuloides Michx., 43; Salix spp., 22; Malus pumila Mill, 14; Alnus spp., 12; Sorbus americana Marsh, 10; Prunus spp., 10; Betula lute a Michx., 6; Fraxinus americana Linne, 5; Populus tacamahacca Mill, 3; Acer spp., 3; Fraxinus pennsylvanica var. lanceolata (Borkh.) Sarg., 1.

P. glaucus is thus a polyphagous species eating plants in the families Rosaceae, Oleaceae, Salicaceae, Corylaceae, Magnoliaceae, Betulaceae, and Acer-aceae, as well as others (see Scudder, 1889).

2. Larval Foodplants of P. rutulus It can be seen in Table 1 that plants in the families Salicaceae, Plata-naceae, and Corylaceae are the only documented foodplants of P. rutulus. In spite of the fact that several authors state that plants of the family Rosaceae 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 larvae died after their first molt. He further 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. Moreover, as mentioned below, Edwards' first instar larvae chose willow over apple, cherry, and Tulip Tree when presented with leaves of these four plants. Further evidence that Prunus is not a foodplant of P. rutulus was indicated from the results of collecting larvae during 1955 in Boulder County, Colorado, when an extensive search of Prunus virginiana Linne produced numerous larvae of P. multicaudatus, 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 (Narrowleaf Cottonwood) along Left Hand Creek at 6,950 feet 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 larvae were found on it. It seems probable that the large cottonwoods along the creek are the principal food of P. rutulus in this area, but due to the large size 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 July 1956; the resultant larvae were positively determined as P. rutulus). Dr. C. L. Remington informed me that he in 1955 had found several larvae 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 larvae on the aspens of the south facing slopes of this canyon and their apparent limitation to the cottonwoods along the creek may be related

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to the presence of larvae of P. eurymedon and P. multicaudatus on these slopes. It is possible that natural selection has favored this microgeographical separation of P. rutulus from the other two due to some adverse larval interaction resulting from their occurrence together on the same slopes. (See, for example, Brower, 1958). In Boulder, Colorado, I obtained two larvae 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 larvae would not eat the leaves. This confirmed the report of Edwards (1884, 1885) who in West Virginia tried to feed the leaves to larvae hatched from eggs sent to him by W. G. Wright from California. According to Edwards, nine hatching larvae ate them sparingly before dying a few days later. He offered larvae from the next lot of eggs a choice of willow, cherry, apple, and Tulip Tree and on the subsequent day all were found feeding on the willow (Golden and/or Weeping), which they ate successfully. Clarke and Sheppard (1955) again indicated that P. rutulus will oviposit on L. tulipifera. They hand-paired a female (ex-pupa from Washington state) with a P. glaucus male (ex-pupa from Illinois) and the captive female laid twenty-one eggs on it. Unlike the pure P. rutulus in the above experiments which refused to eat,

TABLE 1 OPPOSITION AND FOODPLANT RECORDS FOR PAPILIO RUTULUS

Plant Species              Plant Family Region           Source                   Stages, etc.1

pard 1955

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III.

Authors with some or all records definitely or apparently cited from previous publications include Elrod (1906), Wright (1906), Comstock (1921, 1927), Jordan (1924), Holland (1930), Garth (1935, 1950), and Macy & Shepard (1941).

1   In this column: O = oviposition observed in nature; I = oviposition induced in captivity; R= refused to feed; P = fed and then died (plant toxic?) ; E = found as wild egg; LF = found as wild larva; L= reared to larva; C = some reared to pupa; A == some reared to adult; the last item, in parentheses, shows the number of individuals ("n" means several).

2   Inferred from context.

3   Died of "neglect" during fourth instar.

4   Confustion with P. glaucus possible in British Columbia records.

5   "apricot, cherry, prune; wild cherry, wild plum."

6   Comstock in litt. has told me that he was referring to Ptelea baldivinii when he published the vernacular name, "Hop" (1921, 1927). Macy & Shepard (1941) incorrectly iterpreted this "Hop" as Humulus.

or ate leaves and then died, these hybrid larvae accepted them and survived. These results suggest that the ability to eat L. tulipifera is a genetically controlled dominant character. The same larvae also ate Syringa (Oleaceae) for a short period when supplies of Liriodendron were low.

In conclusion, P. rutulus appears to be mainly restricted to the Salicaceae, Platanaceae, and will perhaps eat Alnus viridis DC. of the family Corylaceae. Although it is much more restricted in its diet than P. glaucus, both have main foodplants in common; P. rutulus also differs from P. glaucus in being unable to eat L. tulipifera, even though both will oviposit on it. This difference appears to have a genetic basis with dominance for Liriodendron feeding.

3. Larval Foodplants of P. multicaudatus It can be seen in Table 2 that plants in the families Rosaceae, Oleaceae, and Rutaceae are the only documented foodplant records for P. multicaudatus. The statements that Populus and Salix are foods are not based on data.

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Plants of the family Oleaceae were first recorded by Ruelas (1925) in the Federal District of Mexico, and in a letter Professor J. R. Merritt of the University of Louisville established that Fraxinus is a foodplant in Raton, New Mexico. He found eggs and larvae in various stages throughout most of the summer of 1935 on a species of ash planted about the city library, and from these he reared four males and five females on this foodplant. Recently, Kendall (1957) reported that he had observed a female ovipositing on Ligustrum vulgare Linne in San Antonio, Texas.

In July, 1954, 1 confined two wild-caught females in a gauze bag about a small bush of Prunus virginiana in Left Hand Canyon. One of these, or both together, laid forty-four eggs which hatched and began to eat this food-plant. Since it was necessary to leave Colorado, the larvae were taken during the first instar to North Carolina and subsequently to New Jersey and in both areas readily accepted P. pennsylvanica Linne. Although about half died in the first and second instars, the rest survived and produced adults, with the exception of one which died in forming its pupa. In August, 1955, another wild female confined in a wire screen cage laid four eggs on P. virginiana cuttings at the University of Colorado Science Lodge. During the same summer, a concerted effort was made to collect larvae of P. miilticaudatus, P. rutulus, and P. eurymedon in Boulder County, Colorado. The month of August was devoted to this task and a search was made in Left Hand Canyon from its mouth at 5,700 feet altitude to its head at 9,500 feet on the following plants of the family Rosaceae: P. virginiana (Choke Cherry), P. pennsylvanica (Wild Red Cherry or Pin Cherry), and P. americana Marsh (Wild Plum). Of these plants, only P. virginiana produced eggs and larvae, and a total of 95 were found (excluding parasitized and/or dead eggs). Several of these were preserved before pupation, and the following summer several adults emerged. The preference of females to oviposit on P. virginiana rather than P. pennsylvanica was emphasized by the fact that in Boulder Canyon about three tenths of a mile above the automobile tunnel on the south-facing slope at the edge of the highway, on 1 September, several large P. pennsylvanica bushes had no larvae on them whereas a lone small P. virginiana bush among these did have a fifth instart larva on it. In 1954, Crataegus ery-thropoda Ashe (Hawthorn, Rosaceae) was extensively searched near this canyon, but no eggs or larvae were found on it. Such extreme specialization within the family Rosaceae was also suggested by Pronin (1955) who said that P. miilticaudatus limits its egg laying to P. virginiana in northern California (Shasta County). We also thoroughly examined Populus tremuloides, Kibes inerme Rydb. (Gooseberry, Grossulariaceae), and Ceanothus fendleri Gray (Buckthorn, Rhamnaceae) in Left Hand Canyon, but no larvae of P. multi-caudatus were found on these plants, although numerous P. eurymedon larvae were found exclusively on the latter one. The foodplants of both are largely confined to the dry, south-facing slopes of the canyons in Boulder County.

Kendall (1957) reported finding more than thirty P. multicaudatus larvae on Ptelea trifoliata Linne in San Antonio, Texas, thereby recording the family Rutaceae as known food, as noted earlier by Behr (1884).

1958                                                       The Lepidopterists' News                                                      109

TABLE 2 OVIPOSITION AND FOODPLANT RECORDS FOR PAPILIO MULTICAUDATUS

III.

Authors with some or all records definitely or apparently cited from previous publications include Jordan (1924), Holland (1930), Davenport & Dethier (1937), Essig (1946), and Garth (1935, 1950).

1   For symbols in this column, see footnote 1 of Table 1.

2   "Prunus demissa"

3   "L japonicum", according to Standley (1920-1926), = L. lucidum.

4   Species is certain because no other members of the Papilio glaucus group occur in the Federal District of Mexico.

5   Apparently based on Wright's (1906) suggested possible foodplants.

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In conclusion, it appears that the foodplants of P. multicaudatus are in the families Rosaceae, Oleaceae, and Rutaceae and are thus different from the foods of P. rutulus. As in the latter species, P. multicaudatus is more restricted in its diet than P. glaucus and the two have main foodplants in common. It will be interesting to discover whether P. multicaudatus favors Oleaceae in the southern part of its rangs and Rosaceae in the Northern part, for if it does, there would be a striking parallel with P. glaucus which appears to have these geographic preferences.

4. Larval Foodplants of P. euryrnedon

It can be seen in Table 3 that the only documented foodplants for P. euryrnedon belong to the family Rhamnaceae. As in the case of P. rutulus, plants in the family Rosaceae purported to be the food of P. euryrnedon are not supported by full data.

Of the three western species, females of this one seem the most prone to oviposition errors. Remington (1952) reported an observation made by W. H. Evans in California that females each year laid a few eggs on a cultivated Jasminum (Oleaceae) which the larvae would not readily accept. On one occasion, after the larvae had only nibbled at these leaves for a few days, they were transferred to Rhamnus crocea Nutt., but all died. Moreover, F. P. Sala of Burbank, California, reported in litt. that he had observed females laying on Prunus persica Stokes (Peach) which he said the larvae would not eat. However, he has taken eggs from this plant and reared the larvae successfully on Rhamnus. It is thus apparent that representative plants of two families which comprise the main diet of P. multicaudatus are unsuitable for P. euryrnedon even though females occasionally oviposit on them.

While collecting larvae in Left Hand Canyon during August, 1955, we assiduously searched but failed to find any eggs or larvae on Ribes inerme. However, a female P. euryrnedon apparently erred in laying a single egg on this plant when pursued up the canyon-side by Drs. C. L. Remington and P. M. Sheppard. Remington (1952) reported an observation of D. Eff who saw a female deposit one egg on Crataegus rivularis Nutt. near Rabbit Ears Pass, Colorado. The fact that a thorough search of Crataegus erythopoda near Left Hand Canyon in 1954 yielded no results, suggests that this, too, may have been an error. McDunnough's (1927) report that he observed a P. euryrnedon female ovipositing on Alnus in the Seton Lake Region of British Columbia requires confirmation for the reason noted in Table 3.

During the search for larvae in Left Hand Canyon in 1955, 66 larvae of P. euryrnedon were found on Ceanothus fendleri on the south-facing slopes. Several of these were preserved, and several yielded adults the following year. In spite of the fact that we searched Prunus virginiana, P. americana, and Populus tremuloides as well as Ribes inerme, no P. euryrnedon larvae were found except on C. fendleri. We also looked on the leaves of Ceanothus velu-tina Dougl., which is a more localized species in Boulder County, but found no larvae on it. Their absence on this plant may be due to the gummy substance coating its leaves. The only other member of the family Rhamnaceae in this area is C. ovatus Desf., but this plant was so rare in Left Hand Canyon

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TABLE 3 OVIPOSITION AND FOODPLANT RECORDS FOR PAPILIO EURYMEDON

Plant Species               Plant Family         Region               Source                          Stages, etc1

1927

III.

Authors with some or all records definitely or apparently cited from previous publications include Strecker (1878), Behr (1884), Elrod (1906), Wright (1906), Comstock (1921, 1927), Jordan (1924), Holland (1930), and Davenport & Dethier (1937).

1   For symbols in this column, see footnote 1 of Table 1.

2   Dyar (1893) described the life history of P. eurymedon. It is therefore assumed that he correctly determined these larvae.

3  In litt. from Andrew Anderson of Pateros, Washington.

4   Confusion with P. rutulus or P. glaucus possible if females of the three species were worn.

that no significant observations could be made. In August 1956, 12 additional larvae were found on C. fendleri along the road in this canyon at 6,950 feet.

In collecting larvae of P. eurymedon it soon became evident that plants of a large size and lush appearance were preferred to the abundant small or partly dead ones. Moreover, those which grew in front of large rocks on the steep, south-facing slopes of the canyon in exposed positions seemed preferred to all others.

In conclusion, it appears that P. eurymedon is the most restricted of the three western species, being limited to plants of the family Rhamnaceae. The

E; LF; A; (78) LF; A; (?) E; LF; A; (?) L; C; A; (?) E; LF2; (?) O; (n) O; (?) O; P; (n) O; R; (n) O; (1) O; (1)

O4; (n)

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three western ones thus are mutually exclusive in their foodplants, and are all more limited in their diets than their eastern and northern relative, P. glaucus. P. eurymedon also seems to be more prone to oviposition error than the other two western species.

Summary and Conclusions

1.     This paper is a critical review of foodplant preferences in the five species of butterflies comprising the Papilio glaucus group (P. alexiares, P. eurymedon, P. glaucus, P. multicaudatus, and P. rutulus). New data are also presented.

2.     There are four sources of confusion in the literature which have been considered in arriving at the conclusions presented here: (a) the primary source is that authors have very often failed to indicate whether they reared larvce through to the adult stage for positive identification. This is an absolute necessity due to the combination of the extensive geographic cooccurrence of P. eurymedon, P. multicaudatus, and P. rutulus and the similarity in appearance of their larvae, which have never been figured together until this paper. (b) It has only recently been established that P. glaucus overlaps the northern extremity of the range of the three western species in southern British Columbia, (c) Worn females of P. glaucus, P. rutulus, and P. eurymedon are almost indistinguishable in flight, which means that oviposition records are subject to error, (d) Several later authors have cited earlier ones mistaken for the above reasons.

3.     No foodplant or oviposition references exist for P. alexiares.

4.     Tables 1-3 constitute all data known to the author regarding the three western species. Such a breakdown for P. glaucus is not necessary because records are from the East where it occurs alone and confusion with other members of the group is therefore impossible.

Rosaceae                           + +                          —                         —

Oleaceae                            -f- +                           O                         —

Salicacese                         -(-O                          +                           O

Corylaceas                        +0                          +                           0

Rhamnaceae                      O O                           O                          +

Magnoliaceae                    +0                          —                          O

Platanaceae                       O O                          +                           O

Rutaceas                            O +                           O                           O

Others                              + O                           O                           O

The foodplants of P. alexiares are unknown.

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5.     P. glaucus is a polyphagous species feeding on plants of several families. But unlike it, each of the three western ones is restricted in its food-plant diet even though two of them have foodplants in common with P. glaucus. Moreover, the three western species are mutually exclusive in their foodplant diet (Table 4).

6.     These facts suggest that competition among the western species during the larval stage has resulted in their restricted and mutually exclusive diets. I have elsewhere described how this may have occurred (Brower, 1958), and adult as well as larval competition among them has been discussed in another paper (Brower, in press, b).

ACKNOWLEDGMENTS

I should like to express my appreciation to Dr. C. L. Remington of Yale University for originally suggesting the Papilio glaucus group study and for his help and advice throughout its development; also to Professor G. E. Hutchinson and Dr. E. S. Deevey, Jr., who gave me much useful advice and criticism. I am indebted to Drs. P. F. Bellinger and W. D. Hartman of Yale for criticizing the manuscript. Thanks also are due those members of the Lepidopterists' Society mentioned in the paper. Dr. E. B. Ford, F. R. S., of Oxford University is to be especially thanked for his hospitality at the Genetics Laboratory where considerable revision of the paper was carried out. I am grateful to Drs. J. W. Marr, A. W. Johnson, and W. Osburn for providing me with facilities for the field work at the Institute of Arctic and Alpine Ecology, University of Colorado. Mr. D. Eff and Mr. and Mrs. H. Freudenberg provided invaluable assistance in the field work as did Mr. R. H. Wilson. I also want to thank Mrs. Ina Giles of Amherst College for typing the final manuscript. I am greatly indebted to my wife, Dr. Jane Van Zandt Brower, of Mount Holyoke College, for continual help and encouragement.

This publication represents one aspect of my research at Yale supported during 1954-1957 by grants from the Sigma Xi RESA research fund, the Higgins Fund of Yale University, two predoctoral National Science Foundation Fellowships, and a Fulbright Scholarship at Oxford during 1957-1958.

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Dept. of Zoology, Yale University, New Haven, Conn., U. S. A. Present address: Dept. of Biology, Amherst College, Amherst, Mass., U. S. A.