A MIGRATION PROBLEM - VANESSA CARDUI (NYMPHALID.Te), the PAINTED LADY BUTTERFLY

1962 Journal of the Lepidopterists' Society 229 A MIGRATION PROBLEM - VANESSA CARDUI (NYMPHALID.tE), THE PAINTED LADY BUTTERFLY by CHARLES H. ABBOTT The subject of insect migration, and particularly of butterfly migration, has reached the point where certain laws of migration are beginning to emerge. We are also learning that exact information is lacking on many points which we have taken for granted, and that when we secure this information we shaH be in a better position to formulate laws. As a first basis we must admit, as WILLIAMS (1958) has so many times pOinted out, we cannot arbitrarily divide migration into rhythmic migration and one-way migration (also called emigration or, better, unidirectional migration with no return). Only one butterfly, the Monarch, Danaus pZexippus (L.) is usually considered to have a typical rhythmic migration, in which the same individuals make a two-way flight; while WILLIAMS has collected data showing return flights by successive genera tions in an increasing number of species. Therefore, if two-way and one-way insect migrations are essentially different, we do not have enough knowledge to divide the butterflies between them. Our problem is to find some underlying principles or laws, and there will be time enough to determine how wide can be their application. As has been previously pointed out, the Painted Lady, Vanessa cardui ( Linmcus) was selected for this study, because in migration years it is so abundant that there is limitless material on which to work, and be cause its study has been neglected. The conclusions to date may be found in the author's 1950, 1951, and 1959 papers, and may be very briefly summarized as follows: The species is usually found, so far as North America is concerned, only south of the United States, or possibly, in southernmost Imperial Valley; and comes north only in migration years, which are few in number and at irregular intervals (with possible minor exceptions to be noted later). If one could observe the start of a migration at the point of origin, probably large numbers would be noted all at once, but at points in California very small numbers appear at first with a gradual build-up. The direction of migration is consistent, year after year, in most California localities toward the north-northwest. The geographical ex tent of the migration in anyone year appears correlated with the numbers involved, the larger the numbers the greater the distance covered by the species. It was determined in 1958, when the migration was traced eventu- 230 ABBOTT: migration Vo1.l6: noA ally from the Mexican border to the Cascades of Washington, that several generations were involved, probably four in California, and the number farther north undetermined. This makes one wonder if each generation found an optimum of conditions in the area which it reached and where it laid eggs, and that such a combination of optima over the entire Pacific Coast area rarely occurs. I believe we have already the most complete picture available (of course far from complete) of the migration of a single North American butterfly, except probably the MoriarcliTDanaus plexippus); and from what WILLIAMS says in his latest book Insect Migration (1958), 1 do not believe as complete a picture is available of any species in Europe. What I want to outline now is particular observations which are needed to fill in gaps in our knowledge, which are practicable for field lepidop terists. As a reason for doing it, I want to quote a paragraph from page 2 of WILLIAMS'S Insect Migration (1958): "We can ask for example why Painted Ladies migratc; this is a problem of the origin and evolution of the habit. But we have also to consider the much more detailed problem of how or why a particular individual butterfly is migrating in a particular directiGn at a particular time. In my opinion it is by finding answers to the second that we may be able to solve the first of these questions." l. What is the distribution of this species in western North America in a non-migration year? WILLIAMS (1938, 1958) has rcported, on the basis of published reports from this part of the country, that the species is found north of Mexico only in years of extensive migrations. There seems to be little doubt that the source of our great migrations is south of the Mexican border (Abbott, 1959), although little is known of the exact location or nature of the breeding grounds. One in Baja California, discovered by GARTH and DAWSON in 1949, has been described (Abbott, 1951: p.159). The above statement appears to be open to exceptions, as there are several unpublished reports by competent lepidopterists of small re sident populations in California and Nevada which have been observed year after year. There is a difference of opinion whether these popula tions take part in a general migration. It is important that information on these resident populations should be published, particularly as to the number of generations per year, the length of each stage of metamor phosis, and what stage is inactive in a carry-over from one year to the next. As a possible explanation of this last paragraph, it was suggested by P. F. BELLINGER (personal communication) that V. cardui might have 1962 Journal of the Lepidopterists' Society 231 phases such as those described for locusts, a migratory phase, associated with overcrowding, and a non-migratory phase, called solitary in locusts. (For discussion of UVAROV'S Phase Theory, see Williams, 1958: pp.158- 161. This discussion also reports similar phases in certain moths.) It seems to be more difficult to determine if there is an inactive period in the annual life history of the species in its home in Mexico whence the migrations periodically come. The key point which 'needs to be determined, and is also the most difficult to find out, is what combination of conditions starts a species migrating. Is it a set of conditions which work directly on the adult insect, or is it a combination of favorable conditions resulting in a larger number of eggs laid, a larger number of larvre developing to the pupa stage, or a larger number of pupre surviving to emerge as adults? 2. The only known set of external conditions which appears to favor a very large generation of V. cardui in the desert or semi-desert south of the border is the right combination of rains and temperature to give an unusual crop of desert vegetation, chiefl y annual wild flowcrs . This serves as food for the larva.~, and could result in the next generation of egg-laying and potentially migrating adults being much larger. This makes extremely important the recording of all possible data on rainfall, temperature, and vegetation in subsequent migration years; also facing the fact of the great variation in conditions in different parts of the desert. The past data on this point are rather scarce and inconclusive. The question which needs to be answered is: Is a heavy migration aliways correlated with heavy winter rains in the desert at the right season to produce a large crop of desert annual wild flowers? Data available at present make a "yes" answer to this question doubtful, but the data are inconclusive (Abbott, 1951: p.162.) If this is a correct conclusion, it is probable that large crops of annuals at points somewhat farther north from the Mexican border would help to explain the very large second and third generations such as were noted in 1958. But what determines the large first generation in a migration year? Is it a combination of conditions having a sudden effect on some stage of development? Or does a migration result only when there has been cumulative build-up of nnmbers during preceding generatiom, perhaps since the last preceding migration? 3. An alternative theory, which deserves investigation, is that the instinct to migrate is always present in this species or at least in the migratory phase of this species. In non-migration years they are too few and fly too short distances to be observed by any except trained 232 ABBOTT: migration Vo1.l6: no.4 observers who happen to be in exactly the right localities. This would explain the report by R. C. DICKSON that a small migration occurs in Imperial Valley every year. The following is a suggested pattern for a non-migration ycar. At the beginning of the season only a small number of imagoes trans form from pUplB, and being few in number they do not have to fly far to find vegetation on which to lay eggs; or, owing to a very short growing season of plants, only a very small number of eggs from the last preceding generation develop as far as adults of the first generation of the new season. Parasites in the usual numbers would also be more effective in controlling a small generation. Predators are usually at least one genera tion behind their prey in a fluctuation in numbers (Elton, 1927: p.141 ), and this may be true of the so-called parasitic Hymenoptera and Diptera, which are really midway between predators and parasites. If the instinct to migrate is always present, it would explain why in a migration year the succeeding generations, even if many miles from the base habitat and perhaps from the original cause, continue to migrate until adverse conditions stop the migration. 4. Hew long do female V. cardui live after egg-laying, and do the individuals continue migrating after egg-laying? Do males migrate the same distance as females? 5.

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