The 1998 El Niño event had been expected to substantially affect many insect populations that year, including those of Painted Lady butterflies. An unusually large Painted Lady migration had been expected that year. In March 20, 1998 there had been sightings in southern California indicating that this migration was beginning at that time. Elsewhere in the world, Painted Ladies were also migrating out of East Africa through southern Israel at about the same time (Hansen 1998).
If you are interested in helping collect data on similar migrations, please feel free to contact me. Information on observing procedures can be found under "Observing Migrating Butterflies" and on the Tactics and Vectors website.
Painted Lady migration is different from that of butterflies such as the Monarch and Red Admiral, which have a more or less regular, yearly seasonal migration. Painted Ladies migrate more sporadically. They often do not migrate every year, and the size of their migrations varies much more from event to event. The majority of butterflies which migrate in this way live either in the arid zones of the world or in areas with seasonal extremes of rainfall in the tropical zones (Larsen 1984). In these regions, the climatic conditions can vary greatly from year to year. These are areas where a series of extreme years could wipe out the entire local populations of some species. Under such conditions, it seems to be a good strategy to migrate.
Do Painted Ladies migrate in larger numbers during El Niño years? There is increasing evidence that climatic anomalies such as El Niño trigger large-scale migrations of Painted Ladies and many other migratory butterflies. El Niño and analogous climatic anomalies change the paths of storm systems so that more rain falls over the deserts and arid areas where these butterflies live.
During so-called "normal," non-El Niño years, the butterflies persist at low to moderate levels in relatively limited "source areas" in arid regions such as the desert Southwestern U.S (Tilden 1962). But when an El Niño event triggers unusually heavy rainfall in these areas, the desert greens up and bursts into bloom. Painted Ladies quickly take advantage of these conditions by breeding huge numbers of offspring which feed upon the abundant plant growth. According to Larsen (1984), the caterpillars of Arabian species with similar migration patterns often make such good use of this food supply that they strip much of the vegetation bare by the time they are ready to pupate. So when the next generation of Painted Ladies emerges, there may be far too few food plants left to sustain another generation of butterflies. The best alternative for the vast majority of them is to migrate outward from the source area to seek other areas with more abundant food for their offspring. By dispersing widely, some individuals will reach a locality suitable for breeding and will survive when conditions at the place of origin would have killed the entire population if they had not left (Larsen 1984). Female Painted Ladies that do find a suitable area to breed enjoy extraordinary breeding success which compensates for the loss of many of their siblings which are unable to do so. Another adaptive quality of the Painted Lady is that its larvae can utilize a wide variety of larval food plants. This allows Painted Ladies which have migrated hundreds of miles from the source region to take advantage of an often very different set of potential food plants in their new location.
This outbreak and migrate type of life history is more common in areas which are less hospitable to butterflies. Larsen (1984) compared the climate and the butterfly populations of different areas in the Middle East. He found that more arid areas with more sporadic rainfall had fewer species of butterflies, and higher proportions of the species that were present were migratory.
Myres (1985) pointed out that the correlation between El Niño events and Painted Lady outbreaks is not absolute. Not every El Niño produces precipitation in the source areas, whereas other types of climatic anomalies sometimes do. Namias-Sumner effects (Lamb 1972, pp. 408-410), which involve the growth of an unusually warm area of water farther north and west in the Pacific, may likewise increase precipitation on the southern Pacific coast. According to Myres (1985), these events could explain outbreaks occurring in non-El Niño years. Myres (1985) also said that, because the meridional air flow pattern can be repeated in the eastern Atlantic, Namias-Sumner effects might also contribute to not-infrequent synchrony of Painted Lady outbreaks between North America and Africa. Swengel (1993) commented on Myres' pattern and cautioned that other factors interact with El Niño events to affect Painted Lady abundance (Williams 1998). Williams (1998) says that because of its size, the current El Niño event may be the primary determinant of how abundant Painted Ladies actually become this coming spring.
Return Migration
Once an outbreak of Painted Ladies has migrated northward, do any of the butterflies make a return flight southward? Myres (1985) documented a southward return migration over southern Alberta, Canada in fall 1983. Myres (1985) also suggested that, depending upon wind conditions, the butterflies travel farther north in some years than in others.
Myres (1985) also raised the question of whether or not return migration is necessary to maintain the source populations of Painted Ladies. Whether or not this is so, return migration is necessary to maintain the migratory habit in the source populations. If there were no return migration, all the Painted Ladies that flew north (or their eventual offspring) would simply stay there and eventually succumb to cold weather. Northward migration then would be simply a reproductive dead end. Those butterflies that stayed behind in the source areas, however, would have greater potential to reproduce future generations. Migratory individuals would then become less and less common, and the migratory habit would eventually disappear from the population. Since this is clearly not happening, this implies that Painted Ladies are returning southward in numbers sufficient to maintain the migratory habit in the source populations.
Southward migrations apparently do not need to be as large or conspicuous as the northward ones for this to happen. Evidence prior to Myres' (1985) study suggested "only slight evidence of any such flights in North America" (Williams 1958, pp. 27, 39), suggesting that many return migrations are inconspicuous. Myres (1985) showed that although southward migrations may be smaller and much less obvious to casual observers than the corresponding northward flights, the number of butterflies flying southward can nevertheless be substantial. This evidence suggests that at some point, most of the butterflies which have gone north will die out, but enough will return to the source populations to maintain the migratory habit.
Literature Cited
- Hansen, M.D.D. 1998. Painted Lady Migrations. (Message on the newsgroup sci.bio.entomology.lepidoptera). 19 March 1998.
- Lamb, H.H. 1972. Climate: Present, Past and Future. Volume I: Fundamentals and Climate Now. Methuen, London, 613 pp.
- Larsen, T.B. 1984. Butterflies of Saudi Arabia and Its Neighbours. Stacey International, London, 160 pp.
- Myres, M.T. 1985. A southward return migration of Painted Lady butterflies, Vanessa cardui, over southern Alberta in the fall of 1983, and biometeorological aspects of their outbreaks into North America and Europe. Canadian Field-Naturalist 99: 147-155.
- Swengel, A. (1993). American Butterflies 1(2):
- Tilden, J.W. 1962. General characteristics of the movements of Vanessa cardui (L.). Journal of Research on the Lepidoptera 1: 43-49.
- Williams, C.B. 1958. Insect Migration. Collins, London, 235 pp.
- Williams, E. 1998. Re: Painted Lady Migration? (Message on the newsgroup sci.bio.entomology.lepidoptera). 16 March 1998.