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This is the Red Admiral at the early larva stage
THE RED ADMIRAL, ILLUSTRATIVE NATURAL HISTORY OF
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This is the Red Admiral at the mid larva stage
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This is the Red Admiral at the caterpillar stage
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This is the Red Admiral at the pupa stage
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This is the fully developed Red Admiral with its glorious colours
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The nature of migration in the red admiral butterfly Vanessa atalanta: evidence from the population ecology in its southern range
Constantí Stefanescu, Buttefly Monitoring Scheme, Can Liro, Spain
Introduction
Constantí Stefanescu, Buttefly Monitoring Scheme, Can Liro, Spain
Introduction
Though migration by some butterfly species is well established (Williams, 1930, 1958 ), a detailed understanding is still elusive because of the technical difficulties of studying butterfly migrants (Baker, 1984; Shreeve, 1992). The typical image of true migrants in the north temperate zones is that they simply survive in an overwintering region, then fly north in spring to exploit the temporary availabilty of high quality resources in summer (Dingle, 1996). This is exemplified by the migrant monarch butterfly Danaus plexippus (Malcolm et al. 1993; Brower, 1996) and this same general pattern has been suggested for the red admiral Vanessa atalanta (L.) (Benvenuti et al. 1994). Alternatively, some species time their migration to exploit the availability of high quality resources in their winter range, as well as exploiting the availability of northern resources in summer. Some moths and other insects provide good examples of this type of migration (e.g. Riley et al. 1991; Showers, 1997).
Vanessa atalanta is an Holarctic species, found throughout Europe and Africa north of the Sahara (Emmet & Heath, 1990; Tolman & Lewington, 1997). In its northernmost European range, populations are not permanent and its presence depends each year on migration from southern countries (Tucker, 1997). Although records of its directional flights have been documented many times (e.g. Grant, 1936; Lack & Lack, 1951; Williams, 1951; Roer, 1961; Benvenuti et al. 1994), many unanswered questions remain about the migratory pattern of this butterfly (Baker, 1984). In particular, knowledge about its dynamics in the winter range (presumably in the Mediterranean region) is very scarce.
Two recent papers have emphasised the need for an improvement in knowledge of the biology of V. atalanta in southern Europe. Bryant et al. (1997) investigated how the range margins occupied by four nettle-feeding nymphalid butterflies were conditioned by their thermal requirements for development. Vanessa atalanta did not conform to the same pattern as the other three species, apparently because of its migratory status. They acknowledged that uncertainties over the overwintering biology of V. atalanta constrained their ability to draw conclusions. Using data from the British Butterfly Monitoring Scheme, Pollard & Greatorex-Davies (1998 ) analysed the trend of increased abundance of V. atalanta in Britain from 1976 to 1996. They suggested that this trend was probably due to increased immigration from southern areas in spring, but lack of knowledge on the precise location of source areas prevented them from reaching more definite conclusions.
In the work reported here, data from various sources are combined to obtain a clear picture of the phenology of this long-distance migrant in typical Mediterranean habitats. This information is used to answer the main question: Does V. atalanta simply move south to overwinter and return north to breed in the following spring or does it breed throughout the year (i.e. produce an early spring brood in the south) and exploit high quality resources that are available at different times in the south and in the north of its range?
Results
The work reported here shows that the Catalonia lowlands, and probably the Mediterranean region as a whole, is an area to which adult V. atalanta migrate in order to breed in winter. It is not a region to which the adults retreat simply in order to hibernate successfully. Although the higher autumn than spring peak gives the appearance that overwintering breeding may lead to a decline in population size, this may not be the case: spring adults apparently emigrated soon after emergence and individuals may have been present for a shorter period than in autumn.
As expected, migration (both latitudinal and altitudinal) is key to understanding the phenology of this butterfly in southern Europe. Another crucial point is that autumn breeding and subsequent development of immatures throughout the winter, produces an early spring generation of adults.
In the Mediterranean region, the most conspicuous event in V. atalanta phenology is the mass arrival of autumn migrants from north and central Europe. Depending on the breeding success and population levels reached at northern latitudes, this invasion becomes more or less apparent. In 1997, for example, V. atalanta was particularly abundant in The Netherlands and the British Isles (van Swaay & Ketelaar, 2000) and autumn transect counts at two Catalan Butterfly Monitoring Scheme sites also yielded the highest values ever recorded.
As shown here, migrants start to arrive by late September and early October, and the bulk of migration is recorded by mid October (see also Lack & Lack, 1951). The last migrants are usually seen in early November. This temporal pattern is almost identical to that found by Benvenuti et al. (1994) in central-northern Italy (42–44°N), at a similar latitude to Catalonia.
Pooled data for autumn migrants showed a mean flight direction of 216°, i.e. towards SSW. This direction differs from the predominantly SSE direction found by Benvenuti et al. (1994) for central-northern Italy (but see also Benvenuti et al. 1996). In both cases, however, the preferred direction matched the orientation of the coastline closely, minimising the risk of flying into the open sea, an explanation suggested by Spieth & Kaschuba-Holtgrave (1996) for Pieris brassicae.
Regular monitoring, together with circumstantial observations, suggests that most of the butterflies produced in source areas of northern and central Europe, emigrate to the Mediterranean region in autumn (Pollard & Greatorex-Davies, 1998). Although there are records of overwintering adults at northern latitudes (e.g. Archer-Lock, 1989; Emmet & Heath, 1990), it is generally assumed that overwintering is rare and has a negligible effect on numbers in the following season (Pollard & Greatorex-Davies, 1998 ).
In Catalonia, the arrival of migrants coincided with a period of intensive breeding, and both hilltopping activity and egg-laying increased greatly in October and early November. Adult activity was also recorded regularly in winter months but numbers decreased progressively and only a few, very worn individuals were seen at the end of this period.
This phenological model differs from that in closely related species and may explain why some ecophysiological responses in V. atalanta did not fit well to the predictions raised by Bryant et al. (1997). Aglais urticae, Inachis io, and Polygonia c-album all overwinter as adults and breed in spring (Emmet & Heath, 1990; C. Stefanescu, pers. obs.). In contrast, adults of V. atalanta did not cease their activity completely in winter.
Immatures that hatched from eggs laid in autumn did not enter into a true diapause. Instead, slow growth is still possible whenever temperatures remain above the developmental threshold. In other words, immatures of V. atalanta remain active throughout most of the winter, with occasional quiescence and growth retardation (Mansingh, 1971; Leather et al. 1993).
Bryant et al. (1997) estimated the developmental threshold in V. atalanta at ≈ 8.3 °C though this varied according to instar. Bryant et al. (2000) found that larvae of V. atalanta do not bask and their body temperature depends largely on ambient temperature. In contrast, the gregarious basking larvae of I. io and A. urticae achieve body temperatures 20 °C above ambient temperature when direct sunlight is available.
There is close agreement between development threshold estimates and field observations (Fig. 6). At Can Liro, larval growth was arrested completely between December 1997 and January 1998 , when the mean recorded temperature was 6.21 °C, but growth continued during the same period at Torre del Vent, where the comparable temperature was 9.54 °C. The same pattern was found in the following two winters and, on the basis of recorded temperatures, should have occurred in most recent years (Table 1).
Using weather data for the period 1997–1999 from Torre del Vent and Can Liro, it was found that eggs hatching by late October and early November are predicted to produce adults between 4 and 22 March and 23 April and 5 May respectively. These predictions were confirmed by field data gathered from transect counts of adults (Fig. 2).
Though spring emergence is observed at many lowland Mediterranean sites (Fig. 7c), most of these butterflies leave their natal patches without breeding. This conclusion is reached from two lines of evidence. First, the age population structure was highly biased towards young individuals. Secondly, in 3 consecutive years, early immatures were scarcely recorded in April–May, following emergence of the first generation of adults.
Both latitudinal and altitudinal migration are likely to account for the disappearance of spring butterflies. Northward migration in March–June has been known to occur at northern latitudes for nearly a century (e.g. Williams, 1930, 1951, 1958 ; Grant, 1936; Roer, 1961) but is seen rarely in the Mediterranean region (but see Benvenuti et al. 1994). The recolonisation of central and northern Europe that occurs every season implies that at least part of the population overwintering in the Mediterranean region migrates to the north in spring. Data presented here indicate strongly that the offspring of autumn migrants are involved in this northward migration, and give no support to the hypothesis that recolonisation is a result of wintering adults returning to their original breeding grounds (Benvenuti et al. 1994).
This study also gives some evidence of altitudinal migration in V. atalanta. Intensive monitoring at several sites in the Montseny mountain seemed to indicate that butterflies emerging at low sites in early spring moved uphill to breed at mid and high elevations (Fig. 7a,b). Accumulation of degree-days at the lowest and warmest mountain site from 15 October predicted first emergences of adults between 15 and 31 May in 1997–1999. Adults recorded at this and higher, colder sites before these dates must have come from the lowlands. Additional evidence comes from circumstantial observations of nettle patches in winter. Due to the low temperatures, nettle leaves were killed and became totally unavailable to young larvae until April or May (C. Stefanescu, pers. obs.), as happens in most of central and northern Europe.
Pooled data from several years and transect routes at mid and high elevations showed that a distinct peak of abundance occurred by mid July, ≈ 2 months after the arrival of the bulk of lowland migrants. This increase in population levels was the result of local breeding and the emergence of the second generation of adults in the same season. Predictions of developmental time from first-instar larva to adult coincided fully with the pattern actually recorded.
Likewise, by the end of summer, this second generation of adults can be assumed to have moved downhill to breed again at lowland sites (Fig. 5). A further third generation thus occurs in October, resulting in a mixing of individuals of different ages when autumn migrants start to arrive (Fig. 4). Alternatively, the increase in population levels in the lowlands by late August may be the result of migrants arriving from south or central France, although this seems unlikely considering the lack of observations of southward flights before October (Table 2).
The pattern found in the Montseny mountain can almost certainly be generalised to most of Catalonia and perhaps to many other mountainous Mediterranean areas. An increase in abundance by late August (unexplained by local breeding) seems to be a common feature at lowland sites in the Catalan Butterfly Monitoring Scheme (Fig. 7c). Though detailed data such as those presented here are not available from other areas, Larsen (1976) suggested that, in the south-eastern Mediterranean, V. atalanta may only breed during the winter at lowland sites and from May onwards is mostly found breeding in the mountains.
The complex phenology of V. atalanta in the Mediterranean region, involving altitudinal and latitudinal migration, has certainly evolved as a strategy to track larval resources through space and time. Thus, the arrival of migrants in the autumn, with subsequent breeding and larval development in winter, coincides with the major availability (both in quality and abundance) of the main host plant, U. dioica. The decrease in food quality (Fig. 8) occurring just after the emergence of the first generation of butterflies might have imposed a very strong selective pressure for migration in late spring and early summer. Thus the pressures involved may be departure from a declining resource, movement towards an improving resource, and movement to and from areas where successful overwintering is possible.
Poleward migration in spring has been reported for many insects in temperate regions, including V. atalanta, and is considered to be an adaptive strategy allowing the colonisation of increasingly favourable areas (e.g. Williams, 1958 ; Johnson, 1995; Pedgley et al. 1995; Dingle, 1996; and references cited therein). On the other hand, altitudinal migration is a less well-known phenomenon, though there is circumstantial evidence for many highly mobile butterflies (e.g. Larsen, 1975; Shapiro, 1975, 1980 ) and, more recently, it has been described in great detail in a fairly sedentary species (Peterson, 1997). With the available data, however, it is not possible to know whether both types of migration in V. atalanta could be the result of the maintenance of genetic variation (some kind of dichotomy) in a presumably panmictic population. This seems unlikely and poses an interesting problem that deserves further investigation and would require an experimental approach to be addressed fully.
The phenological pattern described here for the Mediterranean region should help in understanding several aspects of the ecology of V. atalanta, for example those related to changes in abundance in its northern margin of distribution (Pollard & Greatorex-Davies, 1998). Data from several butterfly monitoring schemes are now available (Pollard & Yates, 1993; van Swaay et al. 1997; Stefanescu, 2000) and the integration of all this information will help in understanding the population dynamics of this and other common migrants (see Pollard et al. 1998 , for an integrated study of Cynthia cardui). These widespread and highly mobile butterflies are important in assessing future changes in phenological and migratory patterns in response to global climate change (cf. Bryant et al. 1997).
Reference: Ecological Entomology (2001) 26, 525-536
Grammour Boy
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I am posting it for the edification of the membership concerning the Great Red Admiral--perhaps the greatest creature to ever grace the face of the Earth!!Hobo Jo said:
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You're welcome to profile the Bumble Bee in another thread.Hobo Jo said:
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Why does negative reputation matter to you?Hobo Jo said:
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You did answer back. It's in grey.
Grammour Boy
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This thread is meant to be read, not to ask questions.
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Really?
Grammour Boy
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My neg hits are more powerful than yours, remember that.
