The Afrotropical breeding grounds of the Palearctic-African migratory painted lady butterflies (Vanessa cardui)

Biomass burning smoke pollution stimulates painted lady butterflies (Vanessa cardui L.) to increase flight speed

Smoke from biomass burning significantly degrades air quality due to high concentrations of particulate matter (PM2.5) and trace gases. While the ecological and health impacts of smoke pollution are well documented, its effects on insect migration remain poorly understood. In this study, we conducted two experiments to investigate the flight performance of Vanessa cardui butterflies under varying smoke conditions and identify the mechanisms influencing their behaviour. Butterflies were tethered to flight mills (TFMs) for 6 h, during which flight speed, distance, and duration were recorded across clean-air conditions and three levels of PM2.5 concentrations. Statistical analysis revealed that flight speed increases significantly as smoke concentration increases, although the increased range decreases. At a mean PM2.5 concentration of 120 μg m-3, flight speed increased by 52 % compared to clean-air conditions. To determine whether particulate matter was driving this response, individuals were exposed to smoke with and without particulates. In smoke with particulates retained, butterflies exhibited nearly double the flight speed compared to filtered smoke, indicating that particulates play a key role in altering flight behaviour. Scanning electron microscopy revealed significant deposition of smoke particulates on the antennae and abdomen, suggesting a sensory or physical response triggering accelerated flight. We interpret these findings as evidence that Vanessa cardui accelerates flight in smoky environments as an escape response. This study highlights the remarkable sensitivity of butterflies to smoke pollution and provides novel insights into the ecological consequences of biomass burning, particularly its potential impacts on insect behaviour and migration dynamics.

Trans-Atlantic Dispersal and Introgression Explain Holarctic Disjunct Distributions in Vanessa Butterflies

Species with disjunct distributions have long puzzled evolutionary biologists and biogeographers. Long-distance dispersal can play a pivotal role in generating intra-specific disjunct distributions, initiating early stages of allopatric speciation and leading to eventual interspecific disjunctions. Vanessa butterflies exhibit diverse movement behaviours, from low-dispersal species with restricted distributions to others that engage in annual extensive migratory cycles. The biogeographic history of Vanessa presents intriguing cases of both intra- and interspecific disjunctions. Vanessa atalanta is present in the Nearctic and Western Palearctic but is absent in Asia, while its sister species V. tameamea is endemic to Hawaii. Vanessa indica occurs only in Asia, and its sister species, V. vulcania, is endemic to Macaronesia. Here, we investigate this conundrum through population genomics and demographic analyses of Vanessa atalanta using ddRAD data from 70 samples across its entire distributional range, identifying two genetically differentiated populations separated by the Atlantic Ocean. Demographic simulations and phylogenetic analyses suggest that these originated via long-distance dispersal from the Nearctic to Europe around the Last Glacial Maximum. Hybridisation tests revealed introgression between the Palearctic population of V. atalanta and V. indica, indicating that their distributions overlapped during V. atalanta's colonisation of Europe. We hypothesise that V. atalanta caused a species displacement of V. indica from Europe to Asia, explaining their current allopatric distributions-a scenario that is supported by ecological niche modelling. Together, our results illustrate the role of long-distance dispersal and species interactions in shaping complex biogeographic patterns.

Protected area coverage of the full annual cycle of migratory butterflies

Effective conservation of migratory species relies on habitat protection throughout their annual cycle. Although protected areas (PAs) play a central role in conservation, their effectiveness at conserving habitats across the annual cycle of migratory species has rarely been assessed. We developed seasonal ecological niche models for 418 migratory butterfly species across their global distribution to assess whether they were adequately represented in the PAs across their full annual cycle. PA coverage was inadequate in at least one season for 84% of migratory butterflies, adequate for only 17% of species in one season, and inadequate for 45% of species in all seasons. There was marked geographic variation in PA coverage: 77% of species met representation targets in Sri Lanka, for example, but only 32% met targets in Italy. Our results suggest that coordinated efforts across multiple countries will be needed to develop international networks of PAs that cover the full annual cycle of migratory insects and that conservation measures, in addition to the establishment and maintenance of PAs, are likely to be needed to effectively conserve these species.

Insect-flower interactions, ecosystem functions, and restoration ecology in the northern Sahel: current knowledge and perspectives

Actions for ecological restoration under the Great Green Wall (GGW) initiative in the northern Sahel have been plant focused, paying scant attention to plant-animal interactions that are essential to ecosystem functioning. Calls to accelerate implementation of the GGW make it timely to develop a more solid conceptual foundation for restoration actions. As a step towards this goal, we review what is known in this region about an important class of plant-animal interactions, those between plants and flower-visiting insects. Essential for pollination, floral resources also support insects that play important roles in many other ecosystem processes. Extensive pastoralism is the principal subsistence mode in the region, and while recent analyses downplay the impact of livestock on vegetation dynamics compared to climatic factors, they focus primarily on rangeland productivity, neglecting biodiversity, which is critical for long-term sustainability. We summarise current knowledge on insect-flower interactions, identify information gaps, and suggest research priorities. Most insect-pollinated plants in the region have open-access flowers exploitable by diverse insects, an advantageous strategy in environments with low productivity and seasonal and highly variable rainfall. Other plant species have diverse traits that constrain the range of visitors, and several distinct flower types are represented, some of which have been postulated to match classical "pollination syndromes". As in most ecosystems, bees are among the most important pollinators. The bee fauna is dominated by ground-nesting solitary bees, almost all of which are polylectic. Many non-bee flower visitors also perform various ecosystem services such as decomposition and pest control. Many floral visitors occupy high trophic levels, and are indicators of continued functioning of the food webs on which they depend. The resilience of insect-flower networks in this region largely depends on trees, which flower year-round and are less affected by drought than forbs. However, the limited number of abundant tree species presents a potential fragility. Flowering failure of a crucial "hub" species during exceptionally dry years could jeopardise populations of some flower-visiting insects. Furthermore, across Sahelian drylands, browsers are increasingly predominant over grazers. Although better suited to changing climates, browsers exert more pressure on trees, potentially weakening insect-flower interaction networks. Understanding the separate and combined effects of climate change and land-use change on biotic interactions will be key to building a solid foundation to facilitate effective restoration of Sahelian ecosystems.

Lords of the flies: dipteran migrants are diverse, abundant and ecologically important

Insect migrants are hugely abundant, with recent studies identifying the megadiverse order Diptera as the major component of many migratory assemblages. Despite this, their migratory behaviour has been widely overlooked in favour of more 'charismatic' migrant insects such as butterflies, dragonflies, and moths. Herein we review the available literature on dipteran migration to determine its prevalence, identify key migratory routes and elucidate areas that may prove fruitful for future research. Using 13 lines of evidence to determine migratory behaviour, we determined that species from 60 out of 130 dipteran families show evidence of migration, with Syrphidae fulfilling 12 of these criteria, followed by the Tephritidae with 10. By contrast, 22 families met just two criteria or fewer, underlining the need for more research into the migratory characteristics of these groups. In total, 592 species of Diptera were identified as potentially migratory, making them the most speciose group of insect migrants yet described. Despite this, only 0.5% of dipteran species were found to be migrants, a figure rising to 3% for the Syrphidae, a percentage mirrored by other migratory taxa such as butterflies, noctuid moths, and bats. Research was biased to locations in Europe (49% of publications) and while vast regions remain understudied, our review identified major flyways used by dipteran migrants across all biogeographic realms. Finally, we highlight an unsurpassed level of ecological diversity within dipteran migrants, including ecological roles of huge economic value. Overall, this review highlights how little is known about dipteran migration and how vital their migratory behaviour may be to the health of global ecosystems.

Isotope geolocation and population genomics in Vanessa cardui: Short- and long-distance migrants are genetically undifferentiated

The painted lady butterfly Vanessa cardui is renowned for its virtually cosmopolitan distribution and the remarkable long-distance migrations as part of its annual, multigenerational migratory cycle. In winter, V. cardui individuals inhabit breeding grounds north and south of the Sahara, suggesting distinct migratory behaviors within the species as individuals migrate southward from Europe in the autumn. However, the evolutionary and ecological factors shaping these differences in migratory behavior remain largely unexplored. Here, we performed whole-genome resequencing and analyzed the hydrogen and strontium isotopes of 40 V. cardui individuals simultaneously collected in the autumn from regions both north and south of the Sahara. Our investigation revealed two main migratory groups: (i) short-distance migrants, journeying from temperate Europe to the circum-Mediterranean region and (ii) long-distance migrants, originating from Europe, crossing the Mediterranean Sea and Sahara, and reaching West Africa, covering up to over 4,000 km. Despite these stark differences in migration distance, a genome-wide analysis revealed that short- and long-distance migrants belong to a single intercontinental panmictic population extending from northern Europe to sub-Saharan Africa. Contrary to common biogeographic patterns, the Sahara is not a catalyst for population structuring in this species. No significant genetic differentiation or signs of adaptation and selection were observed between the two migratory phenotypes. Nonetheless, two individuals, who were early arrivals to West Africa covering longer migration distances, exhibited some genetic differentiation. The lack of genetic structure between short- and long-distance migrants suggests that migration distance in V. cardui is a plastic response to environmental conditions.

Trans-Saharan migratory patterns in Vanessa cardui and evidence for a southward leapfrog migration

Some insects, such as the painted lady butterfly Vanessa cardui, exhibit complex annual migratory cycles spanning multiple generations. Traversing extensive seas or deserts is often a required segment of these migratory journeys. We develop a bioavailable strontium isoscape for Europe and Africa and then use isotope geolocation combining hydrogen and strontium isotopes to estimate the natal origins of painted ladies captured north and south of the Sahara during spring and autumn, respectively. Our findings reveal moderate migratory connectivity across the Sahara characterized by a broad-front, parallel migration. We also report evidence of a leapfrog migration, wherein early autumn migrants from higher latitudes cover greater distances southward than their late autumn counterparts. This work represents a major advancement in understanding insect migratory patterns and connectivity, particularly across extensive barriers, which is essential for understanding population dynamics and predicting the impacts of global change on insect-mediated ecosystem services.

A trans-oceanic flight of over 4,200 km by painted lady butterflies

The extent of aerial flows of insects circulating around the planet and their impact on ecosystems and biogeography remain enigmatic because of methodological challenges. Here we report a transatlantic crossing by Vanessa cardui butterflies spanning at least 4200 km, from West Africa to South America (French Guiana) and lasting between 5 and 8 days. Even more, we infer a likely natal origin for these individuals in Western Europe, and the journey Europe-Africa-South America could expand to 7000 km or more. This discovery was possible through an integrative approach, including coastal field surveys, wind trajectory modelling, genomics, pollen metabarcoding, ecological niche modelling, and multi-isotope geolocation of natal origins. The overall journey, which was energetically feasible only if assisted by winds, is among the longest documented for individual insects, and potentially the first verified transatlantic crossing. Our findings suggest that we may be underestimating transoceanic dispersal in insects and highlight the importance of aerial highways connecting continents by trade winds.

Pollen metabarcoding reveals the origin and multigenerational migratory pathway of an intercontinental-scale butterfly outbreak

Migratory insects may move in large numbers, even surpassing migratory vertebrates in biomass. Long-distance migratory insects complete annual cycles through multiple generations, with each generation's reproductive success linked to the resources available at different breeding grounds. Climatic anomalies in these grounds are presumed to trigger rapid population outbreaks. Here, we infer the origin and track the multigenerational path of a remarkable outbreak of painted lady (Vanessa cardui) butterflies that took place at an intercontinental scale in Europe, the Middle East, and Africa from March 2019 to November 2019. Using metabarcoding, we identified pollen transported by 264 butterflies captured in 10 countries over 7 months and modeled the distribution of the 398 plants detected. The analysis showed that swarms collected in Eastern Europe in early spring originated in Arabia and the Middle East, coinciding with a positive anomaly in vegetation growth in the region from November 2018 to April 2019. From there, the swarms advanced to Northern Europe during late spring, followed by an early reversal toward southwestern Europe in summer. The pollen-based evidence matched spatiotemporal abundance peaks revealed by citizen science, which also suggested an echo effect of the outbreak in West Africa during September-November. Our results show that population outbreaks in a part of species' migratory ranges may disseminate demographic effects across multiple generations in a wide geographic area. This study represents an unprecedented effort to track a continuous multigenerational insect migration on an intercontinental scale.

A hydrogen isoscape for tracing the migration of herbivorous lepidopterans across the Afro-Palearctic range

RATIONALE

Many insect species undertake multigenerational migrations in the Afro-tropical and Palearctic ranges, and understanding their migratory connectivity remains challenging due to their small size, short life span and large population sizes. Hydrogen isotopes (δ2 H) can be used to reconstruct the movement of dispersing or migrating insects, but applying δ2 H for provenance requires a robust isotope baseline map (i.e. isoscape) for the Afro-Palearctic.

METHODS

We analyzed the δ2 H in the wings (δ2 Hwing ) of 142 resident butterflies from 56 sites across the Afro-Palearctic. The δ2 Hwing values were compared to the predicted local growing-season precipitation δ2 H values (δ2 HGSP ) using a linear regression model to develop an insect wing δ2 H isoscape. We used multivariate linear mixed models and high-resolution and time-specific remote sensing climate and environmental data to explore the controls of the residual δ2 Hwing variability.

RESULTS

A strong linear relationship was found between δ2 Hwing and δ2 HGSP values (r2  = 0.53). The resulting isoscape showed strong patterns across the Palearctic but limited variation and high uncertainty for the Afro-tropics. Positive residuals of this relationship were correlated with dry conditions for the month preceding sampling whereas negative residuals were correlated with more wet days for the month preceding sampling. High intra-site δ2 Hwing variance was associated with lower relative humidity for the month preceding sampling and higher elevation.

CONCLUSION

The δ2 Hwing isoscape is applicable for tracing herbivorous lepidopteran insects that migrate across the Afro-Palearctic range but has limited geolocation potential in the Afro-tropics. The spatial analysis of uncertainty using high-resolution climatic data demonstrated that many African regions with highly variable evaporation rates and relative humidity have δ2 Hwing values that are less related to δ2 HGSP values. Increasing geolocation precision will require new modeling approaches using more time-specific environmental data and/or independent geolocation tools.