Moth Migration – Philip Strickland
Moth Migration – Philip Strickland
Migration is an evolutionary development and is usually an effort to escape from potentially harmful circumstances. Examples may include a shortage of proper food plants, overpopulation or an unfavourable climate, like cold or extreme rain. Therefore similarly to many other insects, some populations of moths migrate, sometimes long distances, in order to find more suitable conditions. In general moth migration is often seasonal and can be predictable as it often depends on favourable winds during certain months of the year. Moth migration is considered to mean movement in a specific direction as opposed to dispersion which is considered to be undirected movement. In Ireland peak moth migration from Europe and North Africa occurs during the summer months. Favourable winds from an easterly or southerly direction often results in many different species reaching our shores and then spreading out throughout the country. Some of the moths that arrive from other countries are species that already live here. Moths like the Silver Y Autographa gamma and Large Yellow Underwing Noctua pronuba can arrive in huge numbers in the summer and these augment the resident populations here. In the case of the Silver Y there is a discernible difference in the colouring of the locally bred and the migrant individual.
Most of the migrants that come to Ireland are species that do not live here permanently. They may breed here after their arrival, but their offspring do not survive our winter and hence they cannot establish a permanent population. There may be a tendency for some species to attempt to return to their permanent residence at the end of the Irish summer though there is currently little evidence to support this theory. One theory to explain why large numbers of migrants arrive here to breed but fail to survive the onset of colder conditions is the idea that the loss of huge numbers of individual moths is unimportant-the survival of the species is what matters. The fact that large-scale migration to Ireland continues underlines the insignificance for the species of such apparently pointless losses. However, migratory behaviour may well lead to the species’ survival for, in the event of general climate change, inevitable over time, some populations somewhere may be favourably placed for the purpose of establishing a new permanent base from which future generations can migrate (Tolman 2009).
Migratory moths are, in most cases, excellent flyers. Studies of a common migratory species, the Diamond-back Moth Plutella xylostella show that it can migrate 3,000 kilometres and may reach up to altitudes of 100 metres or more. This is despite the fact that these moths are weak fliers, seldom rising more than two metres above the ground and not flying long distances when settled in an area. This species is a passive migrant, being easily transferred by wind over long distances. Therefore this moth does not require strong powers of flight to transfer it over long distances.
Day flying moths use different methods to navigate over long distances. Some species may use coastal lines or mountains to assist orientation. Above sea it has been observed that the flight direction is much more accurate if the landscape on the coast is still visible. Other day flying species may use the sun or in cloudy weather, polarised light to navigate. The polarisation of the sun’s light changes with the angle of the rays, hence they can also navigate with cloudy weather. They may make corrections depending on the time on a day. A study of the migratory behaviour of the Silver Y, showed that this species, even at high altitudes, can correct its course with changing winds, and prefers flying with favourable winds, which suggests a great sense of direction. Night-flying moths may use the moon and stars to help navigate however in cloudy weather they must also be capable of using other means of navigation at night. One study has revealed that Heart and Dart moths use the Earth’s magnetic field to navigate.
Notable Migratory Species
The Vestal Rhodometra sacraria is normally found in Africa, large parts of Asia and Southern Europe. In favourable conditions, it migrates north and can reach Ireland in numbers during the summer months and on occasion can be numerous. In contrast Small Marbled Eublemma parva is an infrequent visitor and has only been recorded around the south east coast on a few occasions.
Many hawk moth species have a tendency to migrate and some spectacular species reach our shores during the summer and autumn. The Hummingbird Hawk-moth Macroglossum stellatarum is a species that is usually found in the Southern Mediterranean and Northern Africa. This distinctive species is often seen flying in sunshine and extending its long proboscis into flowers. In summer, the species disperses north up all the way to Scandinavia and Iceland. In winter it migrates further south, deeper into Africa and to the Indian subcontinent. In Ireland there are usually a few dozen records per year in an average year; however in warm summers, like the years 2006 and 2009, several thousand are recorded here. In mild winters, very small numbers may be able to survive this far north, but these numbers are insufficient to call it a real population. Large nocturnal hawk moths are usually found either in moth traps or feeding at garden flowers such as Buddleia and Nicotiana shortly after dark. They can also be attracted by sugaring, a technique that involves painting a mixture of sugar, syrup and beer onto a post or tree trunk. The Death’s-head Hawk-moth Acherontia atropos is the largest moth found in Ireland with a wingspan of up to 130mm (5 inches). There are usually two or three records here per year and it can be a source of wonderment when found as it flashes its brightly marked abdomen and emits a loud squeak if irritated. Other hawk moths found here are the Convolvulus Hawk-moth Agrius convolvuli, which can be fairly common in some years and the rarer but stunning Striped Hawk-moth Hyles livornica. Perhaps the most exotic looking migrant is the Oleander Hawk-moth (Daphnis nerii), which has only been found in Ireland on a few occasions.
Micro Moth Migration
Given the right conditions many migratory micro moth species can move in vast numbers from mainland Europe, reaching Ireland and spreading out throughout the country. Rusty Dot Pearl Udea ferrugalis and Rush Veneer Nomophila noctuella are two species, along with the previously mentioned Diamond-backed Moth where large influxes regularly occur during the summer months often resulting in scores of individuals recorded in light traps.
The lighthouses scattered around our coastline have in the past added considerably to our understanding of the migration of moths. This was especially the case when the lighthouses were manned and the keepers kept records of all the different species of birds and insects that they encountered while on site at these remote locations. It also became apparent from records received from lighthouses in the north and west of the country that many moth species continued to migrate outwards into the Atlantic in their quest to expand their range. This has been confirmed by studies of species found on oil-drilling platforms in the North Sea. Species such as Dark Sword-grass Agrotis ipsilon and Scarce Bordered Straw Helicoverpa armigera have regularly been found on these platforms. Nowadays a number of different methods are used to track and monitor the flight paths of migrating moths. Low flying species are easily spotted or caught using a light trap. When individuals fly too high for these methods, air balloons equipped with nets are used at times. Alternatively, radar is used to monitor migration.
Global warming has caused an increase of migratory moths that reach Ireland. Research in the United Kingdom confirms that an increasing number of migrants are reaching their shores. Around 40 species of migrant moths have appeared in the UK for the first time in the last 15 years and this trend has been repeated in Ireland to a lesser extent. In some instances these new migratory species may adapt to their new circumstances quite well and this may have a negative impact on our existing species. The vast majority of moths migrate as adults which are not subject to the frequently high level of parasitism the immature stages fall victim to; therefore migrant species may, temporarily at least, escape their parasites. The new colonising species may not be subject to the natural controls that apply to long-standing resident species which may place the latter at a competitive disadvantage. Complex ecological and even evolutionary changes may arise as previously established species attempt to adapt to the changes new species can bring (Parmesan 2006, Hill et al. 2011).There is evidence that newly arrived Lepidoptera can evolve very quickly when in a new environment (Buckley & Brindle 2014). The interaction between the colonists and elements of their new eco-systems is likely to be a fascinating and important area of study in the coming years…
Buckley, J. & Brindle, J.R. (2014) Loss of adaptive variation during evolutionary responses to climate change. Ecology Letters, 17, 1316-1325.
Hill, J.K., et al. (2011) Climate change and evolutionary adaptations at species’ range margins. Annual Review of Entomology, 56, 143-159.
Parmesan, C. (2006). Ecological and evolutionary responses to recent climate change. Annual Review of Ecology, Evolution and Systematics, 37, 637-669.
Tolman,T. & Lewington, R. (2009) Collins Butterfly Guide. Collins, London.
For more information see Philip’s excellent website http://www.moths.ie/
All moth photographs copyright of Philip Strickland except Dark Sword-grass J. Harding.