Insect migration

INSECT MIGRATION IAN WOIWOD Firstly I will give some background information aboul inscct migration and lalk about some of the controversies and thcn providc examplcs of somc of the more spectacular migrants that wc have in Britain.

Records of migration go back to Exodus in the b'iblc wherc locust migrations are referred to. These arc phenomena in which large numbers of insects are seen flying in a particular direction in very straight lines, sometimes in very great numbers, so much so that you cannot miss them. They have been noted over hundreds of years, but it wasn't really until Victorian naturalists in Britain started to takc some of these phenomena seriously that a lot of these data were gathered together and studied in more detail. The first naturalist to do this was J. W. Tutt, a Victorian lepidopterist and editor of the Entomologist's Record and Journal of Variation. He published a series of articles from aboul 1890-1902 under the title 'Migration and Dispersal of Insects' these articles gathered together many of the observations made on these stränge occurrences of insect movement. This was the first attempt to understand these bchaviours.

1t was noticed that there were certain species that cropped up again and again in these observations and these species became known as migrant spccies.

The next person to take up an interest in this was C. B. Williams, a professional entomologist who in the 1920s and before worked in Egypt, Central America and East Africa. As a boy he had heard rumours that butterflies were able to cross the Channel into Britain though at that time he did not take these seriously. However, when he went to work abroad he actually observed some very striking insect movements and as a result became interested in the subject. In 1932 he came to Rothamsted to be head of the Department of Entomology where he updated Tutt's work. This interest in insect movement at Rothamsted has continued since that time. Williams put butterflies and moths into three categories: 1

resident migrants, like the large white, small white and small tortoiseshell - these are species which can breed, and overwinter in Britain successfully.

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annual migrants, species like the Red Admiral, Painted Lady, Clouded Yellow and Silver Y Moth which occur every year in Britain but are not usually successful at overwintering in this country. Their presence must be due to regulär migrations.

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vagrants, species which wc get in very small numbers, and in some years not at all.

Williams produced a book in the New Naturalist series on insect migration in 1954. He was particularly interested in day-flying insects which flew strongly in straight lines, often against the wind direction. Ornithologists argued that this was not true migration in the sense that they understood where birds move to and fro from breeding sites to sites wherc they feed outside the breeding season. Initially, there was very little evidpnee of return migration.

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Williams was concerned that if this behaviour was genetically controlled there would be a "Pied Piper" effect wilh migrants moving one way and the migrant gene would be lost from the source population. He used amateur records of the Clouded Yellow, a regulär migrant to southern counties of Britain, to show that the mean direction of flight changed at different times of year. In August, records showed a northerly movement, later in October they showed a southerly movement apparently back to their source populations. Recently published data from Germany using large whites in cages showed very clearly that at different times of the year the adults behaviour changes in terms of the direction in which they are trying to fly and this behaviour is now well aeeepted for large powerful day-flying insects.

At Rothamsted we have worked for many years on aphids which are small, day-flying insects, not of great interest to amateur naturalists. However, there is a lot of professional interest as they are probably the most important agricultural pests in Britain in terms of feeding damage to crops and the introduetion of viruses. C. B. Williams employed C. G. Johnson and L. R. Taylor to work on aphids in the department. In contrast to powerful insects like the Monarch, aphids are small and weak-flying. Their populations can build up rapidly with parthenogenetic reproduetion producing large numbers of genetically identical individuals. It is therefore surprising to find that amongst these populations there can be both winged and wingless adults i.e. the possession of wings is not genetically controlled but is environmentally determined in response to crowding, plant nutrition and day length. They produce migrant winged forms, but unlike the aforementioned butterflies they do not move in straight lines across the English Channel. Instead they fly upwards in straight lines until they are eventually borne by wind currents. If they do not alight on anything suitable when they come down they take off again and have another go. This is known as wind-bome migration.

C. G. Johnson developed suetion trapping to sample small flying insects.

He wanted to determine how high the aphids flew and what was the behaviour that enabled this movement between host plants. One experiment he used was to tether large suetion traps beneath air balloons going up to a height of 1000 feet. He found that there was a surprising number of insects even at this height.

Furthermore, his studies with Roy Taylor showed that insects that went up into very cold areas where they might not be expected to survive were still viable when they came down. They also found that many other small insects were wind-borne at height. They ran suetion traps up a 100 ft tower at Rothamsted to measure the density height profile of various insects. This work led to Johnson's book in 1969 on insect migration - a comprehensive guide to migrating insects. Dr Johnson and other insect physiologists found that a lot of these long ränge movements took place when the insects were not sexually mature. He called this the 'oogenesis flight Syndrome' and this was originally thought to have applied to most insect migration. However, it has since been found that there are quite a lot of insects where this is not the case. For example, the Monarch Butterfly which lays eggs as it migrates north through North America. Another example is that of parasitic wasps which prey on aphids. Data from a network of aerial insect suetion traps across Britain and

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0 status unknown/data not available Shaded countries are EC members

The network of 12.2 m suction-traps in Europe in 1997 and their sampling strategies Europe have shown migration of female parasitic wasps in search of prey and these are likely to be seasonally mature.

This same network of traps has provided evidence of long distance migration of aphids such as the Peach Potato Aphid (Myzus persicae), an important agricultural pest, travelling from Belgium and Holland to England within the space of two days in July 1982. By looking at the wind patterns at that time we can see that there were very suitable winds for transporting this species into Britain.

C. B. Williams's book raised the question - how far can insects travel? One way to answer this is to mark the insects on the wings in the same way that birds are ringed. Wilhams experimented with putting an address on the wings of butterflies so that they could be returned. Unfortunately, this work was interrupted by the Second World War so recovery was poor. Rates of recovery

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for mark and recapturc experiments on insects are very small. Somc of thc most succcssful results have come from early work by Urquart on the Monarch Butlerfly using tags. He marked 3-4,000 individuals and recovcrcd six, one of which had travelled ovcr 850 miles. Despitc modern technology this is still a problem today. One of the ways being explored at Rothamsted and elsewhere is by studying genetic variability in population to give an indication of how much genetic mixing there is over long distances.

Thc tiny Diamond Backed Moth (Plutella xylostella), a notorious pest on Brassicas, has long been known as a long distance migrant. Movements in this species have been monitored by back-tracking winds to find the probable sourcc location. One pattern from several back-tracking studies in moths has shown source locations in North Africa. Similarly, thc Migrant Locust (Schizocerca gregaria) turned up in Britain in 1954 for what was only the second time that it had been found in this country. Back-tracking studies, including finds of dead locusts in the sea, traced the probable source of this migration to Mauritania. This movement, although of a streng flying insect was almost certainly wind-borne as there wcre winds of a suitable direction at heights of up to 100 m.

The Rothamsted Insect Survey also coordinates a network of a 100 light traps run by volunteers. All macrolepidoptera (larger moths) and some of the migrant lepidoptera caught in thc traps are identified. Two common migrants caught in these traps are the Diamond Backed Moth and the Silver Y Moth.

Both of these species can occur at any time of the year because their source populations in places like North Africa mean that they are always available for migration. Usually there are only a few individuals in January and February with the real migrations not occurring until May/June. In the case of the Silver Y the initial influx of migrants is supplemented by a further population build-up of home-bred individuals.

Numbers of Painted Ladies in 1996 were particularly impressive. On searching thistles in my home area of Bedfordshire in June for caterpillars of this species I found that many of them had been killcd by parasitic hymenoptera. If their migration to Britain had been in search of an 'enemy free space' it was unsuccessful. Nevertheless, enough survived to produce a large population explosion in August. Another species, the Queen of Spain Fritillary is on the verge of going from a very rare migrant to becoming a resident breeding species in this country and it is likely the first populations will arise in Suffolk as recently it has been seen quite regularly at Minsmere and Aldringham Walks.

There is a whole series of very large, showy Hawk Moths that are migrant which don't overwinter here but tum up fairly regularly. The largest and most spectacular of these is the Death's-head Hawk Moth (see Plates 1 & 2). In times past collectors would pay potato pickers to find the larvae or thc pupae so that they could obtain fresh specimens. Another long-range migrant that turns up frequently is thc Convolvulus Hawk Moth. These can be found extracting the nectar from tobacco plants with their long proboscises. A more attractivc, but rarer, migrant is the Bedstraw Hawk Moth which sometimes breeds in this country. The Humming-bird Hawk Moth is another regulär migrant which

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breeds and occasionally tries to hibernatc in this country but thcre arc probably insufficient numbers to establish a viable local population.

As well as Butterflies and Moths there are many Dragonfly species which are migrant. For example, in 1995 good numbers of the migrant Yellowwinged Darter (Sympetrum Ă&#x;aveolum) were seen Coming in off the sea at Felixstowe and elsewhere on the east coast. This species was seen mating and laying eggs as far inland as Bedfordshire and there were several records in the following year of successful progeny, although no permanent populations survived.

A relatively new technique that will provide more information about insect migration is Radar. Most of this work has been done overseas looking at important insect pest such as locusts. The latest development in this area is the Vertical Looking Radar which sends a beam straight up. The data f r o m this can be analysed using Computer techniques to provide information on the displacement of the insects, their mean speeds, their actual weights and the densities of insects at different heights. A new study at Rothamsted using this technique Over the next two years is hoped to provide much new information on the migration of wind-borne insects. (text prepared by D. K. & M. N. Sanford f r o m a recording made at the conference) Ian Woiwod Department of Entomology & Nematology, IACR-Rothamsted.

Harpenden, Hertfordshire, AL5 2JQ.

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Plate 1: Death's-head Hawk Moth. Acherontia (p. 83). A regulär migrant to Britain (p. 4).

atropos L.. Sicklesmere, 1998

Plate 2: Death's-head Hawk Moth larva. Acherontia atropos L„ Sicklesmere. 1998 (p. 83). In times past collectors would pay potato pickers to find larvae so that they could obtain fresh specimens (p. 4).

Plate 3: Catch from CEFAS Research Vessel Corystes (p. 4 & p. 45). Large fish is a Cod, Gadus morhua, others include Sand eels, Ammodytes marinus, Gurnard, Aspitrigla cuculus, Dab, Limandci limanda and Herring, Clupea harengus.