by Peter Davey

Many readers will already have seen a Painted Lady butterfly this year. This is unusual as this fine migratory species is rarely seen in England much before May, and in some years is absent altogether. Much depends on the combination of a large emergence of adults in north Africa plus a persistent airflow from a southerly quarter to bring it to northern Europe in the late spring and early summer.

However, in February 2004 this very combination was established to an extreme degree, and many hundreds duly appeared across mainly southern Britain; indeed, more than five hundred were recorded across Dorset. The Painted Lady butterfly was not the only lepidoptera species to participate in the migration. It was indeed fortuitous that our moth recorders were on the ball and light trapping during the critical period. In Dorset alone, a staggering twenty potential immigrant moth species were observed, many very rare, and one, the Levant Blackneck, was totally new to the county. In fact no less than ten of this rare vagrant were seen all told, and this is equivalent to the total ever seen in Britain. Another species, a well-known migrant micromoth called the Rush Veneer (Nomophila noctuella) tends to occur most frequently in the summer and autumn months. Only ten have ever been recorded in Dorset in February historically, but on this occasion more than one thousand five hundred appeared across the county. Other moth species included the fabulously marked Striped Hawk, the more familiar Humming-bird Hawk and Silver Y, and the minuscule Diamond-back (Plutella xylostella), a tiny micromoth with a six-millimetre wingspan. Another species, the orache-feeding Nutmeg, is double-brooded in Europe and on the wing in May and August. No fewer than twelve of these were trapped across the county, all of an unusually pale or sandy colour, suggesting a very southerly source indeed for this species.

This event therefore was unique. Evidently conditions south of the Mediterranean must have been perfect for a mass emergence of lepidoptera during January. The weather then proceeded to play its part, with a strong high pressure system firmly anchored over the western Mediterranean during the first week of February and an uninterrupted east to south-easterly airflow on its southern flank across much of north-west Africa. This airflow veered to a southerly point to the west of Iberia, and finally to a south-westerly across much of England. Day-time temperatures peaked at 26°C across Algeria, 22°C in Gran Canaria to 13°C over Dorset at this time. The first wave of immigrants passed across the county on 4th and 5th February. The author was fortunate enough to disturb a very pale Painted Lady butterfly from a gravel track in Lower Hyde Heath in cloudy conditions on 4th February and to trap a Striped Hawkmoth at Durlston thirty hours later.

As the east to south-easterly airflow increased across the Western Sahara and Mauritania regions on 4th February, so sand from the desert became airborne and moved on a broad front towards and then across the African coast and finally out into the Atlantic on a turbulent Sirocco wind. A correspondent on the Canary Islands observed the onset of this phenomenon and the sudden arrival of many Striped Hawkmoths and Painted Lady butterflies on 5th February. Although, the offshore airflow over north-west Africa continued unabated, the flow of immigrants to southern England was brought to an abrupt halt on 7th February as polar north-westerly winds swept across northern Europe. The tropical south-westerly winds resumed during 11th February and with it came the second and much larger influx of immigrants. Fantastic totals were recorded on the night that followed: Portland Bird Observatory recorded one hundred and sixty-four Small Mottled Willow moths at light traps; and at Puddletown, twenty five kilometres to the north-east of Portland, three hundred and eighty-four Rush Veneer moths were counted in the light traps there.

Tracing a path back for these individuals lead to a departure point and time coincident with the onset of the Sirocco winds over the Canary Islands a week earlier. The author believes that a large proportion of the insects comprising this second influx travelled at altitude for much of their journey, and that many of these filtered out of the sky across southern Britain as winds fell light at all levels by the evening of the 11th February. Certainly, temperatures that ranged between 12°C over the Canary Islands and 5°C over south-west England at 1500 metres above the ground, would have made flight viable for insects at this level, and the Sirocco a suitable mechanism to transport insects to various levels of the atmosphere at the start of their 4000 kilometre journey.

Interestingly, numbers of moths peaked on 4th and 5th February and again on 11th and 12th February in England, with relatively few seen in the intervening period. However, numbers of Painted Lady butterflies increased between the two peaks. This may have been due to the relatively sunny conditions at the time and the associated stimulation of nectaring and basking behaviour making them more visible to recorders. In addition, the 7th and 8th February fell on a weekend and with it the potential for increased observations.

Dorset's butterfly and moth enthusiasts are to be congratulated on the wealth of county records they submitted. Of the four thousand plus individual immigrants seen across Britain during February, more than two-thirds were spotted in Dorset. At the time of writing in late March, singleton Painted Lady butterflies are still in evidence along the coast, but as every day passes they become more tatty and fewer in number. It will be fascinating to see whether this spectacular immigration manages to spawn resident broods of the respective species over the coming season.

The sequence of meteorological events was also of great interest on this occasion, and the second influx may be the first recorded instance of flight at altitude within the Sahara dust-plume during a winter month. Prior to the onset of the warming trend or global warming, this phenomenon was a relatively rare event and primarily restricted to the summer months. However, in recent years it has become more frequent, and in the hot summer of 2003, no less than four such plumes made it to the UK transporting many rare migrant species, some of which were new to Britain.