Fróðskaparrit - 01.01.1999, Side 163

FRÁBOÐAN UM TVEIR FERÐ AN ÁTTVIRV ALD AR
AGROTIS SEGETUM (L.) OG NOCTUA ORBONA (Hfn) (LEPIDOPTERA, NOCTUIDAE)
167
rent at the 850 hPa level passed over the
Faroes on 2 - 3 July, which coincided with
the sudden occurrence of Plutella xylostel-
la and V. atalanta in the islands. At ground
level, the air current followed the frontal
area of a local low pressure system, which
had originated in southem France on 29
June. During its northward movement on
30 June, it caused a marked depression in
southem Germany with strong, thermal up-
winds and local thunderstorms. During its
further movement to the northwest, the re-
sultant cold front system directed the air
currents over southern Scandinavia. On 2
July at the 850 hPa level, the warm south-
ern air current passed west of Norway with
a speed of 72-108 km/h and reached the
Faroe Islands on 2 - 3 July, then changed its
direction to southwest, Table 1.
The significant occurrence of long-
range, migratory lepidoptera species, such
as P. xylostella and V. atalanta in the Faroes
during these days suggested that they origi-
nated from the Mediterranean site of the
depression on 29 June. The Noctua orbona
might also have entered the air current later
on from a site in central or eastem Europe,
most likely on 30 June, when strong ther-
mal upwinds occurred over central Ger-
many during the deepening of the low pres-
sure area. In this case, the circumstantial
evidence from the data in Table 1 strongly
indicates that airbome transport took place
at 1000-1500 metres above sea-level, thus
enabling a transport of more than 2500 km
from southem Europe to the Faroes, with-
out compromising the viability of even a
small moth like Plutella xylostella.
Furthermore, the present analysis
demonstrates that a main, meteorological
factor governing the insect transportation to
the Faroe Islands is the geographical loca-
tion of the parent high or low pressure sys-
tem over the British Isles. In the case of a
high pressure area, such as in 1996, the mi-
gratory transport is directed clockwise west
of Ireland via the Atlantic Ocean. In 1997,
with a low pressure located in the same
area, the transportation was directed
counter-clockwise via southem Scandi-
navia or the North Sea to the Faroe Islands,
cf. the illustrations in Kaaber et al. (1994).
Interestingly this dispersal mechanism is
used not only by regular migrating insects,
but apparently also by an ever-increasing
number of other lepidoptera species that are
recorded more or less regularly in the North
Atlantic isles far away from their usual
breeding areas.
Acknowledgements
The authors would like to thank Dr. Dorete Bloch, Di-
rector of the Føroya Náttúrugripasavn, Tórshavn, for her
kind co-operation. Professor Peter Esbjerg, The Royal
Veterinary and Agricultural University, Copenhagen,
gave valuable information on Agrotis segetum in Den-
mark. The climatologists, Mr. John Cappelen and Mr.
Stig Rosenøm, both at The Danish Meteorological Insti-
tute, Copenhagen, provided access to the meteorologi-
cal data used. Their kind support is gratefully acknowl-
edged. The fauna studies in the Faroe Islands by SK in
1996 and 1997 was made possible by financial support
and grants from the Schiøtz-Christensens Mindefond,
The Museum of Natural History in Aarhus, and The
Carlsberg Foundation. During both years, Føroya Nátt-
úmgripasavn and Føroya Banki in Tórshavn provided
generous support and excellent working conditions.
Literature
Bretherton, R.F., Goater, B. and Lorimer, R.I. 1983.
Noctuidae. In Heath, J. and Emmett, A.M. (eds.).