Fróðskaparrit - 01.01.1999, Blaðsíða 161

FRÁBOÐAN UM TVEIR FERÐANÁTTVIRVALDAR
AGROTIS SEGETUM (L.) OG NOCTUA ORBONA (Hfn) (LEPIDOPTERA, NOCTUIDAE)
165
Thus, in 1996, Agrotis segetum occurred at
the same time as other migrant species,
such as Vanessa atalanta, V. cardui, Auto-
grapha gamma and Plutella xylostella.
According to observations received at
Føroya Náttúrugripasavn in Tórshavn, the
first two Red Admirals were observed on
14 June, then later on 16 June. From 18 to
21 June, more than twenty specimens were
recorded on many of the islands. From 16
to 19 June, a migration of V. cardui was
recorded on Suðuroy, but apparently not on
the other islands (Mr. Ejler Djurhuus, per-
sonal communication). In westem and
northem Europe, a migration, which was
dominated by V. cardui and other Mediter-
ranean species, occurred at the same time.
The migration reached southem England
on 3 June (Davey, 1997), entered into
southem Scandinavia on 7 June (Hansen,
1997), reached Shetland on 8 - 9 June
(Pennington, 1997), and finally crossed the
Polar Circle around 20 June (Hansen l.c.).
In 1997, the Faroese record of N. orbona
coincided with a small migration of V. ata-
lanta and Plutella xylostella. This migra-
tion was not observed in Shetland, where it
was antedated by a record of an extraordi-
narily rare vagrant moth, Ochropleura
flammatra (Den. and Schiff.), which was
caught on 29 June at Eswick on eastem
Mainland (Rogers and Davey, 1998).
Because of the many verified observa-
tions during both years, this article attempts
to elucidate the possible meteorological
transport mechanisms by which the above-
mentioned lepidoptera species are carried
across the Atlantic from continental Europe
to the Faroes.
Recent research has shown that the
frontal waves that accompany low pressure
weather systems provide the fastest means
of transporting living organisms, both birds
and insects either across the European con-
tinent (Mikkola, 1967; 1986) or the At-
lantic Ocean (Hurst, 1969; Wolff, 1971;
Elkins, 1979; Davey, 1999). By using
“backtracks”, i.e. the daily changes in the
position of the weather fronts along which
air currents occur, it is possible to obtain a
relatively accurate indication of the origin
of migrating or vagrant insects, as the path
of the migration can often be traced back to
the centre of the air currents (Mikkola,
1967; Hurst, 1969; Davey, 1997). Further,
it has been demonstrated that a number of
long-range migrant moth species, such as
Agrotis ipsilon, use low-level jet streams at
an altitude of about 600 m above ground
level (Showers et al., 1989).
The conditions for a wind-bome trans-
port of the two moth species were exam-
ined both at sea level and at higher altitudes
by comparing a number of relevant meteo-
rological parameters prior to and at the time
the migrating lepidoptera were observed in
the Faroes, Table 1. In June 1996, the
weather conditions favoured only a long-
range, aerial transportation between 14-16
June. At that time, a high pressure area
over the British Isles directed fairly strong
southem and southwestem winds clock-
wise from southwestem France, to west of
Ireland, and to the Faroe Islands. The
strong and relatively warm, southem air
currents both at ground level and at the 850
hPa level continued until 17 June, when the
direction of the wind again changed to a