Fróðskaparrit - 01.01.1996, Síða 106

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NOTES ON XESTIA ALPICOLA SSP. ATLANTICA NOV. ON THE FAROES
drucei, Vaccinium myrtillus and Empetrum
hermaphroditum. The moth fauna on that
day was dominated by many Entephria
caesiata Den. & Schiff. and Epinotia mer-
curiana Frol., flying freely together with a
few late Catoptria furcatella Zett. and Xan-
thorhoe munitata Flb. A similar vegetation
characterizes many other mountain slopes
at altitudes between 400-600 metres on the
northern Faroe isles. Presumably such sites
also contain other populations of Xestia
alpicola which are easily overlooked.
The bionomy ofXestia alpicola
in Northern Europe
The present Faroese Xestia alpicola-speci-
mens have all been caught or observed in
the even years 1956 and 1996. In this re-
spect the population corresponds closely
with the population in Northem Scandi-
navia, where it and most other boreal Xes-
íia-species occur in even years, while in
Southern Norway, southwestern Finland
and the Baltic countries it mainly occurs in
odd years (Mikkola, 1976). The British X.
a/p/co/a-population which is restricted to
the northernmost counties in England, the
Scottish Highlands-, northern Ireland, the
Orkneys and Shetland shows a similar pat-
tern. A biennial pattern corresponding with
the Scandinavian and the Faroe Isles has
been observed in the Scottish Highlands
where populations preferably occur in even
years (Bretherton et al„ 1983). Regarding
the population in Shetland very little recent
information exists in the British litera-
ture. According to unpublished material
gathered by Mr. Mike Pennington, Unst,
the available data from the last century in-
dicate that the presumably extinct popula-
tion on Mainland mainly occurred in the
odd years 1881 and 1883, and was found
again on Mainland and Unst in 1893, an-
other odd year. However, the last known
occurrence in Shetland - the only one
known from this century - was in July
1932, an even year (Mike Pennington, i.l.
1996). In the Orkneys the species has only
been mentioned once, from the odd year
1895 (South, 1895; Lorimer, 1983). How-
ever, in 1986 a male was attracted to mer-
cury vapour light on Orphir during a period
with very warm weather (Lorimer, 1993).
In Northwestern Ireland the very few ob-
servations have also been made in both
even (1972) and odd years (1973) (Redway
and Heath, 1973; Redway, 1973). In North-
ern England the local populations also oc-
cur in both even and odd years, preferably
in odd years (Bretherton et al„ 1983).
In the literature there has been some
speculation regarding the background of
this biennial pattern. Kettlewell, in Ford
(1954), proposed that the reason might be
found in common parasitation from other
alpine moth species flying in the interven-
ing years. Thus the parasites might prevent
Xestia alpicola from changing flight years.
He suggested that the alternate flight years
in Scotland of X. alpicola and the geome-
trid moth Psodos coracina could serve as
an indication for this theory. As P. coracina
in Northern Scandinavia does not show any
altemate-year rhythm, this theory does not
seem to be likely. Later on Mikkola (1976)
therefore proposed a more elaborate theory.
He considers that the biennial regulation is
chiefly due to the peculiar biological condi-