Fróðskaparrit - 01.01.2009, Side 126

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PLANT GALLS AND GALL MIDGES OF THE FAROE ISLANDS IN THE ATLANTIC OCEAN
host plants of gall causing organisms. Of
seven plant species that were found to be
host plants of ten species of gall-causingand
associated organisms in the Faroe Islands,
Juniperus communis, Salix phylicifolia and
Viola riwiniana are native to the Faroe Islands,
in contrast to Acer pseudoplatanus, Ribes
rubrum, Sorbus aria and Calium aniso-
phyllum that are alien species introduced to
Faroe Islands intentionally by man in
historical times. In accordance with the
origin of host plants, we may consider the
gall-causing and associated organisms to be
of the same origin, at least the gall midge
Oligotrophus juniperinus as native to Faroe
Islands and Contarinia floriperda as alien to
Faroe Islands. We do not know surely the
origin of other gall midges that are not gall-
causing, but mycophagous or zoophagous,
and their association with host plants is not
so narrow. These gall midges could be
transferred by wind or with air flows from
continental Europe to Faroe Islands.
It is also a question if the species causing
galls on fiower buds of Sorbus aria that has
been identified as Contarinia floriperda is
identical with species developing in flower
bud galls of Sorbus aucuparia. To solve this
problem, it is necessary to examine larvae
and adults reared from galls of both host
plants, and DNA sequencing may also be
necessary.
Conclusions
There are several probable reasons of such
low species number of gall-causing and as-
sociated organisms in the Faroe Islands.
Above all it is the low number of flowering
plants - only about 400 species - and the
absence of wild trees and shrubs that could
be host plants of gall-causing organisms. This
is the result of the geological origin of the
Faroe Islands which came into being through
the activity of submarine volcanoes. The
soils are salt, unfertile and include insuflfi-
cient amount of nutrients for plants. An im-
portant reason is the geographical isolation
of the Faroe Islands which are situated far
from continental Europe - about 600 km
from Norway and 300 km from the Shet-
lands where any investigations of gall-caus-
ing organisms have been done. In contrast,
the gall midge fauna of Norway is rich in-
cluding 290 species (Skuhravá and Skuhravy
in prep.). Also climatic factors, viz. relatively
low temperatures and short growing season
may influence species numbers that devel-
oped in Faroe Islands. It seems that breeding
of sheep and grazing are also important rea-
sons for the low species number of gall-caus-
ing organisms. Sheeps are everywhere walk-
ing in small groups in the wild and feeding
on grasses and wild plants.
Acknowledgements
We thank very much our son-in-law Michal Vanecek
and his wife, our daughter Ivana Vanecková, for their
excellent idea to give us a gift for our wedding
anniversary - the oportunity to spend one week in the
Faroe Islands. Our thanks are also due to Anna Maria
Fosaa, the head of the Botanical Department of the
Faroese Museum of Natural History, Tórshavn, for
information on the plant species number and for
identification of Viola riviniana. We thank Dr. K.M.
Harris, Ripley, Woking, Surrey, UK, for critical reading,
advice and improvements of our manuscript.