Although in the rest of the United Kingdom, and indeed in mainland Europe, the study of plant galls (Cecidology) is increasing and recorders are growing, in Cornwall and the Isles of Scilly, plant gall recorders remain few. The early literature, although extensive, consisted of specialised papers in many scientific publications and books in other languages: Houard (1908-1913 in French), Buhr (1965 in German) and Docters van Leeuwen (1982 in Dutch). Many early works in English such as those by Connold (1901-9) and Swanton (1912) ceased to be in print and were difficult to obtain until Darlington (1968) wrote a small book which introduced many to plant galls.
Seventeen years later, in 1985, the British Plant Gall Society was founded and soon published Provisional Keys to British Plant Galls (1986). Since 1986, a journal Cecidology has been published twice a year, the Society has held field meetings and conferences and has organised a Plant Gall Recording Scheme. In 2002, British Plant Galls Identification of galls on plants and fungi by Margaret Redfern and Peter Shirley, illustrated by Michael Bloxham, Field Studies,10, 207-531 was published and has proved an invaluable aid.
A plant gall consists of plant tissue in which the cells have been stimulated to increase in size or in number (or both) by an intrusive living organism. The relationship is parasitic although a few examples show evidence of symbiosis. A gall is named by reference to both host and parasite or causer. As the study of plant galls becomes more popular, knowledge is continually increasing, but there are still unidentified phenomena which may or may not be caused by responses to factors such as physical damage or climatic conditions.
Galls have been recorded on flowering plants, ferns, algae, fungi, lichens and bryophytes. The causers are bacteria, viruses, phytoplasmas, Protozoa, Rotifera, fungi (uredinales, ustilaginales and others), flowering plants, Nematoda, Acari, Insecta:(Hemiptera: Homoptera including Psyllidae), Thysanoptera, Hymenoptera, Coleoptera, Lepidoptera, Diptera. Most causers are specific to one host, but there are some which affect many different hosts. To add to the complexity of studying galls: in cynipid wasps there is alternation of generations, involving two generations in the course of the life-cycle (often one year) - the agamic generation, exclusively females, and the other with individuals of both sexes (sexual), these occur on the same or different parts of the same host plant; in rust fungi (uredinales), alternate host plants for the same gall species are common, whereas smut fungi (ustilaginales) do not have alternate hosts; and there are inquilines (colonizers of galls) and other parasitic hymenopterans which inhabit galls such as oak marbles and robin‘s pin cushions.
I have made an effort to review and recheck all plant gall records, especially those plant gall species mentioned in the RDB for Cornwall and the Isles of Scilly 1997 Edition:
The number of species recognisable as gall causers in Britain was quoted at just over 1800 in (Spooner & Bowdrey, 1993 and 1994). In Cornwall, in 1997 there were 183 species on over 84 hosts. By 2007, on the database kept by me these have increased to 339 species (including 117 plant galls caused by fungi) recorded on over 218 hosts. Thanks are due to those who sent in records.
(a) There are some plant gall species which are rare or uncommon in the rest of the country which each have a few Cornish records dating from the late 19th century to the 1960s and 1980s. However, there have been no recent records to determine their status in Cornwall. These are:
Andricus albopunctatus (Schlechtendal) (agamic pre 1906), Andricus callidoma (Hartig) (agamic pre 1906 and 1936), A. glandulae (Schenck) (agamic pre 1906), A. inflator Hartig (agamic pre 1906), A. nudus Adler (sexual 1981), Trigonaspis megaptera (Panzer) (agamic pre 1906, 1930s, 1968).
Jaapiella genisticola (F. Löw) (1972 and 1989), Rhopalomyia millefolii (Loew) (pre 1906, 1975).
Aceria erineus (Nalepa) (1975).
Uredinales: Puccinia nemoralis Juel (1985).
(b) Plant Gall species mentioned in the RDB 1997 whose status has changed since or for which further information has been acquired:
Andricus quadrilineatus Hartig (agamic pre 1906, 1930). UK common. Andricus quercuscorticis (Linnaeus) (agamic pre 1906 and 1963). UK fairly common. Cynips longiventris Hartig (agamic pre 1906, 1981, 1982, 1983). UK common.
Contarinia lathyri Kieffer (1901 and 1903). Doubt as to original nomenclature of records. Dasineura violae (F. Löw) (pre 1906). Host species recorded is incorrect for this species. Semudobia betulae (Winnertz) (pre 1906). No records since 1906, may be underrecorded. Macrodiplosis roboris (Hardy) (previously M. volvens Kieffer) (1973 and 1989). May be under-recorded. Geocrypta galii (Loew). Several records since 1997. Rabdophaga cinerearum (Hardy). Locally common on Sallows. There are identification problems of Salix spp. on some records and controversy as to whether Rabdophaga cinerearum and Rabdophaga strobilina Bremi (= R. rosaria (Loew), are different or the same species. Kiefferia pericarpiicola (Bremi) (= K. pimpinellae (F. Löw)). More records since 1997. Mayetiola destructor (Say) (1886). No records since and insufficient information of UK distribution.
Homoptera: Aphididae Brachycolus stellariae (Hardy). No records since 1902, may be under-recorded. Rhopalosiphum insertum (Walker). No records since 1973, easily overlooked. Ceruraphis eriophori (Walker). Now, not regarded as a true gall.
Stenopterapion scutellare Kirby. There are several records of adult species so perhaps the galls are overlooked on the host plants. There are two gall records: from 20/17 (VC2) in 1993 on Western Gorse Ulex gallii and from 10/71 (VC1) in 2003 on Gorse Ulex europaeus. Saperda populnea (Linnaeus). No further records for Cornwall and no records for the adult species.
Aceria geranii (Canestrini). More records since 1997, including two which may have been for Aceria dolichosoma (Canestrini). Aceria geranii has now also been recorded on Herb Robert Geranium robertianum.
(was Mycoplasma) known as clover phyllody, maybe spread by leaf
hoppers. Several records since 1997.
alni (Woronin) Maire & A.Tison (=
Frankia alni) (Actinomycetes). As this causes nodules on roots, it is probably under-recorded and occurs in many places where
Alder Alnus glutinosa grows. Inquilines: Periclistus brandtii (Ratzeburg) (Hymenoptera) in galls of Diplolepis rosae pre 1906. No further records. Clinodiplosis
botularia (Winnertz) (Diptera) in galls
of Dasineura fraxini, pre 1906. No further
records. Subanguina millefolii Aceria artemisiae Cryptosiphum artemisiae Mompha divisella Synanthedon formicaeformis Andricus quercusramuli Biorhiza pallida Cynips disticha Plagiotrochus quercusilicis Tetramesa hyalipennis Chlorops pumilionis Dasineura filipendulae Dasineura galiicola Dasineura odoratae Diodaulus linariae Rhopalomyia ptarmicae Noeeta pupillata Urophora spoliata Aceria anceps Aceria megacerus Eriophyes torminalis Xenodochus carbonarius Taphrina tosquinetii
Frankiella alni (Woronin) Maire & A.Tison (= Frankia alni) (Actinomycetes). As this causes nodules on roots, it is probably under-recorded and occurs in many places where Alder Alnus glutinosa grows.
Periclistus brandtii (Ratzeburg) (Hymenoptera) in galls of Diplolepis rosae pre 1906. No further records. Clinodiplosis botularia (Winnertz) (Diptera) in galls of Dasineura fraxini, pre 1906. No further records.