The wasp and bee fauna of the Ridti archipelago in Lake Milaren, Sweden (Hymenoptera, Aculeata) CONEN E. NILSSON Nilsson, G. E.: The wasp and bee fauna of the Ridd archipelago in Lake Miilaren, Sweden (Hymenoptera, Aculeata). IInventering av gaddstekelfaunan i Riddarkipelagens nat-trrreservat i Malaren (Hymenoptera, Aculeata).1 - Ent. Tidskr. ll2:'79-92. UmeA, Sweden 1991. ISSN 0013-886x. The aculeate fauna (excluding Formicidae, Apidae s.str- Andrenidae, and Halictidae) was surveyed in the Ridd arghipei-ago nature reserve (19 km2) and the small adjacent mainland area Solbacken (0.5 km2). The-Ridi, archipelago is characterized by a mosaic of deciduous forests (including some of nearly primeval type), coniferous forests, wet and d-ry meadows, grazing ground. grazing marshes, and parkland. During 1982 - 1990. a total of I 139 specimens iere c-ollected ii the alchipelago and more than 2000 specimens at Solbacken. 222 species (45 Vo of the Swedish fauna) arelepresented in the material ( I 50 species from the archipelago). Of the species found, 35 occur on red data lists from Britain and/or Central Eurgpe.. The wood-neiting fauna was the richest. Wood-nesting sphecid wasps were_represented by.57 species (72 4o of the wood-nesting Swedish faunaj, a figure that outnumbe-rs most Swedish piovinces (including SkAne. Oland and Gotland) and equ_als_the lqTlgr of Danish. spec-ies. bf-wood-nesting eumenid wasps and megachilid bees, 63Vo and 8lo/o, respectively, of all Swedish wood-iesting species were found. The richness of the wood-nesting aculeate fauna is probably related to the wealth of dead trees and fallen logs, the rich flora of flowe-ring plants, ttui exceptionally mild and dry local climate, and a continuous influence from ireighbouring populationi. Five species are reported as new to Sweden, and l3 new province records are given. G. E. Nilsson, Department of Zoophysiology,IJppsala University, P-O. Box 560, S-751 22 Uppsala, Sweden. Introduction course of this study, it soon became clear that the RidO archipelago contains one of the richest fau- and social wasps and bees re- In general, solitary nas of wood and stem nesting aculeates in nort- types of nesting habits: ( I those present two main ) hem Europe. who dig their nest in sand or soil, and (2) those who nest in cavities in other materials, mainly At least in northem Europe, wood and plant stems. Study area these two ecological groups include similar num- bers of species distributed among most major fa- The Rid6 archipelago is a nature reserve situated milies. in the western part of Lake Miilaren (59'30' N, Sphecidae, the most diverse aculeate family in 16'40' E). The main part belongs to the province Sweden, has 77 species in the first group and 19 Viistmanland (VS), and a smaller part belongs to in the second. The many aculeate species that are Siidermanland (SO). It consists of 160 islands parasites on other aculeates are generally rather ranging in size from 7.5 kmz (Ridttn) to less than host-specific, and, thus, can be referred to the 100 mz (Fig. 1). The total area is about 19 km2. same ecological groups as their hosts' The area, which has been surveyed botanically All recent Swedish regional surveys ofaculeate by Kers (1978), contains a mosaic of deciduous Hymenoptera have dealt with sandy areas domina- forests, coniferous forests, wet and dry meadows, ted by soil nesting species. The present survey is grazing ground, grazing marshes, and parkland. the first study of an area with very few sandy The deciduous forests are rich in oak, elm, lime, biotopes but rich in woodland, notably old, more ash, maple and alder. Probably due to the difficul- or less natural, deciduous forests. During the ties of transporting logs to the mainland, parts of 80 Gδ・′α″E./V′ ′ssο ′ ♪ ６ LИ KE MAιИREAr D“ ビ ″ b9 与~?貫( , ( 、 θ y´ .´ .」 ・ 碑 。・ ´・´ 。 0: ´｀ Q ´・ ぐ ´・ ″・ ヽ S し ・ ・ 、 一 $8. l. Map of the Ridti archipelago. A = Flaten; B-E = Ridctn; F-H = Aggarrin; I = Lilla Aggarcin; J = Hrigholmsskiir; K=Svinholmen; L-Hemmingsskiir; M=AggarciLAngholme; N=Grisslingen;O=Kurci-Nysslingen; P= Ekholmen; Q = Sundbyholmsdn; R = Nysslingen; S = Solbacken. Karta tjver Ridd-arkipelagen med anviind omridesindelning. the forests are in a nearly primeval state. There is a/ri (Fischer v. Waldheim) (Lundberg 1963). The a long history of pasturage (cattle) on the larger Lepidoptera fauna is also rich. For example, Ri- islands (Fig. l: areas B-E, G-H, M, and Q), and diin has the only known population of Ethmia the flora of open land is rich. The only noteworthy dodecea (Haworth) in Fennoscandia (Palm 1989), sandy biotope is an old sand pit (ca 1000 m2; on a beautiful oecophorid moth living on Lithosper- Flaten (Fig. l: area A). mum fficinale. Some notable aculeates from the The climate of westem Lake Miilaren is excep- Ridti archipelago were commented in Nilsson tional for its latitude, the mean summer tempera- ( r986). ture being among the highest (17"C in July) and the precipitation among the lowest in Sweden (Angstrdm I974). Methods The deciduous forests of the Ridti archipelago All families of aculeate Hymenoptera were sur- hold a very rich wood-living beetle fauna inclu- veyed except ants (Formicidae), social bees (Api- ding such rare species as Masosa curtulionide.s dae s.str.) and two families of solitary bees - And- (L.), Chlonphorus herhsti (Brahm), and Diterco renidae and Halictidae. The latter two families The u'asp and bee Jauna oJ'the Ridi arc.hipelago 8l contain only sand and/or soil nesting species. Of becomes even more impressive. After 9 years of the studied families, 489 species are known to collecting, the list of sphecid species from the occur in Sweden. Ridci archipelago (incl. Solbacken) contains the About 55 full-day collecting trips were made same number of wood-nesting species as has ever to the archipelago. Most specimens were collected been recorded from Denmark (57 species accord- with a hand-net at sites which appeared to be good ing to Lomholdr 1916 and Pulawski 1984). Den- habitats for aculeates, like logs, thatch, flowers mark is situated 250 - 600 km S of the archipelago. and sun-exposed foliage. Ca lO Vo of the material is more than 2000 times larger, has a direct conti- were collected in yellow pan traps and trap-nests. nental connection, and, ofcourse, contains a larger Apart from the Ridtj archipelago nature reserve variety of biotopes. The comparatively high abun- (Fig. 1: areas A-R), also Solbacken (Fig. l: area dance of aculeate species in the Ridtj archipelago S) was included in the investigation. Solbacken is is not due to an addition of northern elements. situated at the mainland shore (VS), only 500 m None of the species found in the archipelago has from the nearest island in the archipelago. I have its southemmost Swedish record in VS, whereas a summer cottage at Solbacken, which has allo- l3 species have their northemmost known Swe- wed an intensive collecting activity there since dish occurrence in VS. Indeed, all species found 1982. At Solbacken I have used netting, pan traps, in the archipelago are also known from continen- trap-nests and Malaise traps. This area includes tal Europe. wet and dry meadows, and more or less natural Although all biotopes found at Solbacken ap- deciduous forest, although not as old as some of pear to exist also in the Ridd archipelago (but not the deciduous forest in the Ridij archipelago. In vice versa), the intensive study of Solbacken has this paper, I only present those species from Sol- yielded several aculeate species not found in the backen that have not been recorded from the Ridii archipelago (Tab. 2). These species should be ex- archipelago. pected to occur also in the archipelago. For each species listed in Tabs I and 2, the Abbreviations used for Swedish provinces (from south to north): SK=SkAne, BL=Blekinge, HA=Halland, principal nesting habit is stated. However, the SM=SmAIand, OL=Oland, GO=Gotland. GS=Gotska choice of nest substrate is not always constant for Sandrin, oc=Ostergdtland, .VG=Viistergitland, BO= a certain species. For example, some usually Bohusliin, DS=Dalsland, NA=Niirke, SO=S0derman- ground-nesting species may occasionally nest in land, UP=Uppland, VS=Viistmanland, VR=Viirmland, hollow plant stems, decayed wood or empty snail DR=Dalama. .GA=Giistrikland. HS=Hiilsin&land.. ME= Medelpad. HA=Hiirjedalen. JA=Jiimtland, AI)=Alger- shells. This is true for species like Agenioideus manland, VB=Viisterbotten, NB=Norrbotten. AS=Asele cinctellus, Anoplius c'onc'innus and A. nig,errimus lappmark, LY=Lycksele lappmark, PI=Pite lappmark, (Oehlke & Wolf 1987). Moreover, some chrysidid LU=Lule lappmark, TO=Tome lappmark. The provin- cial division of Sweden is shown on a map in each species that usually parasitize on wood-nesting volume of the series Fauna Entomologica Scandinavica. aculeates may sometimes choose a ground living wasp as host. In Sweden, the SE provinces have the richest Results wasp and bee fauna (e.g. Wolf 1969, Erlandsson During the period 1982-1990, 1020 specimens 1971, Lomholdt 1974, 1976, Erlandsson et al. were collected in the Ridti archipelaeo. In addi- 1988, Svensson et al. 1990, Janzon etal. 1991). tion, Nor6n (1988) collected I l9 specimens of the In Tab. 3, the eumenid, sphecid and megachilid presently studied families. These are included in faunas of the Ridri archipelago have been com- Tab. l. The total number of species collected in pared to those of the 10 south-eastem provinces. the archipelago is 150 (Tab. l). In addition, 72 Especially of the wood-nesting species, Tab. 3 species have been found within 200 m from the clearly shows that the Rid<j archipelago holds an shore of the nearby rnainland area Solbacken exceptionally rich fauna.