© Entomologica Fennica. 5 December 1995 Seasonal occurrence of arthropods as a source of food for birds in Finnish Lapland Simo Veistola, Tapio Eeva & Esa Lehikoinen Veistola, S., Eeva, T. & Lehikoinen, E. 1995: Seasonal occurrence of arthro pods as a source of food for birds in Finnish Lapland. - Entomol. Fennica 6:177-181. The seasonal occurrence in arthropod abundance was studied in Utsjoki, northernmost Finland, from the viewpoint that arthropods form an important food resource for birds. On ground surfaces, Araneae was the most abundant taxon. The abundance peak of spiders occurred in June. Diptera, which was the dominant taxon in samples collected from dwarf shrubs, had their domi nance peak in July. In birch foliage, Homoptera and Coleoptera were most common in June, but lepidopteran and symphytan larvae dominated in late summer. The larvae in pines (Symphyta; Diprionidae and Pamphiliidae) also had their abundance peak in autumn. The role of predation in the seasonality is discussed from the point-of-view of both birds and arthropods. Sima Veistola, Tapia Eeva & Esa Lehikoinen, Laboratory of Ecological Zoology, Department of Biology, University of Turku, FIN-20500 Turku, Finland 1. Introduction 2. Material and methods The annual fluctuation of arthropods may have The study area lies at the northern forest limit strong effects on their host plants, prey and preda dominated by mountain birch (Betula pubescens tors (e.g. Krebs 1985). To understand the rela ssp. tortuosa) and Scots pine (Pinus sylvestris) tionships between organisms, we need studies of in the valleys of the Kevojoki and Utsjoki rivers. the fluctuation of arthropod groups, not only the Details of the vegetation of the study area have fluctuation of some species (Wolda 1988). We been presented by Kallio et al. (1969). are interested in the seasonal variation of arthro The arthropod abundance on the ground and pod density, as they constitute the main food of field layers and in the foliage of dwmf shrubs insectivorous birds. In this paper, we present and trees was studied using pitfall traps, sweep findings on seasonal variation in the abundance netting, the tree-knocking method and frass-col of arthropods grouped in a way which might be lectors. The study sites were situated near the relevant to the foraging habits of birds. Our re Kevo Subarctic Research Institute (69.45'N, sults show the general trends of how the abun 2TOl'E), but three tree-knocking sites were 10- dance of herbivorous and predatory arthropods 20 km away from the station. The detailed loca fluctuates seasonally in the northern forests. tions of the study sites have been presented by Against this background, we briefly discuss the Eeva (1989). consequences of seasonal variation for both in We had eight pitfall traps (diameter 4.2 em) teracting sides. 178 Veistola et al.: Arthropods as food for birds • ENTOMOL. FENNICA Vol. 6 50 50 Birch forest 1 989 Pine forest 1 989 40 40 ->- ~ Other groups n1 "C 30 Q Araneae 30 c. --... n1 20 20 C) E 10 10 I I I I I I I 0 0 I I I I I I 50 50 Birch forest 1 990 Pine forest 1 990 40 40 ->- n1 "C 30 ' 30 --.c... n1 20 20 C) E 10 10 0 0 25.5. 15.6. 5.7. 25.7. 14.8. 25.5. 15.6. 5.7. 25.7. 14.8. date date Fig. 1. Seasonal variation of arthropod biomass in the ground layer (fresh weight) according to pitfall trap samples in birch and pine forests in 1989 and 1990. in a line in each of two birch forests and two pine birches (average height 5.7 m). We present com forests during the years 1989-1990. The traps were bined frass fall data from the years 1986--1991 and emptied once a week and the biomass of arthropods 1993 in this study. Data were collected at Kevo (wet weight) measured. In 1987-89 two sweep from two sites during all seven years. Collectors netting lines (50 hits/line) were run in three birch were emptied once a week. The dry weights (to the forests to determine the abundance of arthropods nearest 0.1 mg) of the faeces were corrected for the on the dwarf shrubs. The sweep collected arthro effect of rain using Tenow & Larsson's (1987) pods from shrubs and flying insects at a height of formula for the frass of Neodiprion sertifer (Geoffroy) 0-30 em. Because of the considerable variance (Hymenoptera) as Y = 20.75(1--e-M91x), where Y is among observers, annual sweep-netting data are the percentage of dissolved faeces and X is the given in percentage of the annual biomass peak. amount of precipitation (in mrn). The seasonal Arthropods living in foliage were collected us fluctuation of arthropod abundance is presented ing the tree-knocking method at four study sites in in 10-day periods (20 May-29 May, and so on). the Utsjoki valley (Eeva 1989). The study birches were 4-6 m high. A plastic blanket (3 x 3 m) was spread under the tree, and arthropods were col 3. Results lected from the blanket after knocking the trunk once. We knocked 10--35 trees per study site during In pitfall samples, Araneae was the most com the summer. Only one sample/tree/summer was mon taxon (89.6% of wet weight). The vast ma taken. jority of the spiders trapped belonged to the The abundance of foliage-feeding larvae was Lycosidae family (cf. Koponen 1975). In both studied by measuring their faeces production with forest types spiders were most abundant in June. frass-collectors (area 0.5 m2 We had 8-10 collec Coleoptera made up the major proportion of the ). tors under pines (average tree height 9.0 m) and group 'Others' (8.2% of wet weight). ENTOMOL. FENNICA Vol. 6 • Veistola et al.: Arthropods as food for birds 179 100 1987 120 1987 Cll Q lr:t:l: 75 -Cll 90 "rC::: C...l.l .:c:I .._ 60 ltl 50 ECl .C::l:l 30 iii 25 Q....). 0 15.6. 5.7. 25.7. 14.8. 0 1881 Dlptera Q Other groups 100 200 1988 • Araneae 1988 CQ ll r~n Coleoptera lr:t:l: 75 Larvae 150 "C Cll = ..:rc::.:I: 50 .~._ 100 == HLeypm. .l alarvrvaaee ltl CEl Coleoptera Q>) - Homoptera 50 == Others ltl 25 Araneae Q....). 0 0 15.6. 5.7. 25.7. date Fig. 3. Seasonal variation in biomass of selected ar- thropod taxa (fresh weight) on the foliage of birches 100 measured using the tree-knocking method. ~ 1989 c ltl 75 "C c abundant in July, but the peaks were caused by ::I ~ 50 the superabundant small Diptera (Simuliidae, .Q:::) Culicidae). The peak in the abundance of iii 25 Coleoptera in sweep-netting samples occurred in Q....). the beginning of July, as it did also in the pitfall 0 trap samples (Figs. 1, 2). Araneae (mainly 5.6. 25.6. 15.7. 4.8. 24.8. Linyphiidae and Thomisidae) seemed to be most date abundant in late summer on dwarf shrubs. Fig. 2. Abundance (fresh weight) of arthropods col Coleoptera (especially Chrysomelidae) was lected by sweep netting expressed as percentage of the dominant group on the foliage of birches in the largest sample in each year in 1987-1989. This the beginning of the summer (Fig. 3). In addi scaling was used because of the change of observers tion, Homoptera (Psyllodea and Aphidodea) were in each year. Larvae = Lepidoptera (16%) and Hy abundant in June 1988. The abundance of Araneae menoptera (84%) larvae. The group 'Others' includes Lepidoptera (ad.), Hymenoptera (ad.), Homoptera, was constant throughout the summer on the foli Heteroptera, Odonata, Ephemeroptera and Tricho age of birches. The biomass of larvae increased ptera. from June to August a4nost linearly (Fig. 4). In Diptera dominated the sweep-net samples (on June, only hymenopteran larvae were common. average 32.3% of biomass, Fig. 2). The abun The most abundant sawfly larvae were Dineura dance of diptera increased from June to July, virididorsata Retzius, but Amauronematus sp. especially in 1987 and 1989. Variation in the and Nematus sp. were also common. In August, size distribution of Diptera was considerable. lepidopteran larvae were also more abundant (the Large and visible craneflies (Tipulidae) were most common genera were geometrids Lycia sp. 180 Veistola et al.: Arthropods as food for birds • ENTOMOL. FENNICA Vol. 6 13 __..._ larvae on pine · • 0 · ·· · larvae on birch Fig. 4. Seasonal variation of frassfall (mean and standard error in ten day 3 periods) on birches and pines. Data from 1986- 1991 and 1993 (n = 7 5.6. 15.6. 25.6. 57. 15 7 25 7. 4.6. 14.B 24.6 years). Date and Archiearis sp.). At the end of the summer, the abundance of arthropods in different parts of larvae made up more than 50% of the arthropod the forest has considerable effects on the northern biomass in the tree-knocking samples (Fig. 3). ecosystems. Wolda (1988) stated that temperature On pines, larvae were rare up to the middle of is the most important factor determining the sea July, but their abundance increased at the end of the sonal variation of insects. Most arthropod taxa, summer (Fig. 4). However, they were less abun therefore, have their peak occurrence quite late at dant than the larvae on birches. The only frass northern latitudes. Furthermore, there are only a producers on pines were found to belong to the few groups which overwinter in stages in which generaNeodiprion sp. (Symphyta: Diprionidae) and they become available for predators in early sum Acantholyda sp. (Symphyta: Pamphiliidae). mer. Vertebrate predators preying upon arthropods, such as insectivorous birds, are therefore food-lim ited in the early summer. By contrast, the arthropod 4. Discussion prey have their peak abundance in late summer, and thereby gain a partial refuge at that time. This Araneae, which is the most important predatory arises from the fact that predator birds are not able arthropod taxon in our study area, was most abun to fully utilize the highest peak because of the food dant in the beginning of the summer on the ground limitation earlier in the breeding season. The mini layer (Koponen 1975 and this study). On the mum time required for fulfilling the breeding sched foliage of trees, the abundance of spiders did not ule of insectivorous passerines, c. 80 days, contrib vary noticeably during the summertime. On the utes significantly to the above pattern. dwarf shrubs, the total peak in arthropod abun Acknowledgements. We thank Lasse Iso-Iivari, Sinikka dance occurred at the end of July every summer. Hanhimaki, Erkki Haukioja, Vesa Koivunen, Martti This was especially due to the late peak abun Komulainen, Seppo Neuvonen, Elina Pallas, Marja Tiirvi, dance of Diptera. and the staff of Research Institute for their help in various On the foliage of birches, Homoptera had their aspects of this work. David Bergen checked the language. peak in June (also Hauldoja & Koponen 1975). In The study was financially supported by the Emil Aaltonen Foundation and the Turku University Foundation. addition, Coleoptera had their peak in the begin ning of the summer (c f. Hauldoja & Koponen 1975). All in all, the total biomass peak occurs in late References summer, because the larvae of many groups did not increase in numbers before mid-July (Koponen Eeva, T. 1989:Elliinravinnon merkitys lapintiaisen (Parus 1973,Hauldoja&Koponen 1975,cf. Tenow 1972). cinctus) ja kirjosiepon (Ficedula hypoleuca) pesinnlin We suggest that the great seasonal variation in ajoituksessa Inarin Lapissa (in Finnish). - Unpubl. ENTOMOL. FENNICA Vol. 6 • Veistola et al.: Arthropods as food for birds 181 M. Sc. Thesis, Dept. Biology, Univ. Turku. Analysis and Synthesis, Vol. 17. Fennoscandian Tun Haukioja, E. & Koponen, S. 1975: Birch herbivores and dra Ecosystems, Part 2:66-72. herbivory at Kevo.-In Wielgolaski, F. E. (ed.): Eco Krebs, C. J. 1985: Ecology: the experimental analysis of logical studies. Analysis and Synthesis. Vol. 17. distribution and abundance. 3rd ed. - Harper, New Fennoscandian Tundra Ecosystems. Part 2:181-188. York. 800 pp. Kallio, P., Laine, U. & Makinen, Y. 1969: Vascular flora of Tenow, 0. 1972: The outbreaks of Oporinia autumnata Bkh. Inari Lapland. 1. Introduction and Lycopodiaceae and Operophthera spp. (Lep., Geometridae) in the Polypodiaceae. -Rep. Kevo Subarctic Res. Stat. 5:1- Scandinavian mountain chain and northern Finland 1962- 52. 1968. -ZooI . Bidrag Uppsala, Suppl. 2:1-107. Koponen, S. 1973: Herbivorous invertebrates of the moun Tenow, 0. & Larsson, S. 1987: Consumption by needle tain birch at Kevo, Finnish Lapland. - Rep. Kevo eating insects on Scots pine in relation to season and Subarctic Res. Stat. 10:20-28. stand age. - Holat·ct. Ecol. 10:249-260. 1975: Spider populations in a subalpine birch forest. Walda, H. 1988: Insect seasonality: Why?-Annu. Rev. - In Wielgolaski, F. E. (ed.): Ecological studies. Ecol. Syst. 19:1-18.