Ann. Zool. Fennici 62: 1–11 eISSN 1797-2450 Helsinki 9 January 2025 © Finnish Zoological and Botanical Publishing Board Population dynamics of the social wasps, Vespula vulgaris and Dolichovespula media (Vespidae), in Finland Atte Komonen1,*, Jouni Sorvari2 & Jyrki Torniainen3 1) Department of Biological and Environmental Science, P.O. Box 35, FI-40014 University of Jyväskylä, Finland (*corresponding author’s e-mail: atte.komonen@jyu.fi) 2) Natural Resources Institute Finland (Luke), Latokartanonkaari 9, FI-00790 Helsinki, Finland 3) Open Science Centre, P.O. Box 35, FI-40014 University of Jyväskylä, Finland Received 14 Oct. 2024, final version received 5 Dec. 2024, accepted 5 Dec. 2024 Komonen, A., Sorvari, J. & Torniainen, J. 2025: Population dynamics of the social wasps, Vespula vulgaris and Dolichovespula media (Vespidae), in Finland. — Ann. Zool. Fennici 62: 1–11. Global change affects species in many ways. To understand spatio-temporal changes in population dynamics, monitoring data are needed. Social wasps (Vespidae) are ubiquitous in many environments. Using beer traps, we studied annual and seasonal population dynamics of workers of two social wasp species, Vespula vulgaris and Dolichovespula media, in south-central Finland over six years. The abundance of both species fluctuated annually. That of Vespula vulgaris displayed some cyclicity, unlike that of D. media; fluctuations were not fully synchronous among locations. Also, sea- sonal fluctuations were spatially and temporally variable. Vespula vulgaris was active later in the season, had longer flight period, and more abundance peaks than D. media, which generally had one short abundance peak. Our study suggests great spatio- temporal variation and species-specificity in the population dynamics of social wasps, which should be considered in designing monitoring protocols. Introduction Global change affects species distribution and abundance, and social wasps (Vespinae) are no exception. Although social wasps are widespread in anthropogenic environments, can reach high local densities, have invaded new regions, and are often considered a nuisance (Masciocchi et al. 2016, Sumner et al. 2019, Santaoja et al. 2023), their population fluctuations are still poorly documented (see Archer 2012). Many extant studies were not species specific, did not use systematic sampling, focused only on a few species or were geographically limited (Akre & Reed 1981, Archer 2012). Knowledge about spatio-temporal variation in population dynam- ics is important for understanding the impacts of environmental change and developing monitor- ing protocols. The annual abundance of social wasps fluc- tuates greatly (Archer 2012). Although there is some indication of cyclic fluctuations, especially over short time periods, the cycle length and amplitude vary in time and space (Akre & Reed 1981, Pawlikowski & Pawlikowski 2006, Sorvari 2013, Masciocchi et al. 2016, Lester et al. 2017). Annual population fluctuations can be driven by extrinsic environmental factors (e.g. weather) and/or intrinsic density dependence; the latter is generally the prerequisite for cyclic fluctuations. 2 Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media • ANN. ZOOL. FENNICI Vol. 62 In social wasps, winter weather affects mortal- ity of overwintering queens, whereas spring and early summer weather affects the number and size of successful colonies (Kasper 2004, Archer 2012, Masciocchi et al. 2016, Sorvari 2018). For social wasps, the most important weather factor is probably temperature. Although individual wasps are rather tolerant to low temperatures (Heinrich 1984, Coelho & Ross 1996, Kasper et al. 2008), cool spring and summer can cause colony failures or slow down colony growth and hence affect the number of individuals. Of the intrinsic density-dependent factors, competition for nest sites and food, as well as parasites and pathogens, are possible reasons for (cyclic) fluctuations. Because social vespine wasps are generalist predators (Torniainen & Komonen 2021), competition for food is less likely a regulating factor than competition for nest sites (see Archer 2012). Parasitoids and predators, such as Lepidoptera, Diptera, Hymenoptera and Coleoptera, are common in nests but are rarely so abundant that they could be the primary cause of fluctuations (Donovan 1991, Gambino 1995, Komonen 2022, Komonen 2023). Little is known about microbes, fungi and viruses in wasps’ nests (but see Rose et al. 1999). As suggested by Paw- likowski and Pawlikowski (2006), it seems likely that there is some density-dependent factor(s) that cause inherent cyclicity, but weather eventually determines if the abundance peak is realized or not; favourable weather can also compensate for inherently low-abundance years. The abundance of social wasps varies also seasonally. A new colony is established in spring by an overwintering, mated queen (Archer 2012). After the first workers emerge, the queen remains in the nest and focuses on egg laying. Colonies are annual and grow larger until the late summer or autumn collapse (Archer 2012, Masciocchi et al. 2016). In the northern temperate and boreal zones, there are evidence that colony growth is affected by weather (Akre & Reed 1981, Paw- likowski & Pawlikowski 2006, Sorvari 2018; but see Fox-Wilson 1946). Just before the collapse, colonies produce sexual castes, which mate in autumn; only the queens overwinter. In Finland, the number of workers reach their peak in late July to mid-September, depending on the species and year (Pekkarinen & Huldén 1995); especially those species, which are readily beer-trapped, typically peak in August (Sorvari 2013, but see Komonen et al. 2020). In Finland, there are 12 species of social ves- pine wasps (Douwes et al. 2012). Most species have a nation-wide distribution, whereas Vespula germanica and Vespa crabro have a southern dis- tribution. Probably due to climate change, both species are extending their ranges (Teräs et al. 2003, Sorvari 2013, 2018, Komonen et al. 2020) which may also affect their dynamics calling for regular monitoring. Our goal was to study the annual and seasonal population dynamics of social wasps (Vespinae) in south-central Finland using a standardized sampling with beer traps. Annual wasp dynam- ics are poorly known in the boreal zone, and seasonal dynamics have rarely been studied over many years. We asked (1) how much the worker populations fluctuate annually and are the fluc- tuations cyclic; (2) are the annual fluctuations spatially consistent; and (3) are the seasonal fluctuations spatially and annually consistent. We focus on the two most common species, Vespula vulgaris and Dolichovespula media. Material and methods Study sites The study was conducted in south-central Fin- land. The main study site (Jyväskylä) was sam- pled for six years (2019–2024). To evaluate generality of the wasp dynamics, less intensive trapping was conducted in Hyytiälä and Lammi for four years (2019–2022) and Kuopio for five years (2017–2021) (Appendix 1); the data from Turku (2008–2014) were largely published in Sorvari (2018) so we used it only when compar- ing wasp density and occupancy, as well as occu- pancy dynamics among sites. The southernmost site Turku is 390 km SW from the northernmost site Kuopio. Traps (see Sorvari 2013: fig. 2) were hung in trees or bushes 1.5 to 2 m above- ground. Each trap was filled with 2 dl of beer and a hint of brown sugar and dry yeast (see Dvorak 2007, Sorvari 2013). The mean daily tempera- ture and precipitation from April to August near the main study site are listed in Appendix 2. Indi- ANN. ZOOL. FENNICI Vol. 62 • Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media 3 viduals were identified to species using Douwes et al. (2012). Trapping At the main study site, we studied worker wasp dynamics during six years. Traps were kept in the same locations except for two traps which had to be moved 80 m and 100 m, respectively, in 2022. Traps were set for somewhat variable periods during the peak flight season of worker wasps from July to September (Appendix 1). Annual fluctuations were studied with 28 traps, of which 22 were operating for 11 days from mid-July and 11 days from mid-August onwards, and 6 traps were operating continuously and those were emp- tied and refilled once a week. In case of the con- tinuously operating traps, we used the data from a 14-day periods in mid-July and in mid-August to match the sampling period of the other traps; this was also done in Hyytiälä and Lammi, where only continuously operating traps (n = 3 in each) were used. In Kuopio and Turku, 20 traps were operating for a week, starting from mid-August (Appendix 1). Seasonal fluctuations were studied with continuously operating traps, which gener- ally were used from the second week of July to late September/early October in Jyväskylä, and to mid-September at other sites. Over the six years and all the study sites, only 0.03% of the trapping periods (= time between set up and emptying of a single trap) included damaged and dried traps. If the dried traps contained no wasps (n = 14 trap- ping periods), they were excluded from the data for that trapping period; if they contained wasps (n = 5), they were included. Statistical analyses Annual fluctuations were analysed separately for V. vulgaris and D. media, using statistical tests and visual interpretation of graphs. Statistical analyses were conducted only for Jyväskylä and Kuopio, as they had sufficient data. Both sites were analysed separately due to different trap- ping years. Because of some damaged traps and variation in trapping times, wasp abundance was standardized to mean number of individuals per trap day for each trapping period (i.e. 7, 11 or 14-day period depending on the analysis). Differences in the numbers of individuals among years were analysed with generalized linear mixed models (GLMM), as implemented in IBM SPSS Statistics ver. 28. Trap (random factor) was considered a subject and Year and Month (fixed factors) repeated measures; an inter- action between Year and Month was included. The normal distribution with an identity link, and diagonal as the repeated covariance structure, were used, as it gave the smallest AICc-value and/or best model fit, as judged by residuals. We also modelled the response with the Poisson and negative binomial distributions using counts of individuals without standardization but with an offset variable controlling for the difference in trapping times. These models did not fit the data as well as did the chosen model, yet they all provided qualitatively similar results, which indi- cated that the statistical analyses were robust. For post-estimation settings, degrees of freedom were estimated using the Satterthwaite method, and the tests of fixed effects and coefficients were esti- mated using robust covariances. Estimated means were compared using pairwise contrasts, and the sequential Bonferroni was used to adjust for mul- tiple comparisons. Based on the visual inspection of the residual plots, residuals were adequately normally distributed and homoscedastic. Interspecific differences in annual and sea- sonal abundance and fluctuations were analysed statistically, but also evaluated visually. Interspe- cific differences in the total number of trapped individuals each year were tested with a χ2-test (df = 1), using exact p values that were Bonfer- roni adjusted within each site. Coefficient of variation (CV) was used to quantify and com- pare annual variation in abundance at different sites and species over a comparable period in August. In Jyväskylä, CV was also used to quan- tify the magnitude of seasonal (weekly) variation for each species (11 to 16 trapping weeks each year in 2019–2024; n = 6 continuously operated traps); interspecific differences in CV were ana- lysed with a t-test (Sokal & Brauman 1980). To find out if the weekly abundances of V. vulgaris and D. media fluctuated in synchrony, we cor- related the first differences in the six-year time series; the first difference being the change in 4 Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media • ANN. ZOOL. FENNICI Vol. 62 value from a point in the time series to the next point. Significant correlation would indicate that the weekly abundances covary. Results At the main study site (Jyväskylä), the mean number of V. vulgaris individuals per trap day varied among years (GLMM: F 11,59 = 8.71, p < 0.001; Table 1 and Appendix 3) and among trap locations (random effect Z = 2.54, p = 0.01). There was also some tendency for annual cyclic- ity (Fig. 1). The mean number of individuals did not differ consistently between July and August (contrast estimate = 0.025; Table 1 and Fig. 2). The mean number of D. media individuals per trap day varied among years, as well as between July and August (GLMM: F11,66 = 10.98, p < 0.001; Table 1 and Appendix 3) and among trap locations (Z = 2.09, p = 0.036), and there was no indication of annual cyclicity (Fig. 1). The mean number of individuals was higher in July than in August (contrast estimate = 0.046; Table 1 and Fig. 2). The total number of trapped V. vulgaris individuals was each year greater than that of D. media (χ2 > 13.9, p < 0.01; Appendix 1). Coeffi- Table 1. GLMM summary of the standardized number of V. vulgaris and D. media workers in beer traps in Jyväskylä in July and August 2019–2024. Term Vespula vulgaris Dolichovespula media F df1 df2 p F df1 df2 p Year 12.58 5 49 < 0.001 8.16 5 44 < 0.001 Month 0.45 1 17 0.510 7.46 1 68 0.008 Year × Month 6.30 5 55 < 0.001 9.70 5 59 < 0.001 Year 202420232022202120202019 A bu nd an ce 0.6 0.4 0.2 0 V. vulgaris D. media Fig. 1. Mean ± 95%CI numbers of Vespula vulgaris and Dolichovespula media workers per trap-day in Jyväskylä 2019–2024; traps (n = 28) operated each year for about two weeks from mid-July and mid- August onwards. Year 202420232022202120202019 A bu nd an ce 1.0 0.8 0.6 0.4 0.2 0 August July V. vulgaris Year 202420232022202120202019 0.6 0.4 0.2 0 August July D. media Fig. 2. Mean ± 95%CI numbers of Vespula vulgaris and Dolichovespula media workers per trap day in July and August in Jyväskylä 2019–2024. Traps (n = 28) operated for about two weeks from mid-July and mid-August onwards each year. ANN. ZOOL. FENNICI Vol. 62 • Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media 5 cients of variation across the years were 121% and 210% for V. vulgaris and D. media, respectively. For both species the mean number of individuals and trap occupancy showed rather synchronous annual fluctuations (Fig. 1 and Appendix 4). In Kuopio, the mean number of V. vulgaris individuals per trap day varied among years (GLMM: F 4,43 = 15.65, p < 0.001; Appendix 3) but not among trap locations (Z = 0.84, p = 0.40). There was also a tendency for annual cyclicity (Fig. 3). The mean number of D. media individuals per trap day varied among years but or among trap locations (GLMM: F 4,34 = 3.02, p = 0.031; Z = 1.14, p = 0.25; Appendix 3), and there was no indication of annual cyclicity (Fig. 3). The total number of trapped V. vulgaris individuals was each year larger than that of D. media (χ2 > 21.6, p < 0.01). Coefficients of vari- ation across the years were 73% and 98% for V. vulgaris and D. media, respectively. For both species the mean number of individuals and trap occupancy showed rather synchronous annual fluctuations (Fig. 3 and Appendix 4). The patterns at other sites were partly similar. In Turku, the total number of trapped V. vulgaris individuals was each year greater than that of D. media (χ2 > 11.8, p < 0.01; Appendix 3), except in 2017 and 2013, respectively, when no dif- ference was observed. Coefficients of variation across the years were 97% and 82% for V. vul- garis and D. media, respectively. For both species the mean number of individuals and trap occu- pancy showed rather synchronous annual fluc- tuations (Appendix 4; Sorvari 2018). In Hyytiälä, D. media was more abundant in 2021 and 2022 (χ2 > 19.8, p < 0.01; Appendix 3) but no differ- ence was observed in 2019 and 2020 (Fig. 4). In Lammi, V. vulgaris was more abundant in 2019 and 2022 (χ2 > 8.6, p < 0.05; Appendix 3), D. media in 2021 (χ2 = 14.8, p = 0.004) and no dif- ference was found in 2020 (Fig. 4). Compared with those in Jyväskylä, fluctuations in Kuopio were partly asynchronous, and those in Lammi and Hyytiälä partly synchronous; in Turku, trap- ping years did not overlap with those at the other sites. Seasonal dynamics were variable. In Jyväskylä, the magnitude of variation (measured as CV) in the number of individuals was similar for V. vulgaris and D. media each year (t = 0.16– 1.13, df = 20–40, p > 0.10). Weekly abundance fluctuations of the two species were not corre- lated (r between the first differences = 0.04, n = 83, p = 0.7; Appendix 5). Dolichovespula media had a single peak, generally during the weeks 30 and 31, whereas V. vulgaris either lacked a clear, single peak or had a few peaks (Fig. 5), and also flew later in the season and had a longer flight period. Seasonal dynamics in Hyytiälä and Lammi were highly variable (Appendix 6), but because there were only three traps at these sites, the pattern was sensitive to random fluctuations. The abundance peak of V. vulgaris, and especially that of D. media, was about two to three weeks earlier in 2021 than in the other years. In 2021, the mean daily temperature in Jyväskylä in June and July was about 1 and 2 °C higher, respectively, than in the next warm- est year, and the average daily precipitation was below the six-year average during both months (Appendix 2). Discussion The abundances of V. vulgaris and D. media varied greatly among sites and years, as well as within season. This is a general phenomenon and has been shown with different methods, such as observing foraging adults and count- ing nests, as well as using malaise, suction and bait traps (Akre & Reed 1981, Pawlikowski Year 20212020201920182017 A bu nd an ce 1.5 1.0 0.5 0 V. vulgaris D. media Fig. 3. Mean ± 95%CI numbers of Vespula vulgaris and Dolichovespula media workers per trap day in Kuopio 2017–2021; traps (n = 20) operated for one week from mid-August onwards each year. 6 Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media • ANN. ZOOL. FENNICI Vol. 62 & Pawlikowski 2006, Sorvari 2013, Mascioc- chi et al. 2016, Lester et al. 2017). At the two northernmost sites, Jyväskylä and Kuopio, and the southernmost site Turku, V. vulgaris was clearly more numerous than D. media, whereas in Hyytiälä and Lammi there was more variation annually. Low abundance of D. media in Kuopio and Turku can be partly explained by the fact that traps operated in August, which is generally after the peak flight time of the species, as shown here. Magnitude of population fluctuations of both species varied among locations, but there was no consistent interspecific difference. While some annual fluctuation cyclicity was observed in Vespula vulgaris, this was not the case in D. media. Annually cyclic fluctuations have been documented for Vespula species in Europe (Archer 2001, Sorvari 2013, 2018), yet the amplitude of the cycles tends to vary. For example, in southern Finland the abundance of V. vulgaris and V. germanica fluctuates annually, but occasionally there are years of extreme abun- dances (Sorvari 2018, this study). Our results also support the lack of annual cyclicity in D. media in Finland and, interestingly, two bad years were followed by three good years exactly as in Sorvari (2013). Whether this is a coinci- dence or indicates more complex dynamics than in V. vulgaris, requires further long-term studies. The reasons for different dynamics of D. media A bu nd an ce 0.4 0.2 0 Year 2022202120202019 A bu nd an ce 0.8 0.6 0.4 0.2 0 0.4 0.2 0 0.8 0.6 0.4 0.2 0 Year 2022202120202019 V. vulgaris Hyytiälä Lammi V. vulgaris D. media D. media Fig. 4. Mean ± SE numbers of Vespula vulgaris and Dolichovespula media workers per trap day in Hyytiälä and Lammi 2019–2022 (three traps ran for 56 days from mid-July onwards each year in both sites). Weeks 4039383736353433323130292827262524 A bu nd an ce 2.0 1.5 1.0 0.5 0 2024 2023 2022 2021 2020 2019 V. vulgaris Weeks 4039383736353433323130292827262524 1.5 1.0 0.5 0 D. media Fig. 5. Mean numbers of Vespula vulgaris and Dolichovespula media workers per trap day (n = 6 traps) in Jyväskylä 2019–2024. ANN. ZOOL. FENNICI Vol. 62 • Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media 7 and V. vulgaris (or more generally of Dolicho- vespula and Vespula) are not known but the occasional mass migrations of D. media queens can play some role (Mikkola et al. 2007, Sorvari 2013). Both genera have different parasitoids (Komonen 2022, Komonen 2023) and vertebrate predators (Archer 2012), which could cause dif- ferences in cyclicity (pathogens of vespines are poorly known). The challenge for researchers is that detecting population regulation with a high probability requires decades of data (Woiwod & Hanski 1992). Seasonal dynamics were predictable overall: the number of workers increased from spring and reached peak abundance in late summer. Generally, worker abundance started to increase around the last week of July but the timing of the abundance peak varied annually. Over the six years, the abundance of V. vulgaris did not differ between mid-July and mid-August, whereas D. media was slightly more abundant in mid-July. Yet, in any given year, both species can reach the peak abundance in July or August, most likely depending on the spring and summer weather (Akre & Reed 1981, Pawlikowski & Pawlikowski 2006, Sorvari 2018; but see Fox- Wilson 1946). There was some difference in the timing of abundance peaks between V. vulgaris and D. media in Jyväskylä: the former had more variation in the timing than the latter, and the weekly abundances did not covary. Some of the variation in the number and timing of abun- dance peaks could be explained by weather. For example, in the year 2021, the abundances of both species were exceptionally high, and the timing of the peak abundance was up to three weeks earlier than in the other years. High wasp abundances and early peaks could be explained by the warm and dry summer of 2021, especially in June and July. However, also the year 2024 was very warm from May to August, yet wasp catches were poor. Thus, the causal relationship between weather and wasp abundance is not straightforward, and its reliable documentation would require much longer time series than the present six years (see Woiwod & Hanski 1992). Large variation in wasp abundance among trap locations is expected, since social wasps live in colonies; thus, traps that are near nests can attract considerable number of individuals. Conspecific attraction and learning can contrib- ute to the clumped distribution of workers in food resources and traps (Reid et al. 1995, Lozada & D’Adamo 2011). Foraging distances are generally some hundreds of meters around the nest but vary depending on wasp species, location and availability of food sources (Archer 2012). Because beer traps are generally lethal thus preventing wasps from returning to nest, wasps are unlike to share information about the food source (see Santoro et al. 2015). Thus, lethal traps could give a more accurate picture of the abundance of certain wasp species in an area than non-lethal methods, unless individuals are marked. It may also be useful to accompany any trapping scheme with direct, systematic observation of foraging wasps, but the relation- ship between trap catches and direct observa- tions should be studied. Although only three traps operated in Hyytiälä and Lammi, yet their catches roughly reflected the abundance of V. vulgaris and D. media in Jyväskylä, which was extensively sampled. Thus, although the more traps the better, smaller number of traps can be partly compensated with longer trapping periods. Longer sampling periods are also encouraged due to the intra- and interspecific variation in the seasonal peak abundance. Our study shows that different social ves- pine wasp species have different annual and seasonal dynamics. These differences should be considered in developing monitoring schemes for research or environmental change. Given the high variability in wasp abundance, even among nearby traps, spatio-temporally extensive trap- ping is needed to get reliable species-specific national or regional data over years. Despite its limitations, long-term standardized monitoring of social vespine wasps is easy to conduct and increases our knowledge about insect population fluctuations. Acknowledgements This study was facilitated by the nationwide sampling scheme jointly organized by the Finnish research stations (RESTAT; www.researchstations.fi). We thank Silja Vuoren- maa and Antti Uotila at Hyytiälä Forestry Field Station and Esa-Pekka Tuominen, Joni Uusitalo, Jussi Vilen and Janne Sundell at Lammi Biological station. We also thank Juraj 8 Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media • ANN. ZOOL. FENNICI Vol. 62 Švajda, Anna Lähde, Aleksi Nirhamo, Salla Pitkänen, Tatu Koponen, Joona Hirvensalo and Lauri Viitanen for their help in the field. The study was funded by the Kone Foundation (grant 201800577). References Akre, R. D. & Reed, H. C. 1981: Population cycles of yellowjackets in the Pacific Northeast (Hymenoptera: Vespinae). — Environmental Entomology 10: 267–274. Archer, M. E. 2001: Changes in abundance of Vespula germanica and V. vulgaris in England. — Ecological Entomology 26: 1–7. Archer, M. 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Trapping period and effort by site and year, and the number of trapped wasp individuals as well as that of the two most numerous species. Sites are ordered from the south to the north. Site and year Trapping period Traps Cumulative Total Vespula Dolichovespula trapping number vulgaris media days of wasps Turku 2008 13–20 Aug. 20 140 505 208 0 Turku 2009 1–8 Aug. 20 140 65 45 0 Turku 2010 30 July–6 Aug. 20 140 602 429 39 Turku 2011 4–11 Aug. 20 140 119 73 37 Turku 2012 7–14 Aug. 20 140 422 189 50 Turku 2013 5–12 Aug. 20 140 41 20 19 Turku 2014 8–15 Aug. 20 140 922 605 20 Lammi 2019 16 May–26 Sep. 3 405 93 59 30 Lammi 2020 6 July–14 Sep. 3 213 55 18 36 Lammi 2021 15 July–16 Sep. 3 192 154 54 102 Lammi 2022 11 July–12 Sep. 3 192 136 84 50 Hyytiälä 2019 1 July–2 Sep. 3 192 56 24 23 Hyytiälä 2020 6 July–14 Sep. 3 213 25 7 13 Hyytiälä 2021 12 July–13 Sep. 3 192 85 22 63 Hyytiälä 2022 11 July–12 Sep. 3 192 75 7 66 Jyväskylä A 2019 13 July–30 Sep. 6 846 250 223 16 Jyväskylä A 2020 11 May–4 Oct. (7 Oct.) 6 891 237 188 37 Jyväskylä A 2021 13 May (17 May)–30 Sep. 6 815 377 229 129 Jyväskylä A 2022 11 July–2 Oct. (3 Oct.) 6 507 364 154 210 Jyväskylä A 2023 12 July (13 July)-31 Aug. (4 Oct.) 6 405 322 162 150 Jyväskylä A 2024 12 July–27 Sep. 6 462 97 67 25 Jyväskylä B 2019 15–26 July & 12–23 Aug. 22 517 226 196 16 Jyväskylä B 2020 13–24 July & 13–24 Aug. 22 473 71 61 6 Jyväskylä B 2021 12–23 July & 9–20 Aug. 22 528 366 247 103 Jyväskylä B 2022 11–22 July & 8–19 Aug. 22 484 123 43 75 Jyväskylä B 2023 13–24 July & 11–22 Aug. 22 506 167 114 45 Jyväskylä B 2024 12–23 July & 12–23 Aug. 22 528 140 115 16 Kuopio 2017 16–23 Aug. 20 140 11 6 3 Kuopio 2018 6–13 Aug. 20 140 129 106 23 Kuopio 2019 12–19 Aug. 20 140 63 57 6 Kuopio 2020 9–16 Aug. 20 140 134 131 1 Kuopio 2021 9–16 Aug. 20 140 62 48 12 10 Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media • ANN. ZOOL. FENNICI Vol. 62 Appendix 2. Spring and summer weather (mean daily temperature and precipitation) at the Jyväskylä airport during the trapping years 2019–2024 (data: Finnish Meteorological Institute, www.ilmatieteenlaitos.fi; the weather station is located 17 km north of the study area). Temperature (oC) Precipitation (mm) 2019 2020 2021 2022 2023 2024 2019 2020 2021 2022 2023 2024 April 4.7 1.7 2.8 1.5 2.5 0.9 1.8 0.8 1.7 1.7 0.9 1.1 May 8.8 7.6 9.2 9.0 10.0 12.7 2.7 0.6 2.8 1.1 1.1 0.5 June 16.1 17.9 18.6 15.6 15.4 16.5 1.3 1.9 1.5 2.3 2.5 2.1 July 15.7 15.1 19.6 16.8 15.9 17.8 0.3 2.7 1.5 3.9 5.3 2.3 August 14.6 14.8 14.0 16.8 16.0 16.2 2.2 0.6 4.8 1.8 3.6 2.5 Appendix 3. Spatio-temporal variation in wasp abundance in roughly equal periods in mid-August (in Jyväskylä also mid-July). Given are the standardized mean number of individuals (density) per trap day (min–max) and per- centage of occupied traps (min–max) for Vespula vulgaris and Dolichovespula media workers in beer traps. Occu- pancy is not shown for Hyytiälä and Lammi due to small number of traps. Site Years Density Occupancy (%) V. vulgaris D. media V. vulgaris D. media Jyväskylä July 2019–2024 0.29 (0.11–0.72) 0.15 (0.02–0.46) 76 (64–96) 53 (12–86) Jyväskylä August 2019–2024 0.26 (0.11–0.39) 0.11 (0.01–0.33) 69 (38–86) 38 (11–82) Kuopio 2017–2021 0.51 (0.04–0.98) 0.06 (0.01–0.16) 69 (25–95) 21 (5–30) Turku 2008–2014 1.60 (0.14–4.32) 0.17 (0–0.36) 90 (50–100) 41 (0–75) Lammi 2019–2022 0.23 (0–0.40) 0.14 (0.05–0.38) na na Hyytiälä 2019–2022 0.07 (0–0.17) 0.23 (0–0.64) na na Tr ap o cc up an cy (% ) 100 80 60 40 20 0 100 80 60 40 20 0 Year 20 24 20 23 20 22 20 21 20 20 20 19 20 18 20 17 20 14 20 13 20 12 20 11 20 10 20 09 20 08 Year 20 24 20 23 20 22 20 21 20 20 20 19 20 18 20 17 20 14 20 13 20 12 20 11 20 10 20 09 20 08 TurkuKuopioJyväskylä JulyJyväskylä August A B Appendix 4. Percentage of traps occupied by (A) Vespula vulgaris and (B) Dolichovespula media workers in Jyväskylä (mid-July and mid-August 2019–2024; n = 28 traps), Kuopio (mid-August 2017–2021; n = 20 traps) and Turku (early to mid-August 2008–2014; n = 20 traps). ANN. ZOOL. FENNICI Vol. 62 • Komonen et al.: Population dynamics of Vespula vulgaris and Dolichovespula media 11 Appendix 6. Mean numbers of Vespula vulgaris and Dolichovespula media workers per trap day (n = 3 traps) in Hyytiälä and Lammi 2019–2022. 37343128373431283734312836333027 Week 243835322926393633302724 A bu nd an ce 2.0 2019 2020 2021 2022 2023 2024 1.5 1.0 0.5 0 V. vulgarisD. media Week 383736353433323130292827262524 A bu nd an ce 1.5 1.0 0.5 0 Lammi 2022 Lammi 2021 Lammi 2020 Lammi 2019 Hyytiälä 2022 Hyytiälä 2021 Hyytiälä 2020 Hyytiälä 2019 V. vulgaris Week 383736353433323130292827262524 2.0 1.5 1.0 0.5 0 D. media Appendix 5. Mean numbers of Vespula vulgaris and Dolichovespula media workers per trap day (n = 6 traps) in Jyväskylä in 2019–2024. Vertical lines indicate the first trapping week in each year. This figure is based on the same data as Fig. 5 but trapping periods arranged continuously to allow for easier interspecific comparison.