In social vespine wasps, the nest characteristics and associates vary intra- and interspecifically. I studied the nest characteristics and nest associates of three Dolichovespula wasp species in boreal Finland. The average number of combs and cells per nest was similar among D. media, D. saxonica and D. norwegica, and similar to Dolichovespula in the temperate zone. Most D. media and D. saxonica colonies produced both gynes and males, whereas D. norwegica produced mainly either one. The average colony lifespan of D. saxonica was a bit over two months, i.e. shorter than documented for Dolichovespula in the temperate zone. The proportion of nests parasitised by the ichneumonid wasp Sphecophaga vesparum was 36% in D. saxonica and 50% in D. media; S. vesparum has not been recorded from D. media before. The proportion of parasitized cells was 0.6–11% for D. saxonica and 0.2–22% for D. media. The proportion yellow, overwintering cocoons of S. vesparum was highly variable among the nests. The proportion of nests parasitised by the moth Aphomia sociella was 36% in D. saxonica and 13% in D. media. Sphecophaga and Aphomia were not recorded from D. norwegica colonies, which supports studies in the temperate zone. The study adds up to the previous research on the nest structure and nest associates of vespines, and together they indicate that there is geographical variation within Europe and globally in the host use of Sphecophaga.

Aphomia sociella, colony structure, social wasps, Sphecophaga vesparum, Vespinae

Social vespine wasps (Vespinae) are divided in four to five genera (Persson 2015; Lopez-Osorio 2017), of which three occur in Finland: Dolichovespula (Rohwer, 1916), Vespa (Linnaeus, 1758) and Vespula (Thompson, 1869). Biology of Dolichovespula, especially that of D. norwegica (Fabricius, 1781) and D. media (Retzius, 1783), is poorly known compared to Vespula, partly because of the global invasions of the latter (Beggs et al. 2011). Dolichovespula and Vespula also differ in their sociality as well as colony and nest characteristics (Foster et al. 2001; Archer 2012) but there are also differences within Dolichovespula (Archer 2006). Colony and nest characteristics are biologically interesting as such, and they are needed to interpret possible intra- and interspecific differences in the identity and prevalence of nest associates (parasites, parasitoids, predators and scavengers), which can influence the size, activity and success of vespine colonies (Archer 2012).

In Finland, cuckoo wasps Dolichovespula adulterina (Du Buysson, 1905) and D. omissa (Bischoff, 1931) parasitize D. saxonica (Fabricius, 1793) and D. sylvestris (Scopoli, 1763), respectively (Douwes et al. 2012). Cuckoo wasps do not feed on the host larvae but the queen replaces the host queen and use the host workers as slaves to produce reproductive castes. The wasp Sphecophaga vesparum (Curtis, 1828) (Hymenoptera: Ichneumonidae) is nearly a cosmopolitan ectoparasitoid of Vespinae (Archer 2012). Two subspecies are known: the European S. vesparum vesparum and the North American S. vesparum burra (Cresson, 1869). The endoparasitoid beetle Metoecus paradoxus (Linnaeus, 1761) (Coleoptera: Ripiphoridae) and related taxa are common parasites of Vespula but have rarely been observed from Dolichovespula nests (see Carl and Wagner 1982; Makino 1982). Similarly, the syrphid Volucella pellucens (Linnaeus, 1758) (Diptera: Syrphidae) parasitizes Vespula but not Dolichovespula. The moth Aphomia sociella (Linnaeus, 1758) (Lepidoptera: Pyralidae) is a common inhabitant of Dolichovespula nests (Gambino 1995). It predates eggs, larvae and cocoons, but feeds also on waste found in nests. The species generally attacks declining colonies and thus is likely to have a marginal effect on colony success (Gambino 1995; Archer 2012). The overall prevalence (proportion of occupied nests) of nest associates in vespine nests is usually high but the nest-specific prevalence (proportion of occupied cells or the number of individuals) is usually low (Archer 2012).

The nest characteristics and associates of Dolichovespula are poorly studied in comparison with Vespula, especially in the boreal region. The extant studies have been conducted in very different geographical locations and suggest geographical variation in the biology of Dolichovespula (e.g. US, Greene et al. 1976; Gambino 1995; Japan, Makino 1982; UK, Archer 2000, 2002; Poland, Pawlikowski and Pawlikowski 2010; Nadolski 2012, 2013). I studied the colony and nest characteristics, as well as the nest associates, of D. saxonica, D. media and D. norwegica in boreal Central Finland.

The study was conducted in the province of Central Finland, which belongs to the middle boreal zone. During 2019–2022 Dolichovespula nests were located based on public information and active searching. All nests were within 60 km (mostly within 5 km) from the Jyväskylä city centre. Nests were in a variety of natural and anthropogenic locations, mostly in semiurban broadleaved or mixed forests. Most D. saxonica nests were inside wooden bird nest boxes (1.3–2.0 m above ground) that were used for a bird study (Komonen and Torniainen 2022). The nests were removed and their characteristics (number of combs and cells), and the number of nest associates or parasitized cells, were recorded in the laboratory. Small supplementary combs (one D. saxonica nest) between the main combs were not counted as separate combs but they were included in the total cell counts. The number of small and large cells in each comb was also counted or estimated but because they are difficult to distinguish in combs having intermediate-sized cells (Greene et al. 1976; Archer 2006), I do not provide quantitative estimates of small-to-large cell ratios. Interspecific differences in the colony size (total number of combs and cells) were analysed with the non-parametric Kruskal-Wallis test. The sample size may vary in different analyses and data summaries because not all characteristics were recorded from every nest for varying reasons. Nest characteristics are available in the Suppl. material 1.

Lifespan was documented for seven D. saxonica colonies in bird nest boxes (n = 7), which were monitored about once a week for a bird study from April 2021 onwards. Colony lifespan was determined as the difference between the first date the wasp was observed and the date of colony removal. Because the colonies were initiated earlier than observed at the first time, I estimated the date of colony initiation: the number of days between the date a nest box was observed occupied and the previous date the nest box was empty, divided by two). At the time of removal, four colonies had just ceased and three were still active, although at their final phase with males and gynes.

To make colonies comparable they should be in the same developmental phase. This was not fully possible. Thus, in analyses I focused on the mature colonies, which were determined as having large cells containing sealed brood or large cells from which the adults had emerged (see Archer 1981). All D. media (n = 8) and D. norwegica (n = 4) nests were mature, whereas two D. saxonica (n = 31) nests had only small cells, one had only large cells and two were destroyed by A. sociella. To document, if the nests produced both males and gynes, I recorded the adults and opened the closed cells. I also examined each nest for nest associates, of which S. vesparum and A. sociella were abundant and could be identified to species. All open and closed host cells were examined for S. vesparum cocoons. I recorded the number of cocoons (one host cell may host several parasitoid cocoons), the colour of the cocoon (white or yellow), and whether the parasitoid had emerged from the cocoon (i.e. there was an exit hole; Fig. 1). Sphecophaga vesparum lays three kinds of eggs: those that produce 1) white cocoons and then brachypterous females; 2) those that produce weak, yellow cocoons and then winged females and possibly males; and 3) resistant yellow, overwintering cocoons (Donovan 1991). I classified the cocoons as white or yellow because I was not confident in separating the two types of yellow cocoons, except in one nest. Aphomia sociella larvae were counted or the past presence of the species was determined by the silk spun by the larvae.

Figure 1. 

White Sphecophaga vesparum cocoons from which the parasitoid has emerged, and closed yellow cocoons in a nest of Dolichovespula media. A host cell may have both white and yellow cocoons. Photo: Atte Komonen.

The number of combs and cells was variable among nests but there were no interspecific differences (H = 1.1, p = 0.57 and H = 0.7, p = 0.72, respectively, df = 2, n = 38; Table 1). Of the basal combs (n = 32), 88% had only small cells, and of the newest comb, 47% consisted only of small cells. The middle combs had generally large or medium cells, and if there were small cells (63% of nests), they were peripheral. The mean ratio of cell numbers in adjacent cells was similar among the species and increased from the basal (oldest) towards the outer (newest) combs (Fig. 2). The mean ± SD lifespan of D. saxonica colonies was 70 ± 4 days (n = 7).

Figure 2. 

The mean ratio (± 1SD) of the cell numbers in adjacent combs (1 is the basal, oldest comb). Numbers give the number of nests.

The most frequent and abundant nest associates were S. vesparum and A. sociella. These attacked D. saxonica and D. media, whereas no nest associates were found in D. norwegica nests. Both in D. media and D. saxonica, S. vesparum parasitized large and small cells. In D. media, there was an average of 1.5 (max. = 8) parasitoid individuals per host cell, and in D. saxonica 1.1 (max. = 4). In both species, there was generally more yellow than white S. vesparum cocoons (Table 2). In D. media colonies, the ratio of white to yellow cocoons was highly variable. The proportion of white or yellow cocoons, from which the parasitoid had emerged, was less variable. Most of the white cocoons were empty, whereas the yellow cocoons were still inhabited. In D. saxonica colonies, the ratio of white to yellow cocoons was less variable than in D. media. The proportion of white or yellow cocoons, from which the parasitoid had emerged, was more variable than in D. media; exceptionally large or small proportions were associated with small number of parasitoids. Similar to D. media, white cocoons were more often empty than yellow cocoons. No social vespine parasites were observed.

To establish any pattern in biology, studies must be replicated in space and time. This study supports some earlier studies about the biology of Dolichovespula but also suggests that some issues need more studies so that their generality can be evaluated. Particularly, the variation in the host use of S. vesparum needs more research attention, which requires that different Dolichovespula species are studied in the same regions. It is also clear that observed level of parasitism cannot cause significant year-to-year variation in Dolichovespula abundance but can occasionally cause moderate damage in individual colonies. Furthermore, more studies are needed on the other Dolichovespula than D. saxonica, especially since D. norwegica has no documented nest associates.

I thank Kone foundation for a research grant to the wasp project. I thank Lauri Viitanen and Tatu Koponen for the help in the lab, and Jyrki Torniainen for the help in the field.