European paper wasp (Polistes dominula) (Hymenoptera: Vespidae) aggregations in bee hotel nestingblocks

Published April 2023, Olger R. Krischan
(this article has been published in Hymenovaria 27)


Since 2019 the European paper wasp (Polistes dominula) is a returning visitor to our garden, attracted by the Fennel (Foeniculum vulgare) and Trumpet vine (Campsis radicans) nectar sources. Their numbers were limited to one or two females, but in season 2022 more then ten specimen were foraging simultaneously among them two males.
The continuous presence of females in and around the bee hotels nesting blocks was notable. Compared to nests locations of the species seen elsewhere a good nesting site did not seem present here but maybe I had overlooked a nest? The outsides of the nesting blocks were empty, but inside one a number of wasps sat tightly packed in the space behind stems of nesting material. However there was no nest. Armed with a light I have inspected all nesting blocks which revealed another two blocks with an aggregation of more then eight specimen. But here as well a nest was lacking, so why did these wasps gather in large numbers?

Figure 1. European paper wasp ♀︎ (Polistes dominula), gyne leaves aggregation A-2 to forage


The literature provides two reasons for P. dominula aggregation behaviour:

  1. as part of the social process to determine the next generation of queens, the pre-overwinter phase [Dapporto, L.; Palagi, E., 2006], [Dapporto et al., 2006], [Dapporto et al. 2006, [Zanette et al. 2011], [Jandt et al. 2014]
  2. as result of parasitism by Twisted-wing insects (STREPSIPTERA), for Polistes dominula specifically Xenos vesparum [Beani et al., 2007], [Beani et al., 2011], [Beani et al., 2018], [Dapporto et al., 2007], [Hughes et al., 2004]

2.1. Pre-overwinter

When the colony decline starts at the end of the season, from half August to half September [Beani et al., 2011], the potential queens, related (sister) as well as unrelated (cousin and non related) [Zanette et al., 2011], [Queller et al., 2000], will isolate themselves on shared overwinter shelters which ushers in the pre-overwinter phase of their social process [Cini et al., 2009], [Dapporto et al., 2006], [Zanette et al., 2011], [Jandt et al., 2014]. During this phase dominance is established between the females in order to decide who of them will become the next generation queens. As result of the dominance hierarchy a number of females will adopt the role of worker to provide the aggregation with food. This last group has the highest mortality rate coming out of winter [Cini et al., 2009], [Dapporto & Palagi, 2006], [Dapporto et al., 2007]. Often several pre-overwinter aggregations are located closely together [Beani et al., 2011].

2.2. Twisted-wing insects

Twisted-wing insects (STREPSIPTERA) parasitise males and females of seven insect orders among others the hymenoptera [Beani et al., 2007]. One of these parasites is Xenos vesparum (Xenidae), an obligate endoparasite of Polistes dominula [Beani et al., 2007], [Beani et al., 2018]; they develop inside the host but do not kill it. The adult female spends her entire life inside her host. Once full grown her cephalothorax, the fused head and thorax, protrudes between the abdominal plates which enables her to mate. The male develops only a set of hind wings (depictions of male and female can be found in [Kathirithamby, 2003], [Beani et al., 2011]). His pupa protrudes similarly to the female but when he emerges it will leave a hole in the wasp, who will die after sometime as a result of it [Dapporto et al., 2007]. The male usually lives less than five hours [Beani et al., 2011], [Hughes et al., 2004] and in that time frame he will need to find his partner.

X. vesparum is bivoltine with the first generation in May, and the second in June-July [Beani et al., 2018]. The very small free living mobile first instar larvae [Hughes et al., 2004], the planidium [Kathirithamby et al., 2017], is deposited by the female directly on the wasp nest, when the wasp visits the nest, or on flowers, where they are unintentionally picked up by foraging workers who transport them to the nest [Beani et al., 2018], [Dapporto et al., 2007], [Hughes et al., 2004].
Once arrived on the wasp nest the planidium will search for a victim, which can be every larval stage of the wasp, but it is not capable of penetrating the adults [Beani et al., 2018].
A parasitised wasp larva will develop with the internal parasite to adulthood. A single wasp can be a host to more than one parasite [Beani et al., 2018], [Hughes et al., 2004]. Five to ten days after emergence of the adult wasp [Beani et al., 2011] the adult parasite is visible by the deformed abdominal plates due to the protruding cephalothorax or pupa [Beani et al., 2018], [Beani et al., 2011], [Hughes et al., 2004].
Parasitised females do not carry out tasks on the nest but instead abandon it and aggregate from end June in the vicinity on plants [Beani et al., 2018], [Hughes et al., 2004]. There the first mating of the parasite will occur in June/July, followed by the second in August. After the second mating the aggregation will evolve to a pre-overwinter aggregation. Non parasitised females will join after some time [Beani et al., 2018], [Beani et al., 2011].
The fertilized female parasite(s) overwinter in their host and start the cycle again in spring [Beani et al., 2011].

The aggregation behaviour of parasitised wasps is possibly directly influence by the parasite. They form aggregations much earlier then normal in summer. The short lifespan of the male is possibly the driving factor as it facilitates finding a partner [Beani et al., 2011], [Beani et al., 2018]. The aggregations become pre-overwinter aggregations from late-summer onwards and attract non-parasitised congeners. Parasitised wasps remain longer in the aggregation after overwintering than non-parasitised wasps [Beani et al., 2011].


3.1. The aggregations

During the period of August 2022 to February 2023 the aggregations have been observed, with intensive inspections on 29 August, 23 September and 15 February.
The bee hotel nest blocks are all located on a south-east facing wall. The nest blocks containing the aggregations are more or less located next to each-other at a distance of 50 cm.
I observed a Polistes disappearing in the space between a wall and a fence pole. On this location A-4, at a distance of 350 cm of the nearest aggregation (A-1), two specimen had gathered.
In all cases the clusters consisted of females and there was no nest present.

Aggregation A-1

Figure 2. Aggregation location A-1

Aggregation A-1 was formed in a nest block consisting of a wooden frame with a number of sections filled with stems of bamboo or hemp agrimony. Behind the latter (at ±130 cm height) a cluster of 8 visible specimen sat tightly packed in a corner (fig. 2).

Figure 3. Relocated wasps from aggregation A-1

In February 5 specimen were still visible but had repositioned from behind the stems to the opening between the segments of the frame (fig. 3).

Aggregation A-2

Figure 4. Aggregation location A-2

Aggregation A-2 was located in a nest block hanging at a height of ±150 cm. In a deep wide crack, spread out over about half the length of the block, this largest aggregation with 17 visible specimen had found a place in the dark. Possible more specimen were present out of sight deeper in the crack (fig. 4 and 5).
One female from this aggregation has been caught to check for the presence of parasites (fig. 10).
In February the aggregation was still complete with 17 visible specimen.

Figure 5. Aggregation A-2 with 17 visible specimen

Aggregation A-3

Figure 6. Aggregation location A-3

A less wide crack in a nest block hanging at ±200 cm height accommodated aggregation A-3 with at least 10 visible specimen (fig. 6 en 8).

Figure 7. Narrowed location A-3 with 5 visible specimen

During the winter the crack in has closed for the most part as result of humidity, but in February five specimen were still visible there where the opening seemed still wide enough for them to emerge (fig. 7). Possibly more specimen were present but that was not clearly visible.

Figure 7. Aggregation A-3 with 10 visible specimen

Aggregation A-4

Figure 9. Aggregation location A-4

This mini aggregation of two specimen (fig. 9) had dissolved on 23 September and has not been observed after.

3.2. Behaviour

Among the strikingly inactive wasps only a few would leave the aggregations on sunny warm days to forage, usually in the afternoon (fig. 1). In all case the majority of the specimen remained in the aggregation. The time between foraging and returning to the aggregation has not been measured but felt very long, longer than an hour.

3.3. Presence X. vesparum

The observation period started in August, around the time of the second mating of X. vesparum. Then and in the afterwards formed pre-overwinter aggregations parasitised adult wasps would have been visually recognisable. The abdomens of all visible wasps have been checked visually and using photographic materials for the presence of the parasites on different occasions.
On 29 August I sucked away one female from aggregation A-2 and checked her for parasitic presence (fig. 10). Directly after release near the opening she joined the aggregation, which induced minimal activity in few of the other wasps.
In September and February the abdomens have been checked again, but in all cases without results.

Figure 10. European paper wasp ♀︎ (Polistes dominula), gyne from aggregation A-2 without indicators of X. vesparum presence


All aggregations have been observed for the first time in August which coincides with the normal time pre-overwinter aggregations are starting to form, but possibly they were already present earlier.
During the whole observation period from Augustus 2022 to February 2023 all visible wasps within the aggregations and those foraging in the garden were free of indicators hinting at the presence of X. vesparum. If any parasitised wasps were among them they would have been recognisable in August, September and afterwards. Therefore it is likely these are pre-overwinter aggregations that have been developed under normal circumstances.

I’m curious whether a nest will develop in the garden next season.


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