Published November 2024, Olger R. Krischan
(this article has been published in Hymenovaria 31)

Introduction

While walking our garden on 24 June 2023, minute wrinkles in still water lead my attention to a tiny struggling insect. This to the naked eye somewhat insignificant creature underwent a spectacular metamorphosis when magnified, with a graceful, drop-shaped and deep red coloured abdomen (fig 1).

This was my first encounter with the gall wasp family, the Cynipidae. Uncovering the identity and especially the gender of the wasp turned out to be difficult as result of confusing character aspects, and little available detailed image material. Eventually it is identified as a female Xestophanes potentillae. Gall inducing gall wasps lay their eggs in the host plant using an ovipositor. The plant response results in a concrescence, the gall, that functions as shelter and a food source for the larva (Ronquist et al. 2015) (Melika 2009). Gall wasps use different reproduction methods: a) a sexual generation with arrhenotokous parthenogenesis, in which fertilised eggs produce diploid females and unfertilised eggs produce haploid males, b) a generation with virgin females reproducing through thelytokous parthenogenesis, in which unfertilised eggs only produce diploid females, c) alternating generations with cyclic parthenogenesis (heterogony), in which a sexual generation alternates with an agamic generation in a long cycle of two or more years (Melika 2009) (Folliot 1964). Folliot (1964) determined X. potentillae in principe has one sexual generation in June-July of the next year, but a second generation (bivoltine) can occur at the end of August up to the beginning of October in the same year. He categorised the galls in two types based on its location on the plant; type ‘σ’ on petioles and stolons (bivoltinism was observed in this type) and type ‘ρ’ on root and root collar (table 2). X. potentillae induces galls on plants from the genus Potentilla (Jennings & Jennings 2016) (Melika 2006) (Ronquist & Liljeblad 2001) (Folliot 1964). One of its host plants, the Creeping cinquefoil Potentilla reptans, is present in the garden and both genders were found on it, later followed by many galls.
This article provides an overview of the X. potentillae characters including some confusing aspects, the types, shapes and distributie of the galls in the garden population, and some observations on species behaviour.

Distribution and taxonomy

The gender has two species worldwide, X. potentillae and X. brevitarsis, both are restricted to the Western Palaearctic (Pujade-Villar at el. 2019) (Melika 2009) and occur in the Netherlands (Nederlands Soortenregister 2024). Melika (2006) suspects both species are X. potentillae. Possibly this may include another species that uses the stolons instead of the petioles and roots, and is recognisable from the areolet in the forewing (Bowdrey pers. comm.). Possibly the same cryptic species referred to in Jennings & Jennings (2016). Two other Xestophanes species that are frequently mentioned in literature sources, X. laevigatus and X. szepligetti, are synonymised with X. potentillae (Melika 2006). Ronquist et al. (2015) positioned Xestophanes in the tribe Diastrophini.

Identification

The wasp has been identified by Jeremy Bowdrey as Xestophanes potentillae. The following publications can be used for identification. Buffington (2019) provides a key to the tribes within the subfamily Cynipinae. Pang et al. (2020) provides a key to the genera within the tribe Diastrophini. Using Melika (2009) and Eady & Quinlan (1962) the species can be established (note that both publications have Xestophanes listed under Aylacini, see Ronquist et al. (2015)).

Remarks for species characters

X. potentillae, in its current state, has the following characters:

• the notauli are unclear or absent in the last anterior 1/3 (fig 2a)
• the antennal flag segments F1 and F2 are equally long (fig 2b)
• the hindleg tarsal segment 4 is longer than wide (fig 2c)

The colour of the abdominal segments can be dark brown to black posterior dorsally (fig 3). X. brevitarsis is more yellow/orange to amber coloured (Melika 2006) (Bowdrey pers. comm.).

Some character aspects can be confusing so initially the drowning female seemed to be a male. Those insights have been merged in the character list below. The following characters separate the genders (Melika 2006):

• The female abdomen is vertically almost as long horizontally at its widest point, bulging ventrally (fig 3b). In the male the bulge is much less pronounced (fig 3a).
  This character is best assessed in lateral view, it can be less clear from different angles.

• The female abdominal segments 2 and 3 are fused, they are separated in the male.
  Under some angles a separating line can seem visible in the female but viewed laterally this is absent (fig 3b). This is in contrast in the male where a clear deep division is clearly visible from any angle (fig 3a).

• The female has thirteen antennal segments (eleven flag segments), and the male has fourteen (twelve flag segments).
  The antennal segments in some specimens are deeper incised resulting in the presence of a seemingly additional segment (fig 4a) causing gender confusion. However the resulting last segment in this case is equally long to the penultimate, but actually the last segment is always twice as long as the penultimate (fig 4b). This helps identifying this deviation (?). Also the incision is less deep and shaped differently than those between the other segments.
  In the observed specimens from both genders, the females always had a deeper developed incision in the last segment than the males. The males are identifiable in the field from the long antennae compared to the body.

• The antennal flag segment F1 is straight in the female, and arcuate in the male.
  In the female’s can look slightly arcuate (fig 4a), but this is very pronounced in the male (fig 4b).

Activity

On 04 July 2024 I found nine males (fig 5) and two females in and around Potenilla reptans plants, close to the ground. During a check in the beginning of August five specimens were present. The final observation was a male on 09 September 2024.

Galls

On 30 August many galls had developed on the plants (fig 6). The gall inventory (executed on 6 to 8, 13 to 15 and 21 to 22 September 2024) resulted in seven hundred twenty-two galls on fifty-three plants and ten stolons (more than two thousand when the separate galls in the fused gall shape are counted separately, see below) (table 1). Stolons bore galls on the stolon itself as well as on the present nodes. Almost all plants bore galls on at least one petiole (±98%), seven bore galls on the root collar (±13,2%), and five bore them on the roots (±9,4%).

Aspect	Amount
Examined plants 1	54
Plants with galls on petioles	53 (±98%)
Plants with galls on root collar	7 (±13,2%)
Plants with galls on root	5 (±9,4%)
Examined stolons	11
Stolons with galls 2	9 (±82%)
Total galls	722 (±2257 3 )

Of all plant parts used by the wasps in this population, the petiole (figure 8) is clearly the most favourite location (±92,52%), followed by the stolon (±4,99%), the root (±1,52%), and the root collar (±0,97%) (table 2). The majority of galls (±97,5%) were of type ‘σ’ (type ‘ρ’ ±2,5%). On and in the swollen shape of the root the separate galls can be difficult to isolate (fig 7a). Also the examination of the roots was limited to the upper 2-5 centimeters to prevent damaging the plants. As a result the exact amount of galls on the roots may have been higher. Galls on the root collars can be difficult to separate from galls at the base of the petiole (fig 7b). Only galls that clearly could not be associated with a petiole have been counted as a root collar gall (for example sometimes a small deteriorated piece of petiole is still visible), but incorrect ascriptions cannot be ruled out.

Location gall	Gall amount 1	Percentage	Gall type 2
Petiole	668 (±2166)	±92,52%	σ
Stolon	36 (±117)	±4,99%
Root	11 (±22)	±1,52%	ρ
Root collar	7 (±12)	±0,97%

The galls develop in three shapes:
1. Separate; a gall is isolated on larger distance from its direct neighbour(s) (fig 8a),
2. Chained; the galls lie directly next to each other, touching (fig 8b).
3. Fused; the galls are fused into one large gall (fig 8c).
The latter shape contains the largest diameters (up to 12,7 millimeters), and the largest lengths (frequently occupying the entire petiole). It was the most represented shape (55,1%), followed by shape 1 (±30,8%) and shape 2 (±14,1%) (table 3). The amount of galls in the fused shape has been counted using the constrictions on them as an indicator for a gall (fig 8c), albeit with large uncertainty depending on the amount of concrescence (table 1, table 2).

Gall shape	Gall amount	Percentage
Separate	222	±30,8%
Chained	102	±14,1%
Fused	398	±55,1%

Behaviour

The adult wasps were found sitting still or walking around on the leafs of the host plants. The males fanned out into the direct environment of the plants, sometimes a few clustered together. Some males waved their wings rhythmically while walking, which reminded of the courtship behaviour of Picture-wing flies Diptera: Ulidiidae (Marshall 2012), like Seioptera vibrans. Resting specimens were found on the underside of the host plant leaf. The well approachable wasps dropped when disturbed and relocated by foot afterwards. At night the males as well as the females slept, always alone, at the underside of a host plant leaf.

Discussion

Gall type ‘σ’ is the most common in Europa and usually occurs without type ‘ρ’ (Folliot 1964). Folliot’s (1964) study shows the only population with a second generation had a combination of the two types, similar to the population in this publication. The presence of the wasps in June/July fits with a univoltine generation, the male observed on 9 September could fit in a possible bivoltine generation. However during the gall inventory carried out between 6 and 22 September, no other specimens have been observed making that less likely. Folliot (1964) established a rather balanced sex ratio, with somewhat less males than females (40%). From that perspective the abundance of males at the start of June was remarkable. Possibly that is just an optical effect resulting from the reduced observation time. Both aspects will need to be confirmed with more intense field observations.

Acknowledgement

My gratitude goes out to the following persons. Theo Peeters for his help and suggestions in the search for the identity. Charles Davis for his help determining the genus, Miles Zhang for setting up contact with Davis. Jeremy Bowdrey for identifying the species and providing the species related information. Simon Haarder for sharing key character information and setting up contact with Bowdrey.

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Literature

^{Buffington et al. 2020} Buffington, M. L., Forshage, M., Liljeblad, J., Tang, C. T., & van Noort, S., 2020. World Cynipoidea (Hymenoptera): A Key to Higher-Level Groups. Insect Systematics and Diversity, Volume 4, Issue 4, July 2020, 1, https://doi.org/10.1093/isd/ixaa003.

^{Eady & Quinlan 1962} Eady, R. D., & Quinlan, J. (Eds.), 1962. Handbooks for the identification of British insects. Vol. VIII, part 1 (a). Hymenoptera: Cynipoidea. Key to families and subfamilies and Cynipinae (including galls), 81 p.

^{Folliot 1964} Folliot, R.,1964. Contribution à l'étude de la biologie des Cynipides gallicoles (Hyménoptères, Cynipoidea) (Doctoral dissertation, Université, Faculté des Sciences), 564 p.

^Hymenovaria Hymenovaria, Sectie Hymenoptera van de Nederlandse Entomologische Vereniging

^{Jennings & Jennings 2016} Jennings, M., & Jennings, S., 2016. Records of X. potentillae (Retzius) X. brevitarsis (Thomson) (Hymenoptera: Cynipidae) derived from herbarium specimens of potentilla species at the British and Irish Herbarium, Natural History Museum, London, British Journal of Entomology & Natural History 29.

^{Marshall 2012} Marshall, S. A., 2012. Flies: The Natural History & Diversity of Diptera. Verenigde Staten: Firefly Books.

^{Melika 2006} Melika G., 2006. Gall wasps of Ukraine. Vols. 1 & 2. Vestnik Zoologii. Journal of Schmalhausen Institute of Zoology Supplement. 21. 1-644.

^{Pang et al. 2020} Pang Y., Liu Z., Su C-Y., Zhu D-H., 2020. A new species of Periclistus Foerster, 1869 from China and review of the tribe Diastrophini (Hymenoptera, Cynipoidea, Cynipidae). ZooKeys 964: 109-126. https://doi.org/10.3897/zookeys.964.47441.

^{Pujade-Villar et al. 2019} Pujade-Villar, J., Wang, Y., Liu, Z., Chen, X., He, J., & Ferrer-Suay, M., 2019. Discovery of Xestophanopsis gen. n. from China and taxonomic revision of two species misplaced in Ceroptres Hartig, 1840 (Hymenoptera, Cynipoidea: Cynipidae). Entomologica Fennica, 30(3), 126–137. https://doi.org/10.33338/ef.84149.

^{Ronquist et al. 2015} Ronquist F., Nieves-Aldrey JL., Buffington ML., Liu Z., Liljeblad J., et al., 2015. Phylogeny, Evolution and Classification of Gall Wasps: The Plot Thickens. PLOS ONE 10(5): e0123301. https://doi.org/10.1371/journal.pone.0123301.

^{Ronquist & Liljeblad 2001} Ronquist, F., & Liljeblad, J., 2001. Evolution of the gall wasp‐host plant association, Evolution, Volume 55, Issue 12, 1 December 2001, Pages 2503–2522, https://doi.org/10.1111/j.0014-3820.2001.tb00765.x.

^{Soortenregister} Nederlands Soortenregister

Citation

Krischan, O., 2023. Notities bij morfologie, biologie en gedrag van galwesp Xestophanes potentillae (Hymenoptera: Cynipidae) op Potentilla reptans. HymenoVaria, 31, 121-125.