Observations on the behaviour of bethylid wasp Bethylus boops (Hymenoptera: Bethylidae) in ivy (Hedera sp.)


Published November 2023, Olger R. Krischan
(this article has been published in Hymenovaria 28)

Introduction

On 08 August 2021 I found a very small wasp on the inside a window in our house (Figure 1), a bethylid wasp Bethylus. Expert Jeroen de Rond confirmed this to be a female Bethylus boops (= Anoxus boops) [Hedqvist 1975], a species that had the status ‘very rare’ at that time but currently is classified as ‘rare’. She is associated with Ivy Hedera and little is known about host relationships; presumably Psychidae (Lepidoptera) are a possible host group [De Rond, pers. comm.].

Based on earlier experiences with species for which the first sighting occurred in house, there was a good chance the wasp was active in the garden, in which grow four large Ivy Hedera sp. plants. In April 2022 I indeed found the species in Ivy after a week’s search, what was the beginning of a long, and frequently intensive, observation period that ended in August 2023. This article provides an overview of the findings.

Figure 1. A female Bethylus boops

Jackpot

Establishing the focus areas for the search was easy. The four Ivy plants each grow in different spots, the one about two meters across the back door, plant-1, being the nearest to an entrance to the house. There, on 11 April 2022, the search started, armed with a magnifying glass, for a needle about two to three millimeter in size. During the next eight days the plants have been checked a couple of times per day. The focus was on the lower zone near ground level, since the species had been collected there before [Vikberg 1999]. After eight days the search struck gold; on 18 April around 14:17 I found two simultaneously active females (Figure 2) in the plant around the corner from the front door, plant-2. Followed by another female two and a half hour later.

Figure 2. Female Bethylus boops drinking

Observations

In the subsequent period until augustus 2023 many hours have been spent, especially between April and June of 2022, regularly checking the four plants and the leaf litter underneath them for the presence of the wasps. The focus eventually shifted almost exclusively on plant-2 since no specimens were ever found in the other plants. Those plants remained being checked by taking samples.

At the start of the observations in April the observation area at plant-2 could roughly be divided into two zones with low Ivy twines, zone-1 and zone-2, separated with an empty space carrying only a layer of leaf litter of about fourty centimeters. Zone-1 had the deepest leaf litter layer of about fifteen centimeters at the deepest point (Table 3), and initially provided the most observations (Figure 3). As time passed the empty space between the zones gradually closed as the plant grew so only one large zone remained in June (Figure 4). With this the observations in the former zone-2 increased, mainly males.
The leaf litter layer has been investigated four times with a magnifying glass, across the width and depth of both zones, including the vegetation just above and the soil surface. This resulted in two specimen.

In total twenty-two Bethylus specimen have been observed; five males, sixteen females and one unknown (specimen 15). The final observation was made on 02 Juli 2022. The peak was on 26 June 2022 when four specimens, three males and one female, were active simultaneously.

It cannot be ruled out that one or more individuals have been observed on multiple days. Using the photographic materials all specimens on the same day have been checked for uniqueness. Females 8 and 9 are potentially the same specimen.

The period from Juli to August had many interruptions in which no further observations have been made. In the period after no wasps have been observed as well. In the spring of 2023 the observations have been started again until August, but this did not provide any more sightings.

Figure 3. Ivy plant 2 in April with zones 1 and 2
Figure 4. Ivy plant 2 in June

For specimen 15 only the indication photograph has been preserved due to an error, so it has been classified as an unknown Bethylus.

For specimens 11, 12, 14, 15, 20, 21 and 22 the zone has not been recorded.

Female-3 was the most elevated observation on forty centimeters measured from ground level underneath the leaf litter, all others were positioned lower.

Females 5 and 7 were the only specimens found in the leaf litter.

Table 1 provides an overview of the observations.

DateTimeZoneSpecimenDrinkingRemarks
18/iv/202214:17
14:29
16:47
1
1
1
female-1
female-2
female-3
yes
yes
no
females 1 and 2 observed simultaneously
19/iv/202215:491female-4no
22/iv/202210:551female-5noIn leaf litter
27/iv/202217:14
17:57
1
1
female-6
female-7
yes
no

In leaf litter
01/v/202215:35
16:41
2
2
female-8
female-9
no
no
First observation in zone-2
females 8 and 9 possibly the same specimen
06/v/202216:541female-10no
13/v/202219:09
19:38

female-11
female-12
no
no
26/vi/202216:23
16:48
17:19
18:00
18:29
18:33
18:41
1


2
2
2
2
female-13
male-14
15
male-16
female-17
male-18
male-19
no
yes

yes
yes
yes
yes
 
 
 
Specimens 16 up to 19 observed simultaneously


29/vi/202213:26
14:14

female-20
male-21
no
no
02/vii/202218:51female-22noFinal observation
Table 1: Observations overview

Identity

All specimens have been identified using photo graphic material and Archer’s table [Archer 2020], and have been validated by Jeroen de Rond. All females are Bethylus boops (Figure 5).

It is not (yet) possible to determine the males’ species. The male photographic material, which quality leaves room for improvement, does not present clear eye hairs, which can fit B. dendrophilus as well. However as a table for males is currently missing it is unclear whether this feature can be applicable to B. boops [De Rond, pers. comm.].

The present males (Figure 6) differ from the B. boops females in:

  • jaws orange in colour, including base
  • antennal scape orange, including base
  • last three antennal segments darker coloured
  • femur front leg coloured orange for apically up to middle
  • head smoother and shorter

They follow the colour pattern for the B. dendrophilus male as described in Richards [Richards 1939], with an exception of the front leg femur.

Figure 5. Female of Bethylus boops
Figure 6. Male of Bethylus sp.

Behaviour

The wasps are very small but turned out to be rather easy to spot with the naked eye in the local context of the plant by the combination of shape and behaviour. Under normal circumstances they move rather linearly with constant speed, keeping the head in a straight line with the flat body (Figure 5). On the surface of the leaf this translates into a dark, bicoloured, regular moving dash; a short black front, the head and thorax, with a long mat grey rear, the wings.

The small size turned out to be a challenge during the observations as they had the tendency to go up in smoke (and reversed to materialise out of the blue on a leaf). They probably flew away but this has not been directly observed. Their small size makes it easy to hide in a small crevice or fold. This became clear with female-4 who disappeared in a fallen dry leaf. She turned out to sit motionless in the shadow of a fold in the leaf. Others moved to the rear of the leaf where they would hide in an irregularity on the leaf surface or behind a somewhat curled leaf edge.

The majority of the wasps were found on the top of the ivy leafs. The observations played out primarily on the the leaf surface. Rarely they descended towards the leaf litter. Instead they roamed on one or more leafs, especially on the front but also on the back side. Sometimes they repositioned themselves via the leaf stems and the branch to the next leaf.

Female-4 is the only specimen seen descending to the leaf litter.

Female-5 is the only specimen that was found during the search in the leaf litter but it is not clear whether she was already in the leaf litter or had landed on the leaf. Other Hymenoptera, especially very small chalcid wasps Chalcidoidae, would land during the investigation on leafs uncovered by me, so it is possible this was the case for this female as well. She was found in the thickest part of the leaf litter in zone-1. There female-7 also roamed on a leaf on top of the leaf litter, she disappeared without descending into it.
The possibility that a specimen has been missed during the search cannot be ruled out based on the experiences with them hiding and disappearing.

Female-13 has been observed longest. Her journey started on a pile of bags with sand that I had put over the wall so the long observations of the plant and leaf litter, while lying on top of it, would become more comfortable. After roaming for twenty minutes or longer she moved from the bags to the wall. Underneath the bags a humid microclimate had been established with rooting ivy branches and leafs that lay on the surface of the wall. There her behaviour changed. She moved in rather short sprints from cover to cover what gave the impression she was cautious, evading the present Lasius fuliginosus ants. I lost track of her as she entered such a covered position.

The ant L. fuliginosus has her territory in the ivy and is the most abundant insect in the investigated layer of the plant. Female-4 (Figure 7) halted her descent to the leaf litter as she approached a group of these ants that stood about five centimeters further away. The moment they started to approach into her direction she shot away to the back of the branch and had disappeared.
Outside the observation area, at about two meters distance, lay the ivy shoot covered nest entrance of the ant species, about thirty centimeters from the bottom of plant-2. In the vicinity of it no Bethylus wasps have been seen.

Figure 7: Female of Bethylus boops (female-4) halts as she approaches ants

Food

During the observation period both genders fed on multiple occasions on liquids present on the leaf surface, possibly honeydew (Table 1). Large moisture spots, drops, as well as minuscule droplets were consumed. With the head angled downwards they halted at large drops to lick them up (Figure 2), while the smaller droplets were consumed as they slowly walked (Figuur 8).

Figure 8. Female of Bethylus boops licks small droplets while walking

Hosts

No direct interactions with potential hosts have been observed. During the search of the leaf litter an eye was kept out for Psychidae (Lepidoptera) or perhaps another potential host group. This did not result in any specimens of that particular group. In April two Geometridae (Lepidoptera) caterpillars were found in the leaf litter of zone-1. The present groups are shown in Table 2.

GroupSpeciesPosition wrt
leaf litter
Diptera Diptera sp. (1 larva, parasitised by Chalcidoidea sp.)under
Braconidae
(Hymenoptera)
– Braconidae sp. (3+ imago)above
Chalcidoidea (Hymenoptera)– Pteromalidae sp.
– Chalcidoidea sp.
above / in
above
Diapriidae
(Hymenoptera)
– Diapria conica (1 ♀︎)
– Spilomicrus hemipterus (1 ♀︎)
above
above
Formicidae
(Hymenoptera)
Lasius fuliginosus (40+ ♀︎)
Lasius niger (10+ ♀︎)
above / in / under
above
Geometridae
(Lepidoptera)
– Geometridae sp. (1 caterpillar)
Idaea seriata (1 caterpillar)
in
in
Ectopsocidae
(Psocodea)
– Ectopsocus sp. (5+ imago)above
Colembola– Colembola sp. (80+)in / under
Table 2: Present groups above, in and under the leaf litter

Ivy plants

The Bethylus wasps have only been found on plant-2. During the first observation in April the plant was developed less voluminously (Figure 3). As result of growth during the period up until June the volume had increased and covered most of the leaf litter (Figure 4). In spring up into Augustus 2023 the plant had expanded almost over the wall and the leaf litter underneath had become thinner and more sparse.

The large plant grows on the corner of the house. The part to the right connecting zone-2, as well as the part to the left connecting zone-1 and growing around the corner, have been searched for wasps without results. The observations were therefore limited to a specific part of plant-2 in an area with a volume of about 100(l)x50(w)x40(h) cubic centimeters. Table 3 provides and overview of the present ivy plants.

PlantB. boopsLocationLight*Leaf litter**
Plant-1NeeSoutheastPermanent shadeNone
Plant-2JaNortheastPartial shade±3-15cm
Plant-3NeeNortheastPermanent shade / darkness< 3cm
Plant-4NeeNortheastPermanent shade / darknessNone
Table 3: Overview of ivy plants in the garden
* focus on the bottom 50 cm of the plant
** leaf litter of fallen leafs underneath the plant

Discussion

There is no clear answer as to what brought the wasps to this specific, small piece of nature.

The combined observations of 2021 and 2022 show an activity period for the B. boops females that at least starts halve April until the beginning of August. The start corresponds with that of other Bethylus species in the Netherlands [Peeters 2004]. Possibly it can extend longer; it is known that B. cephalotes females overwinter [Peeters 2004]. More fieldwork needs to confirm this.

It was expected that the males would become active in early summer [De Rond, pers. comm.] what corresponds with their appearance end of June.

The observations confirm that moisture stains and drops on the surface of the ivy leafs are a food source for both present genders. The licking of honeydew from the leaf surface is known from other Hymenoptera. The other plants also provided these moisture sources but no wasps have been seen on them. Possibly due to their much darker location they were less attractive.

Like other Bethylidae the larvae of Bethylus are idiobiont ecto parasitoids [Peeters 2004]; they attach themselves to the outside of the host (ecto) and kill the host (parasitoid) in the stage in which they found it (idiobiont). Hosts are often found in leaf litter [Peeters 2004], what probably is also true for B. boops [De Rond, pers. comm.]. Two females, female-5 and female-7, were found in the leaf litter, and one female, female-4, seemed to be descending towards it.

The leaf litter in zone-2 was much thinner than that in zone-1. Initially almost all observations were made in zone-1. After the two zones grew together the observations in former zone-2 increased. That increase can almost entirely be attributed to the males that emerged then. Most of the females kept to zone-1 (Figure 3). This seems to indicate a preference for the females for the part of the observation area with the thickest leaf litter, although for four females the zone has not been recorded.

The potential host group Psychidae was missing among the present fauna in, underneath and above the leaf litter, and no interactions have with other potential host groups have been observed. It is known that the other Bethylus species in the Netherlands use Lepidoptera larvae as their hosts [Peeters 2004]. In this light the two present Geometridae caterpillars could be interesting, although this remains speculation.

The only present organism that induced a visible response in the wasps was L. fuliginosus. The Dutch specimen of B. pilosus was found in the nest of the ant [Peeters 2004]. That species cannot be separated from B. boops and its status is uncertain [Erwann 2022], [Peeters 2004]. B. cephalotes has been found in the nests of these ants as well [Peeters 2004]. In the direct vicinity of a known nest entrance of the ant, about two meters outside the observation area, B. boops has not been observed.

Most time was spend on the leaf surface, of which the larger part in roaming not drinking. For B. fuscicornis it has been suggested they possibly find their hosts on the leaf surface [Peeters 2004], [De Rond 2002].

It is remarkable, and somewhat disappointing, that no further observations have been made in 2023. The grown plant offered more leaf surface and with that more potential food sources. Possibly the covering of the leaf litter and its thinning could be the cause, which would suggest that the leaf litter was the main attraction for the females. A potential relation with the development of vegetation has been suggested for B. cephalotes [De Rond 2002] as well.

No interactions have been observed between the simultaneously active female and three males, and they never approach each other. The fact that both genders were present could just be a direct effect of the food source that they shared; all of them have been observed drinking.
The identity of the males remains unknown for the time being. What did stand out from the photographic materials were the sturdy light coloured hairs on the eyes of the females (Figure 9), that were often clearly visible in bad, unsharp photographs, however in similar photographs of the males they never were. This can potentially be an indication that they are a different species, or that the hairs in B. boops males are possibly less developed. This is speculation and needs to be confirmed with further investigation.

Figure 9. Female of Bethylus boops with hairy eyes

Encore

While observing in May small yellow flies appeared on occasion in a specific part of the observation area. It turned out to be Chyromyidae from the genus Chyromya, a group whose members all are currently classified as ‘very rare’. Two special groups on a stage of about 150 square centimeters, how crazy can it get.

Acknowledgement

I would like to thank Jeroen de Rond for his valuable information on the species and the help offered to identify the specimens. Also I would like to thank my dearest wife Min for her patience and for creating the space for me to conduct this investigation and more general practice this fantastic, albeit slightly out of hand, hobby.



Literature

Archer 2020 Archer, M, 2020. The use of Perkins (1976) to identify species of DEBs (Dryinidae, Embolemidae, Bethylidae) with extra Notes

De Rond 2002 de Rond, J. (2002). Komen en gaan van de platkopwespenfauna in Flevoland. Entomologische Berichten, 62(1), 30-32.

Erwann 2022 MARHIC, Erwann. 2022, Liste preliminaire des Bethylidae de France (Hymenoptera: Chrysidoidea). Osmia, 10: 13-24.

Hedqvist 1975 Hedqvist, Karl-Johan. 1975, Notes of Embolemidae and Bethylidae in Sweden with description of a new genus and species (Hym., Bethyloidea).

Hymenovaria Hymenovaria, Sectie Hymenoptera van de Nederlandse Entomologische Vereniging

Peeters et al. 2004 Peeters, T.M.J., C. van Achterberg, W.R.B. Heitmans, W.F. Klein, V. Lefeber, A.J. van Loon, A.A. Mabelis, H. Nieuwen-huijsen, M. Reemer, J. de Rond, J. Smit, H.H.W. Velthuis, 2004. De wespen en mieren van Nederland (Hymenoptera: Aculeata). – Nederlandse Fauna 6. Nationaal Natuurhistorisch Museum Naturalis, Leiden, knnv Uitgeverij, Utrecht & European Invertebrate Survey – Nederland, Leiden.

Perkins 1976 PERKINS, John Frederick. 1976, Hymenoptera: Bethyloidea (excluding Chrysididae).

Richards 1939 RICHARDS, Owain Westmacott. 1983, The British Bethylidae (sl)(Hymenoptera). Transactions of the Royal entomological Society of London, 89.8: 185-344.

Vikberg 1999 VIKBERG, Veli. Crossocerus assimilis and Bethylus boops, two aculeate wasps new to the fauna of Finland (Hymenoptera, Crabronidae and Bethylidae). Entomologica Fennica, 1999, 10.4: 245-246.

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