Published April 2024, Olger R. Krischan
(this article has been published in Hymenovaria 29)

Introduction

On the morning of 19 August 2023 my eye caught a big black wasp between the erect blades of a tussock on our garden in Krimpen aan den IJssel (ZH). Some days before a similar looking specimen suddenly shot out the vegetation around the pond. Due to the fleetingness of the moment it remained a suspicion that this was the Grass carrying wasp Isodontia mexicana, which could now be asserted with certainty. This introduced species originating from North-America, which for now is still categorised as very rare ^{[Nederlands Soortenregister]}, was first observed in the Netherlands in 2010 ^{[Smit & Wijngaard 2010]}. Of course the wasp had disappeared after the camera had been put into position, but fortunately with the appearance of a flying blade of grass en route to a nest block, the observation period of this nesting female was launched. From 19 august up until 06 September 2023 in totaal seven nests have been constructed in bamboo and stone, which sometimes were provisioned simultaneously, with for as far as has been observed, adult male Southern oak bush crickets Meconema meridionale. This article describes the observed behaviour, prey and material usage, as well as a literature study on prey manipulations.

Identity

The identity is established using Bitsch ^{[Bitsch 2020]} and Smit & Wijngaard ^{[Smit & Wijngaard 2010]}. The within the Dutch context unmistakable combination of characters consists of the long, curved, petiolate first segment of the abdomen, the fully black body with silver hairs and darkened wings frequently covered in a blue-metallic shimmer (figure 1).

Nests

At the start of the observation period on 19 August she had been active for some time already; one nest tunnel had been closed and the final adjustments were being made to the next. In total seven nests were constructed spread over two nesting blocks (figure 2); six of them in bamboo tubes approximately fifteen centimeter in length with diameters of eight, ten and eleven millimeter, and one in a borehole in stone approximately six centimeters in length with an 8 millimeter diameter (table 1). Both nesting blocks have a sunny southern location and are separated about one and a half meter. The lowest and highest rows in the module with bamboo stems hang on respectively seventy and ninety centimeters height. The brick hangs on a height of one hundred and forty centimeters.

At the observation start nest_1 was already finished and nest_2 was being closed with a nest plug. Nest_3 was fit overnight with an unfinished nest plug, judging by the opening in it. Only at the end of the observation period, nine days later, had it been closed. Nest_4 has been observed longest, possibly from the construction start. The full construction was spread out over a period of fifteen days. Nest_5 has been provisioned with grass at least once, but further construction was stopped. The grass, that was visibly protruding for some time, was eventually removed by the wasp or the wind. Nest_6 has been provisioned and closed in two days, but the start of construction is uncertain. Nest_7 was discovered after the wasp had already left for a few weeks. It’s clean nest plug lay about four centimeters deep and missed the characteristic nest plug, which usually has a more messy and rough character (figure 2 & 3). On 25 February 2024 the plug on nest_3 had disappeared, probably as a result of the many stormy days. The remaining plug, on about two and a half centimeters depth, had the same clean character. Nest_8 has been closed overnight after the work on the bamboo tubes was finished, which suggests a construction time of one day.

Prey species

In total three prey species have been photographically recorded, all adult male M. meridionale (figure 4), and one more observation of a Tettigonidae (table 2).

Species	Number	♀︎♂︎	Stage	Antennae	Nest
Meconema meridionale	2 1	♂ ♂	Adult Adult	2 cut off 1 visible	Nest_3 Nest_6
Tettigonidae sp.	1	–	–	–	Nest_3

Plant relationships

The primary construction material was grass, as to be expected, but only dried not fresh [Breugel 2017], [O’Neill 2003]. This was collected on a tussock of Yorkshire-fog Holcus lanatus L., about three meters away from the nesting block, and from other unobserved grass sources outside the garden. Dried leafs of Great willowherb, growing agains the nesting block, were collected twice. Once a leaf was cut from Rosemary Salvia rosmarinus, growing about two meters from the nest, and returned to the nest. Once the wasp has been seen drinking on Fennel Foeniculum vulgare growing one meter from the nesting block. An overview in table 3.

Species	Application
Epilobium hirsutum – Great willowherb	nest
Foeniculum vulgare – Fennel	food
Holcus lanatus L. – Yorkshire fog	nest
Salvia rosmarinus – Rosemary	nest

Isodontia mexicana in Krimpen aan den IJssel

The records in the Waarneming.nl database show the wasp is steadily spreading throughout the country from its first discovery site in Noord-Brabant in 2010 ^{[Smit et al. 2010]}. The nearest observation of nesting activity in Zuid-Holland in that dataset, a grass blade carrying female, is from 2021 in Berkel en Rodenrijs. In 2019 the species has been observed foraging in the neighbouring town Capelle aan den IJssel. The first, and only, observation in Krimpen aan den IJssel on 4 and 28 Augustus 2023, about one kilometer from our garden (Google maps straight line measuring method). The two observations on that last date, a specimen foraging on Fennel, overlap with the observation period in this publication in which the wasp has was not observed on 27 and 28 August. Unfortunately it is not possible to establish whether it is the same specimen based on that photographic material. Is the distance between the gardens at all within the foraging range of the species? The range for I. auripes is more than three kilometers, which has been established with the homing method ^{[Rau 1931, Chlorion auripes]}. In this method a female is caught on the nest, marked and released on several distances from the nest to see if she can find her way back. This gives insights into the theoretical distances in which the wasp can operate, what gives an idea of the potential foraging distances ^{[McCabe & Chandler 2020]}. For Sphex ichneumoneus, a species from a genus related to Isodontia, this distance has been established to be more than two and even almost ten kilometers ^{[Lechner 2015]}. For Cerceris fumipennis, a digger wasp from a different family, the real foraging distance has been established to at least one and a half kilometers ^{[Rutledge 2023]}. With those numbers in mind the distance between the gardens seems in theory to be within range. Whether it is logical to forage for nectar one kilometer away on a plant that flowers one meter away from your nest is a different question, but this wasp seemed to apply some alternative logic anyway.

Total confusion

The wasp behaved very nervously, probably a direct consequence of my present and the big camera rig that rested against the nesting block. There were moments where she kept looking in my direction whilst directing her body diagonally towards the nest entrance. At the slightest movement by me or the camera she took off. Or just left, only to return an hour or longer later. Breugel ^{[Breugel 2017]} experienced similar behaviour. Sometimes she would first enter one or more empty rods next to the nest only then quickly enter it. These shenanigans seemed like diversion to not expose the nest. Very remarkable was the experience when I stood up to stretch after some hours waiting at the nesting block, or left the position to go inside to drink. The wasp would either arrive shortly after to slip inside the nest, or was just leaving again as I was returning to the rig. It seemed as if she was keeping an eye on the situation from a distance waiting for the proper moment. It was remarkably difficult to photograph her. Again and again the focus was on the incorrect bamboo stem, until I realised she was provisioning different nests simultaneously (table 1). The parallel construction of nests is known behaviour ^{[Breugel 2018], [Scaramozzino et al. 1994]}. Her behaviour was very erratic as well with frequent changes in activities, and provisioning interrupted with nesting block inspections. The grass was often collected in the garden, but also elsewhere. It was not unusual for her to return with a small piece of blade after thirty minutes or more. Her activities started late and ended early. Regularly she would disappear for long periods, sometimes even days, only to be at work again the next day. As a result the construction of nest_4 was very erratic (table 1). Altogether this made the photographing a difficult, sometimes frustrating matter.

Plant usage

The use of grass as construction material is a characteristic known for Isodontia species from the subgenus Murrayella ^{[Bohart et al. 1976]}; the closing plug as well as the wall in front of the rear wall and those between the nest cells, consist of grass blades or fibers. Apart from bark fibers ^{[Krombein 1967]} the consulted literature mentions no other non-grass-like species, as reported in this publication. Recent observation in I. elegans, another species from this subgenus, show the use of plastic instead of grass ^{[Orr & Parker 2023]}.

Prey usage

I. mexicana is known to use katydids (Tettigonidae) and crickets (Gryllidae) to feed her offspring, adults as well as immature, with a preference in Europa for Oecanthinae (Gryllidae) and Meconematinae (Tettigonidae) ^{[Schirmel et al. 2020]}, the latter includes Meconema meridionale.

Prey manipulations

In flight the prey is carried in back-belly position, conform Evans ^{[Evans 1962]}, mainly using the middle legs with support of the other legs (figure 5).

Judging by the constant distance between the wasp’s jaws and the grasshopper’s head in the photo material, it shows that, once landed, the prey is carried by the antennae stumps into the nest as to be expected ^{[Evans 1962]}. Caught prey are sometimes first put down in the nest entrance with their hind legs protruding outside (figure 6) ^{[Breugel 2017]}. After some time they are taken inside, probably carried, not pulled, as it is the norm for Sphecidae ^{[Evans 1962]}. This can also be deduced from Spendal et al. ^{[Spendal et al. 2021]} and is observed in the related species Sphex ichneumoneus ^{[Frisch 1939]}. In two of three observations in the current publication based on the photographic material the time past between the two moment was respectively more than twelve seconds and more than one minute, that last time period agrees with the duration mentioned in Breugel ^{[Breugel 2017]}.

To test whether the wasp still held the deposited prey, one of them has been pulled out the nest by the protruding tarsi (figure 7) without any resistance or direct response by the wasp. This seems to indicate there was no contact between hunter and prey. Only one minute later she appeared, clearly searching for the animal. Almost immediately she descended in a wide spiralling flight overlooking the vegetation underneath the nesting block, looking for her prey. But after completing a few spacious circles she gave up, only to return a little hour later with a new prey. I positioned the removed katydid back into the nest entrance, but unfortunately had to leave shortly after so was not able to observe whether the prey was re-used as described in Breugel ^{[Breugel 2017]}. After returning it was disappeared and not found in the vegetation.
Isodontia probably applies a four sting pattern to disable the prey like has been established for other hunters on katydids and crickets, among them I. nigella. In this pattern the prey is first stung in the throat, and then in each leg pair ^{[Steiner 1986]}. The poison of I. mexicana is classified as incompletely paralysing ^{[Piek & Spanjer 1986]}, which means the prey may still show some movement ^{[Breugel 2017], [Lin 1966], [Rau 1935]}. Whether this means the paralysis will be short lived so the prey can eventually recuperate, like is the case with the poison of other wasps, is unclear. Minute movements in the hind legs when touched showed the paralysis in the removed grasshopper was incomplete as well. Burton & Weiser ^{[Burton & Weiser 2019]} describe a nest with a small diameter of five millimeters, which seriously complicated nest provisioning and caused many prey specimens to fall to the ground. The wasp did not show any more interest in them. Burton put some of them aside, and they started showing more movement as time progressed until they fell completely still after a day, even when stimulated.

Parts of the antennae in two of the recorded prey were already absent at arrival at the nest, on respectively the seventh and behind the fifth antennal member (the end is not clearly visible in the photograph). The third specimen has one antenna protruding from the nest entrance, but the presence of the other antenna is uncertain. The loss of limbs in prey has different causes. Firstly these can accidentally break off in the struggle with an attacker, secondly they can be purposely removed by the wasp ^{[Evans et al. 1977]}, and thirdly they can be dropped by the katydid as a defence mechanism by self-mutilation, autotomy. Isodontia actively amputates antennae and legs ^{[Bohart et al. 1976]}, but there was no evidence of the latter here. In one prey the fracture lies in the segment, not between them, which implies a bite (figure 8).

Pham ^{[Pham 2019]} suggests a relation with small nest spacing as a cause for prey mutilation. Spendal et al. ^{[Spendal et al. 2021]} suggest storage efficiency as the main reason for this behaviour. Prey mutilations for I. mexicana reported in consulted literature occurred in the context of nest diameters of seven to ten millimeters, as was the case for the two prey specimen in the current publication (table 4). More observations can confirm a relationship with nest spacing.
Wasps feed on hemolymfe ^{[Evans et al. 1977]}, the prey’s bodily fluids, for example through the wound of cut antennae ^{[Piek et al. 1986]}. Prey may be cut for weight reduction as well. When the transport occurs fully through flight this will prevent the maximum carrying capacity from being exceeded. For this reason Vespula wasps frequently remove the middle and hind legs ^{[Coelo & Hoagland 1995]}. This will almost certainly be no different for Isodontia, especially in case of larger prey. Autotomy is known behaviour for Tettigonidae ^{[Ter Hofstede et al. 2017], [Fleming et al. 2007]}, especially on the grabbed middle or hind legs and seldom on the hearing organ carrying forelegs ^{[Fleming et al. 2007]}.

Publication	Nest ⌀ (mm)	Prey mutilation
		Antennae	Hind leg
Breugel 2017	9	1 (±20%) 2 (50%)	1 (25%)
Breugel 2018	9	2 (most)	–
Breugel 2022	7+8+9	1-2 (±75%)	–
Lin 1966	10	2 (80%)	1 (80%)
Spendal et al. 2021 (I.elegans)	8	2	–

Working hours

The wasp slept in the nest under construction, like described in Breugel ^{[Breugel 2022]}, Amiet ^{[Amiet 2009]}, Scaramozzino ^{[Scaramozzino 1994]}, but sometimes in an adjacent tunnel, possibly preceding the start of construction of a new nest. Her activities started late in the morning, around 10 – 11 o’clock. It seemed she would leave her place to sleep when the sun was visible form the perspective of the nesting block, which hang at the ultimate edge on a corner wall. She stopped her activities early as well, possibly when as result of the shadows cast on the nest by a large tree and a fence. The activities reported by Scaramozzino et al. ^{[Scaramozzino et al. 1991]} were stopped when the nest was covered in shadows.

Summary

Isodontia mexicana was found nesting in our garden between 19 August and 06 September 2023. The skittish and erratic behaving wasp constructed eight nests in total, six in fifteen-centimeter-long bamboo rods and one in a six-centimeter-long borehole in stone. Diameters used consisted of eight, ten and eleven millimeters. The nests were constructed using mainly grass, but different plant materials were used on three occasions. All four recorded prey specimens were Tettigonidae, of which three adult, male Meconema meridionale. Two with amputated antennae. A literature study on prey manipulation is presented. The wasp’s daily activity may be linked to sun visibility and shadows cast on the nest.

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Citation

Krischan, O., 2024. Observaties aan nestbouw en gedrag van langsteelgraafwesp Isodontia mexicana (Hymenoptera: Sphecidae) nestelend in Zuid-Holland. HymenoVaria, 29, 27-32.