Native to North America, it probably reached Europe between 1940 and 1960, likely transported with US supply troops via France. Since then it has spread continuously, both by its own flight and through the unintentional human transport of its brood inside reeds and bamboo canes used in gardens. Since 1962 it has been recorded in at least 16 European countries.

Germany's first record came in 1997 in Tübingen, Baden-Württemberg, and the northernmost reports so far come from Berlin and England. Adults turn up in open landscapes, residential areas, private gardens and city parks, and they build their nests in small above-ground cavities, including the reed and bamboo tubes commonly offered in insect hotels and even old breeding tubes of the violet carpenter bee. Like all digger wasps of its family, I. mexicana is a prey specialist: its larvae are fed exclusively on grasshoppers of the suborder Ensifera, the long-horned orthopterans that include bush-crickets and tree crickets.

The female paralyses its victims and stocks each brood cell with them, separating the cells with walls made from grass stalks, a nesting material unique among European cavity-nesting Hymenoptera. Adults, by contrast, feed on nectar, showing a seasonal preference for Apiaceae and Asteraceae, mostly visiting yellow and white flowers. None of the prey species documented from the wasp's native range occur in Europe, so the question of how the newcomer has adjusted its diet has been open.

Prior to this study, nine Ensifera species had been documented as prey in Europe: seven in Italy, five in Germany and one in Switzerland. The research team built layer trap nests from medium-density fibreboards, carving cavities 13.5 centimetres long in four diameters: 5, 7, 9 and 11 millimetres. Six boards were stacked and strapped together, with each diameter represented twice per panel, giving twelve cavities per diameter per nest.

The nests were installed at sun-exposed locations with openings facing south to south-east, and inspected every four to five days, a frequency chosen because wasp larvae need at minimum four to six days from hatching to feed and pupate. In each brood cell the researchers recorded the number of prey items, identified the species using the DJN observation key, and determined the sex, body length and weight of each individual. Only cells containing an egg and sealed were counted as nests; sealed cells without eggs were treated separately, as possible mock nests.

Two habitats were chosen for their distinct expected prey communities. Meconema species occur in both Freiburg and Ihringen, but the tree cricket Oecanthus pellucens is restricted in the region to the warm, arid vineyard areas around Ihringen at the Kaiserstuhl, whereas Meconema meridionale and Meconema thalassinum are more often found in gardens and near humid forests in eastern Freiburg. Six layer nests were installed in each habitat, at sites at least 300 metres apart, a distance considered plausible for preventing the same female from occupying nests at several sites, given observed flight distances between individual flowers of up to 140 metres.

Deployment on 25 July 2022 was timed to the wasp's flight period, whose observation peak on citizen-science platforms runs from mid-July to early September. The outcome was lopsided. Nests of I. mexicana appeared at four of the twelve sites, all of them in the vineyard habitat at Ihringen, and none in Freiburg, despite existing occurrences and observed nesting activity in neighbouring insect nesting aids there.

In total the researchers recorded 407 orthopterans, of which 297 were Oecanthus pellucens, 238 females and 59 males, and 110 were Meconema meridionale, 57 females and 53 males. Fifty-five completed brood cells contained an egg, holding 352 grasshoppers in all, an average of 6.4 prey individuals per cell. Most of the 55 nests were built in the 11-millimetre cavities; the 7- and 9-millimetre tubes were used at similar, lower frequencies, and the 5-millimetre cavities were never occupied at any site.

Statistical analysis in R, using a generalised linear model with Poisson distribution, showed that the number of prey individuals per cavity increased significantly with cavity diameter, with an estimate of 0.372 plus or minus 0.038 standard error, z = 9.817, p < 0.001. The number of eggs per cavity likewise rose significantly with the number of prey individuals, estimate 0.0916 plus or minus 0.009, z = 9.852, p < 0.001. By contrast, prey size told a different story.

Body length could be measured for 355 of the 407 prey individuals; for the rest, the larvae had already eaten the prey, leaving only highly sclerotised parts, though wings and genitalia still allowed species and sex to be determined. The mean body length was 11.92 millimetres for Meconema meridionale and 12.31 millimetres for Oecanthus pellucens, and mean prey length showed no relationship to cavity diameter. The authors attribute this to the fact that all occupied cavity diameters exceeded the body diameter of the prey, so no size-based selection could become visible.

Two further findings stood out. First, the wasps colonised the lower layers of the stacked nest boards particularly frequently, and while some prey items were deposited in the top layer, brood cells there contained no eggs. The authors consider it unlikely that wasps carrying prey over hundreds of metres would choose a layer only a few centimetres lower to save energy; they suggest instead that upper layers experience more extreme temperature or humidity fluctuations, which the wasps may perceive and avoid, possibly relocating nesting activity to lower layers during the course of a hot day.

Why cavities in the top layer were nevertheless sealed with grass sticks remains open, and the authors raise the possibility that these too are mock nests, perhaps built to distract predators and parasites from egg-bearing cells. Second, the sex ratio of prey was skewed: females made up 52 percent of preyed Meconema meridionale, in line with earlier German findings, but fully 80 percent of Oecanthus pellucens, closely matching the 81 percent female share reported for Oecanthus prey in North America. Whether female tree crickets are simply more abundant, easier to catch, longer-lived in late summer or richer in protein cannot yet be distinguished.

On the central question of competition, the study offers reassurance with caveats. The strong preference for 11-millimetre cavities contrasts with North American reports of 6 to 7 millimetres and German reports of 8 to 10 millimetres, and native bees in Germany generally use diameters of 2 to 8 millimetres. Even when some 11-millimetre tubes remained unoccupied, the wasps partly used 7- and 9-millimetre cavities, where overlap with native species such as Osmia cornuta, O. bicornis and O. adunca could occur.

However, the late breeding onset of I. mexicana, after the main bee season, weakens direct competition, since native bees have already claimed their tubes by then; the potential conflict shifts to the following spring, if wasp pupae still occupy tubes when bees emerge. One genuine rival is another alien, the giant resin bee Megachile sculpturalis, which also prefers large tubes and has been observed robbing out I. mexicana nests to reuse the vacated space. The authors also note that many commercial nesting aids offer mainly very large cavity diameters, a resource that is almost unrivalled for the wasp, and recommend offering smaller and medium diameters to specifically promote native bees.

The authors are careful about a label often attached to the species: invasive. In entomology, the term is usually reserved for insects that establish, spread rapidly and cause substantial ecological, economic or social harm in their new environment, and mere duration of presence is not sufficient for such an assessment. This study found no evidence of competition with native cavity-nesters, but rather successful integration, and the authors propose that, on current evidence, I. mexicana is better categorised as a neozoic species than an invasive one.

They stress that further research is needed on the wasp's natural history and its effects on local Ensifera populations, including whether mock nests serve an anti-parasite function, whether competition with other sphecid wasps occurs, and how prey choice varies across habitats with different orthopteran communities. The study period covered a single flight season and only two habitats, and the wasp occupied nests at only one of four sites where it was already present, so the picture of its ecological impact remains deliberately provisional.

Subject of Research: Nesting cavity and prey preferences of the alien digger wasp Isodontia mexicana in southern Germany

Article Title: Prey and nesting preferences of the alien wasp Isodontia mexicana ( Hymenoptera , Sphecidae ) in southern Germany

Article References: von Adelmannsfelden, S., & Fornoff, F. (2025). Prey and nesting preferences of the alien wasp Isodontia mexicana (Hymenoptera, Sphecidae) in southern Germany. Journal of Hymenoptera Research, 98, 603-619. https://doi.org/10.3897/jhr.98.148947

Image Credits: AI Generated

DOI: 10.3897/jhr.98.148947

Keywords: Isodontia mexicana, alien species, digger wasp, nesting preference, trap nests, Orthoptera, Oecanthus pellucens, Meconema meridionale, cavity-nesting bees, biological invasion, Kaiserstuhl, Hymenoptera

The Entomophthorales and Neozygitales, the two orders that make up the phylum Entomophthoromycota, are dominated by arthropod-pathogenic species, although saprobiontic members also occur. Because they can decimate insect populations in outbreaks known as epizootics, they are regarded as important natural regulators of arthropod numbers and hold considerable potential for biological pest control. Despite this ecological significance, the group has never been the subject of dedicated large-scale survey work, which is why new national records continue to surface from ordinary field collections.

The Swiss material originated from several localities and collectors. At Bümpliz in the canton of Bern, a strikingly high number of infected bush-crickets turned up in a cemetery, on the underside of leaves of a southern catalpa, Catalpa bignonioides. Further samples came from Muttenz and Duggingen in Basel-Landschaft, where cadavers were found on catalpa and on European beech, Fagus sylvatica, and from Eschenz in Thurgau, where hazel, Corylus avellana, and a Prunus species served as collection substrates.

One bush-cricket was even recovered from the ceiling of a house entrance in Regensdorf in the canton of Zurich. The single infected cockroach was found on ivy, Hedera helix, in the city of Bern. Collections spanned September 2020 through September 2021, with the bulk made in August 2021.

Both host species are Mediterranean in origin but are spreading northwards. Ectobius vittiventris has colonized Germany as far as North Rhine-Westphalia and has been recorded in the Alps up to 1400 metres altitude, while Meconema meridionale has spread across western Europe to the North Sea, apparently avoiding higher elevations. The authors suggest that these expanding populations may be encountering fungal pathogens for the first time in their new ranges, a hypothesis that could explain the unusually severe infections observed in Switzerland.

Laboratory work followed classical mycological protocols. Selected infected bush-crickets and the dead cockroach were placed individually in small Petri dishes with water, and microscope slides suspended above the cadavers collected the primary conidia actively discharged by the fungus. Secondary conidia were picked from the water surface.

Fourteen bush-cricket cadavers and the single cockroach cadaver were then dissected, and the fungal material was mounted in lactophenol-cotton blue or lactophenol-aceto-orcein stains. Measurements were taken only from primary conidia lying with their axis parallel to the slide and with the entire spore outline in focus, with twenty-five structures measured per host individual as a standard series, and multiple series examined to capture variation. The fungus attacking Meconema meridionale proved to be Zoophthora giardii.

The host body was completely filled with hyphal bodies, and compound rhizoids anchored the cadaver firmly to the leaf surface. The conidiophores were branched, and their terminal portions, from which the primary conidia develop, detached before spore formation. The primary conidia were subcylindrical with the widest diameter in the middle, averaging 20.0 to 21.6 micrometres long by 6.8 to 7.2 micrometres wide, with a length-to-diameter ratio of 2.86 to 3.08.

These conidia germinated in two ways: some produced a capillary germ tube ending in a type II secondary conidium, or capilliconidium, while others formed a short thick tube giving rise to a type I secondary conidium resembling the primary spore. The fusiform, slightly bent capilliconidia averaged 20.1 to 20.8 by 5.7 to 6.0 micrometres and developed at the tips of capillary tubes averaging 75.1 to 92.4 micrometres in length. Resting spores were absent.

The identification was confirmed by comparison with the original description of Zoophthora giardii published by Bałazy in 1993, which reported primary conidia of 19.5 to 23.5 by 6 to 7 micrometres and capilliconidia of 21 to 25 by 5.5 to 6.5 micrometres. The minor differences in capilliconidium length between the Swiss material and the original account were judged small enough to be attributable to different preparation and staining methods. Zoophthora giardii was previously known from France, Germany and Poland, where Bałazy considered it rather rare, so its appearance in Switzerland is not unexpected.

What is striking, however, is that in Switzerland the fungus was found at several localities and occasionally reached epizootic levels, a contrast the authors attribute to a difference in host. Bałazy had found the fungus on the oak bush-cricket Meconema thalassinum, whereas the Swiss infections occurred on M. meridionale, a newly documented host. It is possible that this species never encountered the fungus within its original distribution area and is therefore more susceptible to infection.

The cockroach pathogen was identified as Conidiobolus gustafssonii. The diseased Ectobius vittiventris was fixed to an ivy leaf with compound rhizoids, wings slightly opened, and surrounded by a white halo of discharged conidia. Inside the body cavity, the hyphal bodies were irregularly rounded, occasionally composed of two or more rounded elements or short hypha-like structures.

In contrast to Zoophthora, the conidiophores were unbranched, each bearing a single conidium, and the primary conidia were round to slightly pyriform with the spore body merging smoothly into a rounded papilla. Conidia were measured three ways: projected conidia averaged 43.2 by 35.6 micrometres, conidia taken from the leaf 40.6 by 31.5 micrometres, and conidia prepared from host tissue 39.9 by 31.4 micrometres. The projected spores were slightly larger, an artefact of compression between slide and cover glass; the other two preparations contained non-fungal material that prevented compression and thus yielded more accurate values.

The authors therefore consider 40 to 41 by 31.5 micrometres, matching Bałazy's original description of 41 to 47 by 30 to 40 micrometres, as the reliable size for this material. Primary conidia germinated with a single short tube to form a secondary conidium resembling the primary one, though no fully developed secondary conidia were observed, and resting spores were again absent. Bałazy had described the species from Ectobius lapponicus, so E. vittiventris likewise constitutes a new host record.

Voucher material has been deposited at the United Herbaria Zurich under the accession numbers ZT Myc 66704 and 66705 for Zoophthora giardii and ZT Myc 66706 for Conidiobolus gustafssonii. With these additions, the number of entomophthoralean fungi known from Switzerland rises to ninety species, a diversity unmatched by any other country for classified arthropod-pathogenic Entomophthoromycota. Pre-alpine valleys have proven especially rich: more than a third of the species recorded in Switzerland originate from the headwaters of the rivers Murg in Thurgau and Töss in Zurich, where relatively undisturbed riverine forests harbour a rich insect fauna that evidently supports a correspondingly rich fungal fauna.

The authors expect a similar effect in alpine ecosystems, which remain unexplored with respect to insect-pathogenic fungi. They conclude that further species of Entomophthoromycota certainly await discovery, not only in Switzerland but worldwide, since the group has never been systematically studied, even though it plays an important role in regulating arthropod populations and carries substantial potential for pest control. The association of these first records with northward-expanding Mediterranean hosts also hints that range shifts may reshuffle host-pathogen relationships, with newly arrived insects potentially meeting pathogens they have never evolved defences against.

Subject of Research: First Swiss records of the entomopathogenic fungi Zoophthora giardii and Conidiobolus gustafssonii on newly documented insect hosts

Article Title:  Zoophthora giardii Bałazy and Conidiobolus gustafssonii Bałazy ( Fungi , Entomophthorales ), two entomopathogens new for Switzerland

Article References: Keller, S. (2022).  Zoophthora giardii Bałazy and Conidiobolus gustafssonii Bałazy (Fungi, Entomophthorales), two entomopathogens new for Switzerland. Alpine Entomology, 6, 19-23. https://doi.org/10.3897/alpento.6.83182

Image Credits: AI Generated

DOI: 10.3897/alpento.6.83182

Keywords: entomopathogenic fungi, Entomophthorales, Zoophthora giardii, Conidiobolus gustafssonii, Meconema meridionale, Ectobius vittiventris, Switzerland, new host records, insect pathogens, biological pest control, biodiversity, Zoophthora