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