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Invasive brown widow spiders avoid parasitism despite high densities

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Invasive species are sometimes less susceptible to natural enemies compared to native species, but the mechanism is often unclear. Here we tested two potential mechanisms for lower parasitism of invasive species: density-dependent parasitism and preference for human-dominated habitats. We investigated how variation in host density and habitat type affect egg sac parasitism in two widow spider species (family Theridiidae). We compared parasitism on the egg sac of the brown widow, Latrodectus geometricus, an urban invasive species, and the white widow, Latrodectus pallidus, a species native to Israel. To investigate variation in host and parasitoid density, we measured nearest-neighbor distance between spider webs and parasitism rates in 16 sites, and in a single site monthly throughout a year. In L. pallidus, denser sites were more heavily parasitized (up to 55%) and parasitism rate increased with population density throughout the season. Extremely dense L. geometricus populations, however, had very low rates of parasitism (0–5%). We then conducted an egg sac transplant experiment in human-dominated and natural habitats. We found no parasitism of either species in the human-dominated habitat, compared to 30% parasitism of both species in the natural habitat. In addition, we found evidence for higher predation of L. pallidus than of L. geometricus egg sacs, particularly in the natural habitat. These combined results suggest that the human-dominated habitats inhabited by L. geometricus have a lower abundance of predators and parasites. We conclude that lower parasitism and predation in human-dominated habitats could contribute to the invasion success of L. geometricus.

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We thank Ishai Hoffmann, Ofir Altstein, and Daniel Alfred J for their assistance collecting spider egg sacs and in the field transplant experiment. Thanks to Vardit Makover as well as Sophia Reve and her family for allowing us to use their gardens for field transplant experiments.


This work was supported by a Zuckerman STEM Leadership Postdoctoral Fellowship to MAM. MS was supported by The Institutes of Advanced Studies at Hebrew University.

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MAM, YL, and MS conceived and designed the experiments. MAM, VA, and TR performed the experiments. MAM analyzed the data and wrote the initial draft of the manuscript. All authors provided editorial comments and made contributions to the final manuscript.

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Correspondence to Monica A. Mowery.

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The authors have no relevant financial or non-financial interests to disclose.

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Communicated by Martin Gossner.

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Mowery, M.A., Arabesky, V., Rozenberg, T. et al. Invasive brown widow spiders avoid parasitism despite high densities. Oecologia 202, 143–150 (2023).

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