The brown widow spider, Latrodectus geometricus, is a hazardous and highly invasive species globally. One of the suggested mechanisms enhancing this spider’s invasiveness is lower susceptibility to natural enemies compared to other widow species. The parasitoid wasp, Philolema latrodecti, is known to attack egg sacs of L. geometricus, as well as those of other species of Latrodectus. Despite its potential importance as a natural enemy of L. geometricus, little is known about the development of this wasp and factors mediating its parasitism success. We investigated the development of this parasitoid in two widow spider host species, one native to Israel, the white widow, L. pallidus, and the invasive brown widow, L. geometricus. We compared wasp developmental success in egg sacs exposed to increasing wasp densities. We found increased wasp emergence and brood size, and smaller emerging wasp body size, with increasing wasp density. Across all wasp densities, more and larger parasitoids developed in the egg sacs of the native host species, L. pallidus, compared to the invasive host species, L. geometricus. Parasitoid sex ratio was highly female-biased and the proportion of males increased with wasp density, suggesting local mate competition. Overall, our results suggest that L. pallidus is a better host for the egg sac parasitoid P. latrodecti, which could give a competitive advantage to the less-affected invasive brown widow spider.

These data were collected at Ben-Gurion University in the Mitrani Department of Desert Ecology. Parasitism of egg sacs was done by MAM and VA. Wasp counting, body length measurement, egg sac diameter measurement, and sex ratio were done by MAM and ADJ. 

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