Parasitoid development and superparasitism in invasive versus native widow spider host egg sacs

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.