Don't put all your eggs in small baskets: Ineffective guardians, incidence of parasitoids and clutch size of Latrodectus geometricus (Araneae, Theridiidae) along an urban gradient

Abstract

Maternal care is common in spiders, but the effects of egg protection by mothers on offspring survival remains understudied for most species, especially against parasitoids. Parasitoid incidence can also be modulated by host clutch size (since larger egg masses provide the opportunity of producing larger clutches for the parasitoids) and urbanization (by creating conditions that may restrict the occurrence of less tolerant species). In this study, we conducted field experiments with the brown widow spider Latrodectus geometricus to investigate the following hypothesis: (1) females are effective in the defense of eggs against parasitoids; (2) large clutch sizes are attacked more often by parasitoids than the smaller ones; and (3) the incidence of parasitoids in egg sacs decreases in a gradient from a preserved area to an urbanized one. The presence of caring females reduced by three times the incidence of two parasitoids, Baeus latrodecti (Hymenoptera, Scelionidae) and an unidentified Eulophidae wasp, compared to unattended egg sacs, but this distinction did not differ from a random distribution. Parasitoids were most often found in egg sacs containing few eggs. The incidence of B. latrodecti in egg sacs was highest in the area with an intermediate degree of urbanization, but Eulophidae wasps were unaffected by the urbanization gradient. We concluded that (1) females are generally ineffective at protecting their egg sacs against parasitoids, that (2) egg sacs containing small clutch sizes are usually more vunerable to parasitoid attacks, and that (3) urbanization does not reduce the incidence of the egg parasitoids identified here. Our study provides an important step towards our understanding of the interactions between spiders with parental care and egg parasitoids, suggesting new aspects to be investigated in future studies.