Invasive brown widow spiders avoid parasitism despite high densities.

Area of Science:

• Ecology
• Invasive Species Biology
• Arachnology

Background:

• Invasive species often exhibit reduced susceptibility to natural enemies, but underlying mechanisms require elucidation.
• Parasitism and habitat preference are potential factors influencing invasive species' interactions with native fauna.

Purpose of the Study:

• To investigate how host density and habitat type influence egg sac parasitism in invasive brown widow (Latrodectus geometricus) and native white widow (Latrodectus pallidus) spiders.
• To test density-dependent parasitism and habitat preference as mechanisms for reduced enemy impact on invasive species.

Main Methods:

• Compared egg sac parasitism rates in relation to host density (nearest-neighbor distance) across 16 sites and monthly for a year.
• Conducted egg sac transplant experiments in human-dominated versus natural habitats.
• Assessed predation rates on egg sacs in different habitats.

Main Results:

• Native L. pallidus showed increased parasitism with higher host density (up to 55%).
• Invasive L. geometricus populations had very low parasitism rates (0-5%) even at high densities.
• No parasitism occurred in human-dominated habitats; 30% parasitism was observed in natural habitats for both species.
• Higher predation of L. pallidus egg sacs was noted, particularly in natural habitats.

Conclusions:

• Human-dominated habitats harbor fewer parasites and predators, benefiting invasive L. geometricus.
• Lower parasitism and predation in urban environments contribute to the invasion success of L. geometricus.
• Habitat preference and density-dependent effects on natural enemies are key factors in invasive species dynamics.