Colony size and parasitoid load in two species of colonial Metepeira spiders from Mexico (Araneae: Araneidae).

While a number of advantages may result from group living, it may also lead to increased levels of attack by parasites because groups may be easier to find. This leads to the prediction that levels of parasitism should increase with colony size. We test this prediction by comparing colony size and parasitoid load for two species of colonial orb-weaving spiders from Mexico, Metepeira (undesc. sp., tentatively named atascadero) and Metepeira incrassata, which exhibit contrasting levels of social organization and utilize different habitats. For M. atascadero, which occurs solitarily or in small groups in desert/mesquite grassland habitat, rates of egg-sac parasitism fluctuate widely from year to year, and are closely tied to spider egg output. There is no relationship between colony size and rate of parasitism. For colonial M. incrassata, which occur in tropical rain forest/agricultural habitat, rates of parasitism are relatively constant from year to year. However, there is a positive relationship between colony size and rate of parasitism in this species. These differences are discussed with regard to the stability of the two habitats, prey availability, and the foraging behavior of the respective parasitoids.