Dominant insectivorous polyphagous predators in winter wheat: High colonization power, spatial dispersion patterns, and probable importance of the soil surface spiders (Araneae)

Abstract

Spiders inhabiting the soil surface of a winter wheat field were examined using pitfall traps near Zurich, central Europe. The soil surface spider community was dominated by two species of wolf spiders Pardosa agrestis (Westring) and Pardosa palustris (L.) (Lycosidae) and three species of sheet-web weavers Erigone atra (Blackwall), Erigone dentipalpis (Wider) and Oedothorax apicatus (Blackwall) (Linyphiidae). The identity and average number of spiders collected in the traps over time between field border and center were compared revealing that the number of wolf spiders on the border did not differ significantly from the center. However, the sheet-web weavers expressed a consistent trend of slightly greater numbers in the field's center compared to the border. Computation of the dispersion index (after Morisita) indicates aggregated patterns throughout the field for both Lycosidae and Linyphiidae caught in the pitfall traps. The combined number of spider individuals showed an overall increase as the season progressed. Over 12000 spiders were collected from the 2 ha winter wheat field by means of 45 (7 cm diam.) pitfall traps in less than 3 months, whereby this high number is providing additional evidence for previous work reported in the literature of winter wheat field colonization by large numbers of spiders. The capacity of soil surface spiders to build up these high numbers, potentially consuming field crop insect pests at elevated rates suggests that they may play a significant role as natural control agents. The effectiveness of these spiders as natural enemies may be limited only by their apparently low feeding frequency reflecting a low rate of metabolism.