Effects of manipulated density of the wolf spider, Pardosa astrigera (Araneae: Lycosidae), on pest populations and cabbage yield: a field enclosure experiment

We manipulated the density of the wolf spider, Pardosa astrigera L. Koch in enclosed cabbage plots (2 x 2 m) to assess the impact on the herbivore density and crop yield. P. astrigera was introduced to half of the enclosures at densities of 25 and 50 individuals/m(2) in the spring and autumn experiments, respectively; spiders were removed from other enclosures. The results indicated that P. astrigera significantly suppressed the densities of the diamondback moth, Plutella xylostella (L.), in the spring, and the green peach aphid, Myzus persicae (Sulzer), in the autumn experiments. The cabbage yield did not increase, despite the suppression of these herbivores. P. astrigera had no impact on three other lepidopteran species: Pieris rapae crucivora Boisduval, Autographa nigrisigna (Walker), and Mamestra brassicae (L.). The number of spiders increased by 23 % but decreased by 62 % at the end of the spring and autumn experiments, respectively, compared with the initial numbers introduced. Our results show that wolf spiders (Lycosidae) would be effective biocontrol agents of certain herbivore species if spiders abundantly occur early in the crop season and remain stable throughout the season. However, further investigations are required to determine the effect of spiders on the crop yield.