Effects of spider presence on Artogeia rapae and host plant biomass

Manipulated field experiments were conducted during two seasons to evaluate the impact of spiders on the survivorship of the cabbage white butterfly, Artogeia rapae, inhabiting broccoli plants. The proportion of A. rapae egg cohorts surviving to their first and fifth (final) instar stages were evaluated during two and three field seasons, respectively. The treatments included check (spider removal) or spiders (spider present). During 1 year of the study, the proportion of A. rapae eggs surviving to the first instar stage was significantly reduced on spider compared to check treatment plants. The percentage of A. rapae eggs surviving to fifth instar stage was significantly reduced on spider compared with check treatment plants during each field trial. During the three field trials, opportunity for survival to the fifth instar stage was increased 1.7, 2.7, and 1.3 folds, respectively, on check compared to spider plants. Additionally, by experiment completion, vegetative plant biomass of spider “protected” plants was increased by 80, 121, and 28% contrasted with check plants.