Manipulating the abundance of natural enemies in ornamental landscapes with floral resource plants

We manipulated densities of floral resource plants to test the hypothesis that natural enemies would be more abundant in artificial ornamental landscapes that contained high densities of floral resource plants than in landscapes without these plants. We established an experimental landscape consisting of 3 x 3 in plots that contained a central bed of Euonymus fortunei (Turcz.) and either a low or high density of four species of perennial flowering plants, or no flowering plants. The cultivars of perennials, Trifolium repens L., Euphorbia epithymoides L., Coreopsis verticillata L. var. 'Moonbeam,' and Solidago canadensis L. var. 'Golden Baby,' were chosen because their staggered bloom periods would provide nectar and pollen for natural enemies throughout the summer months. We used yellow sticky cards and a vacuum sampler to collect arthropods from experimental plots in 2000 to 2003. Natural enemies in general, and spiders and parasitic wasps specifically, were usually most abundant in euonymus beds surrounded by flowering plants. The number of parasitic wasps and total natural enemies on sticky cards was positively correlated with biomass of E epithlymoides within plots, and that of all flowering plants combined. Removal of inflorescences from plants in a subset of study plots did not affect patterns of natural enemy abundance or dispersion of natural enemies within the landscape, suggesting that vegetative characteristics of plants, rather than flowers, influenced the abundance of natural enemies. We conclude from these findings that the presence of floral resource plants in landscapes enhances the abundance of natural enemies. (c) 2005 Elsevier Inc. All rights reserved.