Habitat Associations of Euphorbia and Aphthona Species from Europe: Development of Predictive Models for Natural Enemy Release with Ordination Analysis

Abstract Biotic and abiotic factors can have a profound influence on the occurrence, species composition, structure, distribution, relative abundance, and dynamics of plant species and their associated natural enemies. Elucidation of some of the more relevant habitat characteristics for a pestiferous plant or phytophage species in its place of origin may enable better prediction of its rates of colonization and spread should it accidentally be introduced into new areas. A similar analysis of the habitat requisites for the associated natural enemies of the plant/phytophage should enable the development of predictive models that identify habitat factors conducive to the establishment and impact on the particular plant or phytophage species. In this study, habitat associations were characterized for four different Euphorbia species and their associated flea beetle species in the Aphthona complex from 17 field sites in Europe, representing xeric, mesic, and hydric habitats. Micro- and macronutrient analyses were conducted on soil and spurge foliage and roots; physical properties of the soil were analyzed; plant cover of grasses, forbs (a broad-leaved herbaceous plant), and Euphorbia species (and total plant productivity) were estimated at each of the sites, and relative abundance counts were made for each of the flea beetle species at each of the 17 sites during spring, early summer, and mid-summer 1991. Spurge species included Euphorbia cyparissias, E. lucida, E. seguieriana, and E. virgata. Flea beetles species included Aphthona cyparissiae, A. czwalinae, A. lacertosa, A. nigriscutis, A. pygmaea, A. venustula, and A. violacea. Ordination models generated for the spurge species suggested that E. virgata and E. lucida were associated with higher levels of soil matric potential, clay, organic matter, Ca, Fe, K, Mg, N, P, Zn (soil layer B ), and plant productivity. In contrast, E. cyparissias and E. seguieriana were associated with relatively lower levels of plant productivity and higher levels of sand, CaCO 3 , and Zn (soil layer A ). Ordination models developed for the Aphthona species suggested that A. czwalinae and A. lacertosa were associated with sites containing higher levels of clay and plant productivity and higher levels of Mn in the Euphorbia roots; A. violacea was associated with sites with intermediate levels of sand, clay, and plant productivity, in addition to spurge roots with higher levels of Cu, Fe, K, N, P, and Zn. A. nigriscutis was associated with sandier soils with lower levels of plant productivity and higher levels of Ca, K, and N in the Euphorbia roots. A. cyparissiae, A. pygmaea, and A.venustula were associated with moderately sandy soils with relatively lower levels of plant productivity and Euphorbia species with higher levels of Ca and N in the roots. The ordination models generated from this study provide the diagnostic framework for the identification of appropriate habitats and key site requisites that might be conducive to the establishment and impact of the Aphthona species on leafy spurge in North America.