A common feature of many agroecosystems is a reduction in species richness coupled with high populations of selected other species. With parasitic Hymenoptera, for example, the phenomenon may affect not only the interspecific relationships within certain parasitic spectra, but also the intraspecific diversity of individual parasitoids (Unruh and Messing, 1993). There may be changes in the gene flow between populations from different host species as well as certain dominance of features in populations from highly populated dominant hosts. These factors are important in the drift of parasitoids between different hosts. One of the main problems in agroecosystems is the lack of hosts for host alternation and relationships (Nemec and Stary, 1984; 1985; Powell, 1986).
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