ABSTRACT
The presence of weeds within or around crop fields influences the dynamics of the crop and associated biotic communities. Studies over the past thirty years have produced a great deal of evidence that the manipulation of a specific weed species, a particular weed-control practice, or a cropping system can affect the ecology of insect pests and associated natural enemies (Van Emden, 1965b; Altieri, Schoonhoven, and Doll, 1977; Altieri and Whitcomb, 1979a,b; Thresh, 1981; William, 1981; Norris, 1982; Andow, 1983a). Weeds exert a direct biotic stress on crops by competing for sunlight, moisture, and some nutrients, thus reducing crop yields. Weeds indirectly affect crop plants through positive and/or negative effects on insect herbivores and also on the natural enemies of herbivores (Price et al., 1980). Herbivore-natural enemy interactions occurring in a crop system can be influenced by the presence of associated weeds or by the presence of herbivores on associated weed plants (Altieri and Letourneau, 1982). On the other hand, herbivores can mediate the interaction between crops and weeds, as in a natural community where the competitiveness of two plant species was altered substantially by the selective feeding of a foliage-consuming beetle (Bentley and Whittaker, 1979). Such relationships have been little explored in agricultural systems.
