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Conventional agriculture is highly dependent on energy, fertilizer and pesticide subsidies (Pimentel et al., 1975; Reganold et al., 1990; Barrett et al., 1990). Intercropping, the simultaneous cultivation of two or more crops in the same field, has potential to reduce the use of high-subsidy inputs, to prevent soil erosion and to provide habitats for wildlife (Altieri et al., 1983; Horwith 1985; Rodenhouse et al., 1992). Intercropped agroecosystems also have been shown to limit insect pest populations (Root, 1973; Bach, 1980a, b; Risch, 1981; Karieva, 1983; Letourneau and Altieri, 1983; Risch et al., 1983; Kemp and Barrett, 1989). Risch et al. (1983), for example, reviewed 150 studies of insect pest populations in intercropped systems and found that of 198 insect species reported in these studies, 53% had lower populations in the intercrops, 18% had higher populations, 9% showed no significant difference, and 20% had a variable response.
Natural enemies and the resource concentration hypotheses (Root, 1973) are two mechanisms proposed to explain lower densities of insect pest populations in intercropped systems. The natural enemies hypothesis predicts that predators and parasites of insect pests are more abundant in vegetationally diverse systems and that their increased presence reduces populations of insect pests. Predator and parasite populations presumably benefit from alternative sources of food and refuge available in these more diverse systems. The resource concentration hypothesis attributes lower populations of insect pests in diverse systems to interference by nonhost plants with host-locating (e.g., chemical) mechanisms in insect pests. In other words, insect pest populations in monoculture systems are greater because each individual can more easily locate and forage on crop species. Nonhost plants may also indirectly hinder the host-locating process by altering microclimatic factors such as light and humidity (Root, 1973; Risch, 1981). Replicated, large-scale studies are needed, however, to test these hypotheses at the agroecosystem level.
Kemp and Barrett (1989) tested these hypotheses within experimental soybean agro-ecosystems at our study site. They found little evidence that intercropped strips increased numbers of insect predators in the soybean crop. They did find, however, that grassy strips located between soybean strips significantly increased the proportion of the green clover-worm (Plathypena scabra) infested by the fungal pathogen (Nomuraea releyi). Grassy inter-cropped strips also appeared to confer an "associational resistance" to invasion by migrating adult potato leafhoppers (Empoasca fabae). The "resistance by association" hypothesis suggests that biotic, chemical and microclimatic structure of nonhost plant species lowers herbivorous pressure in intercropped or diverse systems as compared to monoculture systems (Tahvanainen and Root, 1972; Root, 1973; Letourneau, 1990). For example, in a complex cropping system or habitat the chemical stimulus responsible for the attraction of a particular insect pest may become lost or diluted (Schoonhoven et al., 1981). Because of these differences in dispersal behavior, additional studies are necessary to quantify the effects of intercropping on rates and patterns of insect dispersal within soybeans.
We selected a generalist herbivore species, the Japanese beetle (Popillia japonica Newman), to quantify dispersal behavior and rates of movement in monoculture vs intercropped …