Ecological Entomology (1996) 21, 141-149 Movement of an insect parasitoid in simple and diverse plant assemblages MOSHE COLL and DALE G. BOTTRELL Department of Entomology, University of Maryland, College Park, Maryland, U.S.A. Abstract. 1. It has been proposed that herbivore populations are lower in diverse than in simple plant habitats because of greater abundance andor higher efficiency of natural enemies in mixed plant stands. However, higher enemy colonization is expected in mono- rather than multispecific vegetation if the response of specialistnatural enemies to habitat diversification is similar to that of monophagous herbivores. 2. We used release-recapture experiments to determine how the presence of maize (non-host plant) influences the movement of the parasitoid Pediobiusfoveolatus in the absence of hosts. We then assessed how vegetation diversity affects wasp reproduction (parasitism) and subsequent density in the presence of its hosts, Mexican bean beetle larvae. 3. Fewer female wasps immigrated into and more emigrated out of a bean-tall maize intercrop than bean monocultures. Bean plant density and the presence of maize per se did not significantlyaffect parasitoid immigration. Instead, maize height was the primary factor contributing to lower female immigration into the bean-tall maize plots. However, tall maize plants did not impede the wasps’ within-habitat movement. 4. When wasps were released outside the plots, higher parasitism was recorded in monocultures,irrespectiveof host density. In contrast, when wasps were released within the plots, significantly higher parasitism rates were found in the bean-tall maize habitat. 5. Results suggest that female wasps accumulate in the bean-tall maize habitat in response to resources other than hosts and, ultimately, wasp density may be determined primarily by differential emigration rather than by immigration rates. Key words. Colonization, vegetation diversity, dispersal, intercropping, parasitoids, natural enemies, Pediobiusfoveolatus. introduction Vegetation diversity often influences the abundance of insect herbivores by altering their movement (Turchin, 1986; Bergelson & Kareiva, 1987; Power, 1987; Bach, 1988; Elmstrom et al., 1988; Andow, 1990; and studies reviewed by Kareiva, 1983). This phenomenon is central to Root’s (1973) ‘resource concentration hypothesis’, in which he argues that monophagous insect herbivores are more likely to locate and remain in taxonomically pure plant assemblages than in diverse ones. If specialist natural enemies respond to habitat diversification in a manner similar to that of monophagous herbivores, then higher rates of immigration and lower rates of emigration would be expected in mono- than in multi-specific plant communities Correspondence: Dr Moshe Coll, Department of Entomology, Faculty ofAgriculture, The Hebrew University of Jerusalem, P.O. Box 12, Rehovot 76100. Israel. (Sheehan, 1986; Russell, 1989). Yet the ‘enemies hypothesis’ (Root, 1973)predicts, and limiteddata (Andow, 1991; Coll, 1995) suggest, that natural enemies are often more abundant in diverse than in simple habitats (but see Andow & Risch, 1987; Helenius, 1990;Van Huis, 198 I). To compare the impact of natural enemies on herbivore populations in simple and diverse habitats, it is therefore important to understand how vegetation texture influences the movement of parasitoids and predators in the field. A high concentration of host food plants in large monocultures may attract parasitoids that use plant cues to locate their hosts’ habitat. Emigration, on the other hand, may be induced after parasitoids encounter non-host plants in diverse plant stands. Further, based on the behaviour in other small insects, it is likely that the presence of non-host plants may interfere with the within- habitat (‘trivial’) movement of parasitoids by altering wind patterns or presenting physical barriers (cf. Bottenberg & Irwin, 1991). Such interference may occur especially when plants of 6 1996 Blackwell Science Ltd 141