Modelling herbivore movement and colonization: pest management potential of intercropping and trap cropping John E. Banks 1 and Barbara Ekbom 2 1 Interdisciplinary Arts & Sciences, University of Washington, Tacoma, 1900 Commerce Street, Tacoma, WA 98402, U.S.A. and 2 Department of Entomology, Swedish University of Agricultural Sciences, PO Box 7044, S-75005 Uppsala, Sweden Abstract 1 Using a stochastic simulation model, we explored the effects of agroecosystem diversity on herbivore densities. 2 Using parameters that included reproduction, colonization, and local movement rates, we simulated an insect herbivore population colonizing rows of plants in an agricultural setting. 3 Plant rows were comprised of either principal crop, intercrop, or trap crop. Herbivore parameters varied for different plant types. 4 Percent crop cover and movement rates were varied, and ensuing herbivore densi- ties on crop rows were recorded. 5 In trap cropping schemes, both percent crop cover and movement rates were criti- cal in determining herbivore densities. Intercropping schemes were governed pri- marily by colonization rates. 6 These results suggest that trap cropping schemes merit more attention than inter- cropping systems in the design and analysis of mixed cropping systems. Keywords Emigration, insect behaviour, simulation model, vegetation diversity. Introduction Despite the fact that for decades vegetational diversity has been suggestedasastrategyforcropprotectionagainstinsectsbothin the form of intercropping (Andow, 1991) and trap cropping (Hokkanen, 1991; Javaid & Joshi, 1995), it is still anomalous in commercial agriculture. One reason that farmers have not yet embraced diversity as a cultural control, apart from economic and political reasons (Brummer, 1998), is that any prescription for the reduction of insect damage in today's agricultural systems must be accompanied by an indication of the predictability and reliability of the suggested technique. Unfortunately, the results of decades of ®eld studies in agroecology do not bode well for the establishment of a generalized, reliable protocol for deploying diversity in crop ®elds. For example, a review by Andow (1991) indicated that population densities of insect pests were lower in polyculture than in monoculture in only approximately 52% of the reviewed cases.Inamorerecentmeta-analysis,Tonhasca&Byrne(1994) found lower herbivore densities in polyculture in 60±70% of the studies used in their analysis. Despite these tolerable results, the reduction of herbivorous insects in diversi®ed crops were, according to the authors' interpretation, too low to be a reasonable value for practical conclusions. More recent work has attempted to elucidate mechanisms underlying the ambiguous results of past ®eld studies. Vandermeer (1989) proposed two mechanisms to explain how vegetation diversity can directly affect herbivore populations: (1) the disruptive crop hypothesis, which is equivalent to Root's (1973) resource concentration hypothesis, and (2) the trap crop hypothesis. The ®rst mechanism stipulates that insects will have dif®culty in ®nding, and be more likely to leave, crop plants associated with one or more taxonomically or genetically different plants than crop plants in monoculture. The second mechanism is the opposite to that of the ®rst, in that it supposes that pests will be attracted to rather than repelled by associated plants and hence be less likely to leave the trap crop and wander into the principal crop. In this paper we explore the tenets of the disruptive crop/ resource concentration (DC/RC) and trap crop (TC) hypotheses and their impact on insect herbivore pest populations. A related mechanism that indirectly affects herbivores in diversi®ed agroecosystems, is the natural enemies hypothesis (Root, 1973), which stipulates increased herbivore mortality due to enhanced enemy abundance or heightened rates of predation or parasitiza- tion in diverse systems when compared to moncultures. This hypothesis has received less support in the literature than the R Correspondence: John E. Banks. Tel:+1 253 692 4550, e-mail: banksj@u.washington.edu ã 1999 Blackwell Science Ltd Agricultural and Forest Entomology (1999) 1, 165±170 Agricultural and Forest Entomology (1999) 1, 165±170