Movement of potato tuberworm, Phthorimaea operculella (Zeller), and diamondback moth, Plutella xylostella (L.), was examined indirectly by estimating their distribution, and directly by using mark-recapture techniques. Trapping techniques including trap tubers and trap plants were used to assess the distance that potato tuberworm moved from infested crops. These experiments suggested that a low proportion of moths foraged beyond 100–250 m to infest tubers or plants. Light traps indicated that the number of moths moving out from crops diminished over a 40-m distance. Dispersing moths penetrated 30 m into new crops to infest the foliage. Direct movement of potato tuberworm and diamondback moth between crops was estimated using mark-recapture experiments. Fluorescent dusts were more effective than felt pen for marking, and moths were recaptured with sweep-nets followed 1 d later by pheromone trapping. From sweep-net collections of potato tuberworm, a mean of ≈17% of moths was shown to move between crops. Only 1.2% of diamondback moth was recaptured by sweep netting outside the release area and very few moths were caught in pheromone traps. These results, together with the literature, suggest that sufficient potato tuberworm would forage between adjacent treated and untreated crops to minimize the development of insecticide resistance. The use of refuges to conserve susceptible pest populations is recommended for managing resistance that may arise from any future use of Bt-transgenic potatoes or Brassica spp. in New Zealand. Refuges that are intended to dilute potential resistance of potato tuberworm to transgenic crops should be placed close to transgenic potato crops.