Associational susceptibility has been predicted to arise when a target plant's neighbor is more palatable to an herbivore than the target plant itself, resulting in attraction and spill-over of the herbivore onto the target plant. This prediction was tested on two hosts of the invasive pyralid moth Cactoblastis cactorum Berg. A combination of observational surveys and experimental plantings were used to test for differences in herbivore damage and plant size on isolated Opuntia humifusa (Raf.) Raf. plants, O. humifusa paired with O. stricta (Haw.) Haw., and O. humifusa in conspecific pairs. Based on general patterns of associational effects and specific C. cactorum behavior, it was predicted that O. humifusa would have higher levels of C. cactorum damage when growing near O. stricta, as compared with isolated plants or those growing near conspecifics. However, little support for associational resistance or susceptibility was found. Surveys revealed that isolated O. humifusa plants had on average more cladodes than isolated O. stricta plants or paired plants. Additionally, O. humifusa paired with O. stricta had more C. cactorum larvae and old damage than pairs of O. humifusa, but did not have more than isolated O. humifusa plants. Plants from the site used for the experimental plantings had little to no C. cactorum damage, regardless of plant neighbors, and revealed no associational effects for native herbivores. The occurrence of associational susceptibility driven by a spillover mechanism may require a higher herbivore density and a sparser distribution of the herbivore's preferred host than was present at the sites use here.