Oligophagous herbivores that move between host species may face tradeoffs between reduced reproduction and improved dispersal capabilities. We tested the hypothesis that, in oligophagous insects, larvae reared on a suboptimal host will eclose to adults with greater flight tendency but poorer reproductive output than adults that develop on optimal hosts. Accordingly, we studied the effects of larval food on the potato tuberworm, Phthorimaea operculella (Zeller) (Lepidoptera: Gelechiidae), an oligophagous herbivore in which some populations seem to have recently increased their host range to include tomato. Larvae that were fed tomato foliage eclosed to adults with a higher tendency to fly (i.e., were more likely to disperse) than those that developed on potato leaves, an optimal host (61.7 ± 8.7 versus 31.0 ± 3.3% were classified as “flyers” when reared on tomato versus potato foliage, respectively). A higher tendency to disperse from poor host plants would retard the evolution of host races in herbivore populations. Variations in larval food plants and in the tendency of adults to fly correspond with female wing loading, energy reserves (lipid content), and reproductive output. We suggest that the effect of host quality on adult dispersal is mediated primarily by biomechanical factors, such as wing loading.