Technical Abstract: Elevated tissue concentrations of metals have been shown to defend metal-hyperaccumulating plants against both herbivores and pathogens. We examined the effects of host switching on growth and survival of a generalist herbivore. Specifically, we tested the hypothesis that early larval development on non-toxic hosts could improve caterpillar performance on high-Ni Streptanthus polygaloides, a Ni hyper-accumulator. Initial larval performance (weight gain) was lowest for insects switched to high-Ni hosts. Decreased initial larval performance was also noted for insects switched from lettuce to low-Ni plants, but these larvae recovered quickly. Original host identity (lettuce or low-Ni plants) did not affect subsequent larval performance. By day 8 of the feeding trials, all larvae switched to high-Ni hosts had died. We concluded that polyphagous caterpillars are unable to counter Ni-based defenses via host switching.