Herbivorous insects generally choose host plants that optimize growth. However, many specialists choose chemically defended host plants that slow growth, prioritizing protection from natural enemies. Invasive specialist herbivores may also be unpalatable in their non-native range where they lack co-evolved natural enemies. Here, we explore the diet choices of the invasive yellowmargined leaf beetle, Microtheca ochroloma. We measured M. ochroloma preferences among six Brassica plants varying in glucosinolate concentrations. In both field and laboratory experiments, M. ochroloma preferred the less-toxic host plants mizuna and Chinese cabbage. In the field, predatory insects were least common on mizuna, suggesting that less-defended plants may not increase the invader’s susceptibility to predation. The positive preference-performance relationship we observed suggests that M. ochroloma prioritizes growth over protection against natural enemies. By identifying mizuna as a highly-preferred host for this invasive pest, our results may inform trap cropping strategies that limit damage in agricultural systems.