1. Plant invasions can alter the behavior and performance of native herbivorous insects because the insects are evolutionarily naïve to the novel plants. An ecological trap results when native insects prefer invasive plants over their native hosts but suffer reduced fitness on the invaders. Although such traps are predicted to occur frequently given the prevalence of invasive plants, empirical support for ecological traps and their underlying mechanisms remains sparse. 2. We examined the potential for the invasive plant Spartina alterniflora to act as an ecological trap for the native moth Laelia coenosa, which previously fed mainly on the indigenous plant Phragmites australis in a Chinese saltmarsh. We surveyed Laelia egg densities on Spartina and Phragmites in the field, and determined adult oviposition preference and offspring development on the two plant species. To investigate the causes of adult preference and offspring performance patterns, we compared resource abundance in the field, plant-odor attractiveness, and leaf nutritional and defensive traits between Spartina and Phragmites. 3. We found that Laelia egg density and female preference for ovipositing were higher on Spartina than Phragmites. However, performance of offspring was poorer on Spartina than Phragmites. Spartina dominated a larger area and had greater leaf biomass than Phragmites in the field, and volatile odors released by Spartina were more attractive to Laelia females than those released by Phragmites. Although leaf C, C:P ratio, and terpenoid content did not differ significantly between the two plant species, Spartina leaves were tougher and more waxy, had lower N, and had higher concentrations of alkaloids and phenolics than Phragmites leaves. 4. Synthesis: Our data suggest that invasive Spartina can create an ecological trap for the native insect Laelia. This trap appears to result from environmental cues (resource availability and leaf odors) that attract the herbivore to the plant, but do not reliably predict the dietary qualities (nutrition and defenses) that negatively affect herbivore offspring performance. These findings reveal an important negative effect of plant invasions on resident herbivores and highlight the roles of resource availability and plant traits at different life stages of the insect.