Alien invasive plant species can invade various habitats and be grazed upon by different insects. Therefore, habitat conditions may affect the strategies of alien invasive plant species to defend against insect herbivory. We conducted a greenhouse experiment in which we grew ramets of the invasive herb Alternanthera philoxeroides collected from three different habitats (aquatic, semi-aquatic and terrestrial) and subjected them to two levels of simulated leaf herbivory, i.e. no defoliation (control) and 50% leaf removal. Simulated leaf herbivory significantly reduced root biomass, stem biomass, leaf biomass and total biomass of A. philoxeroides, but had no significant effect on root/shoot ratio, root mass ratio, stem mass ratio and leaf mass ratio. Root biomass, stem biomass, leaf biomass and total biomass of A. philoxeroides were higher in terrestrial environments than in semi-aquatic and aquatic environments, but root/shoot ratios in terrestrial habitats were significantly lower than in semi-aquatic and aquatic habitats. Simulated leaf herbivory significantly decreased lignin content of A. philoxeroides, but had no significant effect on the contents of tannins and phenols. Habitat types had no significant effect on lignin content of A. philoxeroides, but tannin content was significantly higher in terrestrial habitats than in semi-aquatic and aquatic habitats, and phenol content was also significantly higher in terrestrial habitats than in semi-aquatic habitats. These results suggest that the chemical defense of A. philoxeroides to insect herbivory may be higher in terrestrial habitats than in aquatic and semi-aquatic habitats. Total biomass of A. philoxeroides was significantly negatively correlated with phenol content under simulated leaf herbivory, and was significantly positively correlated with tannin content under both control and simulated leaf herbivory treatments. However, total biomass was not correlated with lignin content. These results indicate that insect herbivory can trigger a trade-off between growth and chemical defense in A. philoxeroides, i.e., herbivory results in investing in chemical defense at the cost of reduced growth, and habitat conditions have little effect on such a trade-off.