Assessing the impact of temperature changes on insect population and overwintering on nontarget hosts is important for prediction of nontarget effects in weed biological control. Agasicles hygrophila is a beetle used for biological control of the invasive plant alligator weed, Alternanthera philoxeroides, with nontarget damage to a native plant, Alternanthera sessilis. In this study, we monitored plant growth and phenology along with beetle population and overwintering on these two hosts along a latitudinal gradient from subtropical to temperate climates (Guilin, Wuhan and Kaifeng) in China. We found only annual A. sessilis seedlings in temperate Kaifeng, but both annual seedlings and perennial ramets of A. sessilis were found in Guilin and Wuhan. However, in winter, living A. sessilis plants were found in only subtropical Guilin. Beetles at the Guilin site could successfully maintain their populations and overwinter on A. sessilis. Although the beetle could sustain its populations on A. sessilis at the other two higher-latitude sites during the growing season, it failed to overwinter on either species, indicating that the temperatures in different climate zones may directly and/or indirectly affect the development of insect biological control agents and their population sizes on nontarget hosts. Therefore, it is important to consider the shifting plant–insect interactions induced by climate when assessing potential nontarget effects of species introduced as biological control agents.