Abstract: Epiblema strenuana is a gall inducing lepidopteran that was introduced into China as a biological control agent against ragweed, Ambrosia artemisiifolia. In Linxiang, Hunan Province, E. strenuana feeds on two species of host plants, A. artemisiifolia and Xanthium sibiricum. Ostrinia orientalis, an endemic insect that originally fed on X. sibiricum, is also a borer of A. artemissifolia. In order to assess the ecological impact of E. strenuana where it has been released, especially in locations where it coexists with O. orientalis on A. artemissifolia, a factorial design was used to conduct an experiment in a field cage. The results showed that both E. strenuana and O. orientalis reduced the biomass, seed production and height of the host plant. Biomass was reduced maximally by 94.3% in host plants with a 1.0 cm main stem following inoculation with 63. larvae of E. strenuana and 8 larvae of O. orientalis. Plant height was reduced maximally by 31.7% when 17 larvae of E. strenuana and 8 larvae of O. orientalis were inoculated on plants with a stem diameter of 1.0 cm. Seed production was reduced maximally by 99.3% when 40 larvae of E. strenuana and 5 larvae of O. orientalis were introduced onto a plant with a 0.7 cm main stem diameter. When 40 larvae of E. strenuana and 5 larvae of O. orientalis were inoculated on the host plant with a 2.1 cm main stem diameter, biomass and seed production were reduced by 2.0% and 6.1% respectively, but height increased by 2.4% Control efficiency decreased as the host plant grew because of the increasing capacity for compensatory plant growth. Feeding impact on the biomass, seed production and the height of the host plant, A. artemisiifolia exceeded compensatory growth during the early growth of the plant (main stem < 0.7 cm), but the situation was reversed in the oldest plants. The impact of E. strenuana on seed production was greater than O. orientalis, while the impact of O. orientalis on biomass and plant height was greater than E. strenuana. Furthermore, when these two Insects fed on A. artemisiifolia at the same time, their feeding behavior was independent, and no undesired consequences occurred. In order to utilize these two biocontrol agents efficiently, the insects should be introduced onto the target weeds during the early developmental stages of the host plant.

Key words: Epiblema strenuana, Ostrinia orientalis, Ambrosia artemisiifolia, biological control, control efficiency, risk assessment