Pre-release assessment of Gadirtha inexacta, a proposed biological control agent of Chinese tallow (Triadica sebifera) in the United States

Abstract

Native to China, Chinese tallow, Triadica sebifera (Euphorbiaceae) is an aggressive woody invader in the southeastern United States. The noctuid, Gadirtha inexacta (=Iscadia inexacta), is a multivoltine herbivore attacking this plant in China. To evaluate its potential as a biological control agent in the United States, we conducted experiments on the host-specificity and impact on the target weed in China. The host range was tested on 78 plant species in 21 families through no-choice larval development tests and field surveys. The results showed that G. inexacta could only complete development on T. sebifera, T. rotundifolia and T. chihsinianum, suggesting a narrow host range. Attack by G. inexacta larvae significantly reduced the growth of seedlings of T. sebifera and the impact was greater when the plant was repeatedly defoliated and in shaded conditions. The plants were entirely defoliated after being repeatedly damaged in the treatments of the highest larval density (six larvae per seedling) in shade, causing 80% seedling mortality and the total biomass was reduced to 60% of the control. The results of this study suggested that G. inexacta may be a suitable biological control agent of T. sebifera though further tests on native North American species are needed.