Ailanthus altissima (Mill.) Swingle and its variant A. altissima var. Qiantouchun are notorious invasive weeds. Two weevils, Eucryptorrhynchus scrobiculatus (ESC) and E. brandti (EBR) are considered as candidates for biological control of A. altissima. The aim of this study was to model the potential distributions of ESC and EBR using CLIMEX 4.0. The projected potential distributions of ESC and EBR included almost all current distribution areas of A. altissima, except Southeast Asia. Under historical climate, potential distribution area of EBR is larger than that of ESC, 46.67 × 10⁶ km² and 35.65 × 10⁶ km², respectively. For both ESC and EBR, climate change expanded the northern boundary of potential distributions northward approximately 600 km by the middle of 21st century, and 1000 km by the end of 21st century under RCP 8.5. However, the suitable range decreased to the south in the Southern Hemisphere because of heat stress. The modelled potential distributions of ESC and EBR in the United States demonstrated that the climate was suitable for both weevils. Therefore, considering only climate suitability, both ESC and EBR can be considered as potential biological control agents against A. altissima with some confidence that climatic conditions are likely suitable.

In Northern China, the tree-of-heaven root weevil (TRW), Eucryptorrhynchus scrobiculatus (Motschulsky) (Coleoptera: Curculionidae), is a serious borer pest of Ailanthus altissima (Mill.) Swingle (Sapindales: Simaroubaceae) (tree-of-heaven) is an important tree species for landscapes and afforestation. This study evaluates the efficacy of trunk trap nets (TTN), thiacloprid spray (TS), and cypermethrin spray (CS) alone, and in combination with TTN and thiacloprid or cypermethrin spray (TTNTS or TTNCS) for controlling TRW in A. altissima plantations in 2016–2018 in three sites (Haojiaqiao, Wutongshu, and Taojiajuan), which are located near Lingwu city in Ningxia, China. TTN, TTNTS, and TTNCS treatments significantly reduce marked TRW captures (more than 93% with respect to the untreated trees) and wild TRW captures in population monitoring trunk trap nets (MTTN) deployed in stands (more than 55% with respect to the untreated trees). Further field trials demonstrate that these TTN, TTNTS, and TTNCS treatments significantly reduce damage to A. altissima in stands; there are no significant differences between TTN and combination treatments. In addition, significantly more TRW are captured in MTTN within Taojiajuan than within Haojiaqiao or Wutongshu in field trials 2018. There were no significant differences between sites in 2016 and 2017. This study indicates that the TS and CS treatments used had very little to no impact on TRW populations and tree health in the timeframe examined. TTN alone are clearly effective for suppressing populations of TRW and stabilizing A. altissima tree health. This physical control technique, using TTN, could be sufficient to manage TRW on tree-of-heaven with no insecticide.

Temporal, spatial, and trophic niche differentiation usually occurs among species living in similar habitats which can reduce interspecific competition under limited resources. Eucryptorrhynchus scrobiculatus and Eucryptorrhynchus brandti are sympatric and closely related species and are host-specific to Ailanthus altissima (Mill.) Swingle in China. However, the possibility of niche differentiation between the two species is not clear. Therefore, we investigated the density of the two weevils temporally, spatially, and trophically (plant parts of A. altissima) at Lingwu farm and Haojiaqiao town, Lingwu city, Ningxia Hui Autonomous Region. No differences were found in temporal Levin’s niche breath index values ranging from 3.74 to 3.95 and temporal Pianka’s niche overlap values (0.98–0.99). There was little differentiation in the vertical spatial niche. The vertical distribution of E. scrobiculatus was mainly located from 0 cm (ground) to over 250 cm on tree-of-heaven. Eucryptorrhynchus brandti was found predominantly from 0.1 to 50 cm to over 250 cm on tree-of-heaven. The overlap of the cardinal direction spatial niche was 0.90–0.98, indicating no significant overlap in the cardinal direction at Lingwu farm and Haojiaqiao town. Significant differentiation of trophic niche occurred with E. scrobiculatus feeding on 1-year-old branches, perennial branches, and petioles, while E. brandti feed on the stem.