The tree-of-heaven trunk weevil, Eucryptorrhynchus brandti (Harold) (Coleoptera: Curculionidae), is one of the most harmful pests that damage the tree of heaven, Ailanthus altissima Swingle (Sapindales: Simaroubaceae). Aggregation behavior tests of E. brandti adults were conducted in laboratory conditions. The effects of temperature and light on the aggregation behavior of adults were tested, and the effect of sex and host was conducted with binomial choice experiments. The results showed that (1) the adults aggregate in both light and dark environments but preferred the dark environment, (2) temperature can drive the aggregation of E. brandti adults, (3) host plants could trigger E. brandti adults’ aggregation behavior, which is probably related to phytochemicals and insect feeding and localization, (4) there was mutual attraction of males and females and chemical attraction of crude intestinal extracts of males and females, and (5) aggregation behavior of E. brandti adults may also be related to the mediating of physical signals in insects. In this study, aggregation behavior can help us understand conspecific interactions and discover some strategies for effective control.

Eucryptorrhynchus scrobiculatus (Motschulsky) (Coleoptera: Curculionidae) is a notorious pest of Ailanthus altissima (Mill.) Swingle (Sapindales: Simaroubaceae). E. scrobiculatus adults typically aggregate under leaves and in soil crevices at the base of A. altissima in the field. We hypothesize that the environmental factors and conspecific signals determine their aggregation behavior. To test this, we investigated adult numbers in light-exposed and shaded areas of the sample trees and conducted experiments in both field and lab settings. Results revealed that (i) greater adult distribution in shaded areas; (ii) significant influence of temperature and illumination on aggregation tendency in the field; (iii) no gender-based difference in aggregation degree and maximum aggregation between light and dark; (iv) the host plant triggering the aggregation tendency, negatively affected in the absence; (v) the aggregation tendency of E. scrobiculatus weakened with the temperature gradually changing to ordinary temperature; and (vi) mutual attraction and chemical attraction between males and females. Thus, the aggregation behavior was influenced by factors including temperature, light intensity, host plant, and conspecific signals, but light’s role was not obvious in the lab.

Eucryptorrhynchus scrobiculatus (Motschulsky) and E. brandti (Harold) are 2 serious pests inflicting damage on Ailanthus altissima (Mill.) Swingle. In the field, these species exhibit aggregation behavior. We hypothesized that this behavior facilitated the contact and horizontal transfer of Metarhizium anisopliae (Metschn.) Sorokin in weevil species. Little or no mortality in both E. scrobiculatus and E. brandti adult individuals exposed to low concentrations M. anisopliae (1 × 105, 1 × 106 conidia/ml). A mortality rate of 100% was observed in both E. scrobiculatus and E. brandti adults after 3 and 5 d of treatment with M. anisopliae at concentrations of 1 × 109 and 1 × 108 conidia/ml. The aggregation behavior of E. scrobiculatus and E. brandti adults was unaffected by M. anisopliae. In the transfer test of M. anisopliae, the mortality rate of recipients mixed with donors treated with M. anisopliae conidia was significantly higher compared to untreated donors and control recipients in laboratory and field experiments. Furthermore, the mortality of secondary recipients in both E. scrobiculatus and E. brandti was significantly higher than that of the control group. The findings suggested that horizontal transfer of M. anisopliae occurred in both species, which was further supported by microscopy observation and the activity of immune-related enzymes in the donor, recipient, and secondary recipient. Our findings demonstrated a specific method for improving pest control by combining aggregation behavior with the use of biopesticides, thereby enhancing the understanding of biological management strategies.