An Unusual Attack Pattern of Scolytus Multistriatus Beetles in Chinese Elm

In autumn, some Chinese elm (Ulmus parvifolia Marsham) trees, especially those with large seed crops, attract Scolytus multistriatus beetles. The beetles land on a single tree in masses and attempt to penetrate the inner bark. The tree responds with a copious flow of sticky sap that kills beetles at entrances or repels them. As the sap flow increases, Scolytus beetles aggregate on the tree in even larger numbers and continue to excavate very short, erratic, and shallow tunnels in the outer bark. The basal to mid-bole section was attacked with highest density (17 to 18 tunnels per dm2, with a male-to-female ratio of 1:8.7). Perhaps due to a high concentration of host volatiles and insect-produced attractants, S. multistriatus beetles shifted their attack to adjacent elms. However, the beetles avoided penetrating the inner bark of these trees and excavated only shallow tunnels in the outer bark. The beetles also showed a preference for tunneling into the basal to mid-bole section, but high mortality was observed (36.3% to 40.7%, with a male-tofemale ratio ranging from 1:3.8 to 1:4.3). The number of female beetles and number of feeding scars in the crotches of small twigs were dependent on the number of females in the stem and branches (P> 0.001).