Olfactory orientation responses of the eucalyptus woodborer, Phoracantha semipunctata, to host plant in a wind tunnel

Abstract

The eucalyptus woodborer, Phoracantha semipunctata Fabricius (Coleoptera: Cerambycidae), attacks mainly species of Eucalyptus (Myrtaceae). This study investigated walking and flight behaviour of P. semipunctata males and females exposed to an odour plume originating from a log of E. globulus placed vertically in the upwind end of a wind tunnel. In control experiments, beetles were exposed to a PVC drainpipe in the same position as the log, providing a visual stimulus without host-tree odour. No statistical differences were found between behavioural responses of either sex when exposed to the log or PVC pipe. No beetles landed on the PVC pipe, whereas 49% of the beetles exposed to host-tree odour plume landed on the log. Beetles aged over 24 days after emergence from the host tree were more responsive than beetles aged 20-24 days, and accounted for 86% of the beetles that landed on the log. While walking, host-tree odour affected the behaviour of the beetles that landed on the log as follows: upwind movement and path linearity increased, whereas turning rate, stopping frequency, mean stopping time and time to take-off flight decreased. During flight, host-tree odour affected the behaviour of the beetles that landed on the log as follows: increased upwind flight, turning rate, flight time, flight distance, and decreased flight speed. For beetles that never lost contact with the odour plume, flight progressed upwind with narrow zigzags, and showed higher directedness upwind, path linearity, faster flight speed and lower turning rate than for beetles that lost contact with the odour plume. After loosing contact with the plume, beetles tended to decrease their upwind progression, exhibiting a sharp turn or quick counterturns followed by crosswind or downwind excursions. This led to regaining contact with the odour plume and resumed upwind progression at higher speed provided they flew within the boundaries of the plume. The results showed that host-tree odour affects both walking and flight behaviour of P. semipunctata beetles, inducing a more directed upwind movement and landing on the visual stimulus of a tree trunk.