Within-Tree Distribution and Survival of the Eucalyptus Longhorned Borer Phoracantha semipunctata (Coleoptera: Cerambycidae) in a Mediterranean-Type Ecosystem

Abstract

The attack patterns, infestation success and larval development of woodborers within living trees are complex and are largely shaped by host tree characteristics. Following a severe drought in a native eucalypt forest where outbreak densities of a native Australian beetle, the eucalyptus longhorned borer (Phoracantha semipunctata), occurred, a tree dissection study was conducted in Australia. This involved felling 40 trees each of jarrah (Eucalyptus marginata) and marri (Corymbia calophylla) that were cut into 1-m sections and neonate larval galleries, larvae in pupal cells and adult borer emergence were measured and added to give total numbers per tree to determine the within-tree distribution and survival of P. semipunctata. There was a significant impact on larval survival in both species, in contrast, pupal survival remained high. Within-tree distribution of P. semipunctata was directional with borer emergence and incidence of larval galleries both negatively associated with tree section height above the ground and positively associated with section diameter and bark thickness, reaching a maximum towards the base of trees. High incidence and survival in lower thicker tree sections indicate a more conducive environment for larval development, in contrast to poor larval survival in smaller thinner sections at the top of trees. The dependence of larval survival on tree characteristics controlling the within-tree distribution of borer emergence is emphasized, and needs to be considered when estimating the spread of borer populations during outbreaks.

Impact of braconid wasps on larval performance of longhorned borer Coptocercus rubripes Boisduval (Coleoptera: Cerambycidae) in Eucalyptus forest of southwestern Australia

“…This is the first report of a major infestation of Co. rubripes in eucalypt trees, where logs at the clearfell and patch sites in the NJF of Southwestern Australia were only infested by Co. rubripes and not by P. semipunctata as previously thought to be the main borer infesting stressed trees in the region (Seaton et al 2015;Seaton et al 2020). Previously, only specimen records showed that Co. rubripes could attack Eucalyptus in Australia (Froggatt 1923) and was present in Glenn Forest in the Darling Range near Perth, Western Australia (Wang 1995a).…”

“…There was a lack of infestation of the clearfell logs by other cerambycids such as P. semipunctata (Seaton et al 2015), although the survey showed evidence that potential populations of both Co. rubripes and P. semipunctata were residing in old dead branches surrounding the clearfell site. In contrast, P. semipunctata generally occur in high numbers in summer (Seaton et al 2015(Seaton et al , 2020 and thus did not coincide with the time of the clearfell.…”

“…In contrast, P. semipunctata generally occur in high numbers in summer (Seaton et al . 2015, 2020) and thus did not coincide with the time of the clearfell.…”

“…Eliminating parasitism of larvae using caged logs increased the chance of larvae to develop into adults, although the high incidence of larval feeding increased the demand on phloem/sapwood resources (food), and larval mortality was still observed. As sapwood resources become depleted, there can be increased potential for galleries to overlap, and larvae that were weak or slower in development could have died from feeding on poor substrates including frass from other larvae as what occurred by P. semipunctata in logs of Eucalyptus species (Hanks et al 1993b(Hanks et al , 2005Seaton et al 2020). Cannibalism between Co. rubripes larval conspecifics may have also occurred in a similar way to P. semipunctata, where the head of one larva was chewed off at the junction of where two galleries crossed each other (Powell 1982).…”

“…2015; Seaton et al . 2020). Previously, only specimen records showed that Co. rubripes could attack Eucalyptus in Australia (Froggatt 1923) and was present in Glenn Forest in the Darling Range near Perth, Western Australia (Wang 1995a).…”