Behaviour of woodborers (Coleoptera: Cerambycidae) and sapwood moisture in a drought affected Northern Jarrah (Eucalyptus marginata) Forest

A severe drought event during 2011 in the Northern Jarrah (Eucalyptus marginata Don Ex Smith) Forest of southwestern Australia, provided ideal conditions for an outbreak of the eucalyptus longhorned borer Phoracantha semipunctata Fabricius (Coleoptera: Cerambycidae). Located in a Mediterranean climatic zone, the jarrah forest region is expected to experience increased frequency of droughts causing tree mortality, yet little is known of endemic borer activity in response to tree drying. A detailed study was conducted within the jarrah forest following the drought, with the overall aim to provide an understanding of the role of sapwood moisture in controlling the infestation and development of longhorned borers. This was achieved using a field-based approach within a natural habitat and addresses the following key objectives: (a) assess the influence of seasons on infestation by key borer species, (b) determine the role of parasitoid wasps in controlling borer infestations, and (c) evaluate the influence of the two main Eucalyptus species, jarrah and marri (Corymbia calophylla L.A.S Johnson) on borer behaviour. Deterioration in crown health during the drought was associated with increased emergence of P. semipunctata and this was higher in larger lower tree sections where differences in emergence occurred between tree species and favoured marri with thicker bark compared to smaller higher tree sections with thinner bark. Poor survival of neonate larvae in both species was associated with overlapping galleries at high larval incidence, indicating intense competition for sapwood resources among developing larvae, while pupal survival remained high. This is the first study to show Coptocercus rubripes Boisduval to be an important borer along with P. semipunctata in the jarrah forest. Coptocercus rubripes mainly attacked in spring, prefers jarrah, has singular galleries per oviposition site, and grew slowly over spring to autumn, emerging in autumn-winter. Phoracantha semipunctata mainly attacked in the dry warm summer months, preferred marri, has many galleries per oviposition site, and grew rapidly over summer-autumn, emerging in summer the following year. Once sapwood moisture content in water stressed trees decreased by 19 % below turgid levels (independent of tree species), coinciding with a change in colour of canopy leaves of trees from green to yellow, conditions were suitable for borer larvae to establish. As sapwood dried, the larger P. semipunctata developed quicker and galleries were more extensive than C. rubripes. Larvae of various ages contributed to total sapwood consumption and reached a maximum at higher moisture contents in marri than jarrah. The native parasitic wasps Syngaster lepidus Brulle and Callibracon limbatus Brulle attacked maturing borer larvae of C. rubripes and P. semipunctata in stressed jarrah and marri hosts. Parasitism averaged 67 % and was inversely related to larval density. In the absence of parasitoids, borer survival in caged logs was greatly improved, although 30 % mortality still occurred and was attributed to increased competition among developing larvae. This research shows falling sapwood moisture has a key role for borer infestation and development in response to drought stress within forests and can be influenced by borer and tree species, parasitoids, and season. This knowledge will enable better management of cerambycids in forests subject to drought