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The interaction of drought and the outbreak of Phoracantha semipunctata (Coleoptera: Cerambycidae) on tree collapse in the Northern Jarrah (Eucalyptus marginata) Forest

Seaton, Stephen (2012) The interaction of drought and the outbreak of Phoracantha semipunctata (Coleoptera: Cerambycidae) on tree collapse in the Northern Jarrah (Eucalyptus marginata) Forest. Honours thesis, Murdoch University.

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The Eucalyptus longhorned borer Phoracantha semipunctata Fabricius is an endemic Cerambycid beetle to the Northern Jarrah Forest (NJF) of south-western Australia that attacks stressed trees. The population dynamics, distribution and biology of P. semipunctata in its native habitat are poorly understood. Following a recent drought event in 2010 patches ofjarrah and marri trees throughout the NJF suffered high mortality with a noticeable increase in the borer populations. The aims of the present study were to determine the effect of this drought on P. semipunctata populations in jarrah (Eucalyptus marginata Donn ex Smith) and marri (Corymbia calophylla Hill) trees across the NJF and to determine if they would attack healthy adjacent trees, when their population levels were high. The association between the health of trees and P. semipunctata infestation was determined in standing dead, dying or healthy trees located in intact and collapsed areas of forest. Life cycle stages of P. semipunctata in terms of egg clutches, neonate feeding galleries, mature larvae and pupae in heartwood and adult emergence holes were determined by felling jarrah and marri trees. Distribution of P. semipunctata across the NJF was investigated by counting adult emergence holes in the bark of standing trees dead, dying or healthy trees at a number of collapsed sites.

The health of trees in collapsed areas had clearly deteriorated where 76% of trees had lost all or part of their canopy and 18% had recently died. In contrast, the health of the trees in the surrounding intact areas where 3 7% had lost all or part of their canopy and less than 1% of trees had recently died. There was a strong association between P. semipunctata infestation and the health of the trees, with the borers concentrated in trees in collapsed sites with an average of 4.5 emergence holes (m-2) of P. semipunctata for the first 2m of the tree in collapsed areas compared to an average of 0.1 emergence holes (m-2) in the healthy intact areas. Phoracantha semipunctata were attracted to trees that had lost all or part of their canopy or had died since the drought with 94% of individuals in these trees and less than 1% of P. semipunctata in healthy trees with an intact canopy. The low levels of P. semipunctata in the trees that had lost their canopy in the intact areas tends to indicate this may be the normal or 'background' level of infestation in the NJF. These background levels may increase with the occurrence of more collapsed areas under increased frequency of droughts, due to the predicted declines in rainfall in the south-west of Western Australia.

Destructive sampling in April showed that infestation levels within trees were very high with a maximum of 429 emergence holes per tree for jarrah and 345 emergence holes per tree for marri. Averaged across four collapsed sites, marri had higher levels of infestation with 15.42 emergence holes (m-2) compared to jarrah with 10.55 emergence holes (m-2) for the entire tree. In jarrah and marri an increase in clutch points (i.e. oviposition sites) and more neonate larvae (i.e. number of larval tracks) resulted in a higher of number of emergence holes. Most of larvae in the heartwood had emerged at the time of sampling being low in number but were higher in jarrah compared to marri. The differences in the total P. semipunctata emergence holes (m-2) between jarrah and marri was a result of a complex interaction of tree height and diameter over bark (DOB), where number of emergence holes (m-2) decreased with height and tree species, giving a range of responses at different sites. These differences in P. semipunctata infestations between jarrah and marri varied with sites. The distribution of P. semipunctata infestations within trees were strongly associated with billet size as it varied with tree height with higher infestations of P. semipunctata occurring closer to the base of the tree where trees had a large diameter (15-22 em). Bark thickness ranged from 10-14 mm and was found to be positively correlated with billet diameter (r2 = 0.372, P<O.OOl). It was also thicker in marri (averaging 12 mm) than jarrah (averaging 10 mm). As jarrah stem diameter decreased less with height than marri, lower levels of infestation by P. semipunctata occurred for marri at the same height of tree as compared to jarrah. The damage (on a surface area basis) to the sapwood from larval feeding galleries was considerable with up to 100% for jarrah and 76% for marri with an average of 48.2% and no difference between jarrah and marri across all sites. Sapwood damage decreased 30 to 50% with tree height above 3m where there were less emergence holes (m-2).

Considerable variation in terms of site characteristics (rainfall, soils, stand density and proportion of jarrah to marri) occurred across the NJF and no one factor was critical in determining which sites had trees that were most affected by the drought. Sites where trees had suffered the most canopy loss had the highest (6 emergence holes (m-2)) or 20,000 borers/hectare. This relationship accounted for most of the variation of P. semipunctata populations differences between sites across the NJF. A consistent feature of P. semipunctata infestation in the NJF was the occurrence of collapsed patches of various sizes and different numbers of P. semipunctata across the NJF. Green height (GH) (the proportion of the original tree height for which green foliage remained) and canopy remaining on the tree were found to be good estimates of tree health and canopy remaining was highly correlated with epicormic shoot occurrence. Trees that lost most of their canopy during the 2010 drought had recovered by producing a large number of epicormic shoots. However, some of these epicormic shoots died, indicating that recovery may be short term either from continued drought events in 2012, for example the low July rainfall and possibly the effects of damage from borers. Densities of jarrah and marri varied between sites and had a large influence on the total number of P. semipunctata that occurred at a site.

The location, health of trees and stand characteristics determine the population levels of P. semipunctata in the NJF. The thesis demonstrates that (a) P. semipunctata infestation levels and sapwood damage was concentrated in drought affected trees in collapsed patches, (b) levels of infestation varied and depended on stem diameter, bark thickness and tree species jarrah or marri), and (c) levels of infestation varied across the NJF and depended on the health of trees at individual sites and their stand characteristics. This study has increased our understanding of the ecology of P. semipunctata and its infestation dynamics in jarrah and marri. It will provide a basis for more detailed studies of the relationship between tree moisture deficits and the resistance to infestation by P. semipunctata in relation to tree characteristics and increased drought levels.

Publication Type: Thesis (Honours)
Murdoch Affiliation: School of Biological Sciences and Biotechnology
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
Supervisor: Hardy, Giles and Matusick, George
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