Environmental influences on stem borer incidence in Australian subtropical Corymbia plantations

Population dynamics of forest insect pests are a key determinant of forest health globally, but there is often insufficient information on tree susceptibility and environmental drivers of attack to construct pest population models that inform management. We investigated susceptibility of Corymbia citriodora subsp. variegata (spotted gum) plantations to two stem borer families (Cerambycidae, Cossidae) for which the drivers of attack are largely unknown. We surveyed 20 plantations for borer incidence (proportion of trees attacked), severity (number of attacks/attacked tree) and height and collected environmental variables in six categories (site characteristics, physical soil characteristics, foliar nutrition, landscape structure, climate and management). Most attacks on tree stems were associated with the cerambycid beetle Phoracantha solida, which were positively related to secondary attack from the cossid moth Culama australis. Mean incidence of borer attack was 3.5% and highly variable across plantations (0–22%). Phoracantha solida incidence was highly correlated with severity at a plantation scale. Relative tree diameter was the most important associate of P. solida attack incidence; proportionately larger diameter trees were more likely to be attacked where no thinning had occurred. Foliar nutrients, specifically lower concentrations of potassium, iron and nitrogen, were associated with higher P. solida attack incidence. We suggest that measuring incidence can be used as a proxy for severity of P. solida-associated damage at a plantation scale in operational pest surveillance. The results indicate that thinning may reduce the occurrence of proportionately larger diameter trees being attacked. We also recommend further research to determine the effectiveness of adding fertiliser to remediate sites that are deficient in potassium, nitrogen and iron.