Life history and behavioural traits of Mnesampela privata that exacerbate population responses to eucalypt plantations: Comparisons with Australian and outbreak species of forest geometrid from the Northern Hemisphere

Abstract

AbstractThe larvae of the native Australian moth Mnesampela privata (Geometridae) sometimes defoliate plantation eucalypts, causing concern to industry. Typically, populations of M. privata maintain innocuous numbers in native forests, but outbreak populations can occur in plantations. Certain of the life history and behavioural traits of M. privata exacerbate population responses in simplified systems. Specifically, M. privata exhibits indiscriminate oviposition behaviour, for example, females oviposit upon leaves where conspecific egg clutches are already present. Combined with large egg clutches, this trait can lead to heavy exploitation of natal hosts. Complete defoliation of the natal tree necessitates that larvae disperse to undamaged hosts. In native forests, dispersing larvae would have a lower probability of locating a new host of suitable species and phenotype than they would in a plantation. Larval tolerance of a wide variety of species of Eucalyptus facilitates utilization of non‐natal eucalypts. A lower rate of encounter between larvae and natural enemies is achieved by means of leaf shelters and autumnal seasonal activity. Good dispersion by gravid females from plantations with large moth populations ensures that nearby plantations will be located. Given these characteristics, continued outbreaks of M. privata are likely in monospecific eucalypt plantations, especially when management practices reduce populations of natural enemies and overall vegetational complexity. However, because outbreaks only arise in plantations, population fluctuations of M. privata cannot be considered ‘self‐driven’. Hence, this moth is not a true eruptive species in the manner of other geometrids such as the autumnal moth (Epirrita autumnata autumnata) from Europe or the hemlock looper (Lambdina fiscellaria fiscellaria) from North America, but rather a gradient outbreak insect.