A theory of phasmatid outbreak release.

Abstract

The paper attempts to explain the causes of phasmatid outbreaks in a theory of outbreak release which is based on direct observations of the phasmatid Didymuria violescens (Leach) and a subjective analysis of several outstanding features associated with the history, distribution, and development of phasmatid outbreaks in the highlands of south-eastern Australia. The outbreaks of D. violescens are largely confined to mountain forests in southern New South Wales and Victoria where a 2-yr generation period predominates and a 2-yr cycling of population prevails with peaks and troughs of hatching, and hence "peak" and "off" years of phasmatid abundance, in alternate years. Birds greatly limit the survival rate of D. violescens during off-years but are ineffective during peak years because of their limited total response to increase in numbers of the prey. The theory implies that D. violescens has two systems of control: one at low density when increase is limited largely by egg parasites or birds or both; and another at high density when increase is ultimately prevented by intraspecific competition for food. The theory postulates that very cool summer weather conditions may occasionally disrupt the low-density system of control and thereby initiate outbreak release either directly in some situations or indirectly in others.

The first fossil leaf insect: 47 million years of specialized cryptic morphology and behavior

“…It must have been caused by vigorous selection pressure by visually oriented predators. Common predators of adult phasmids are insectivorous birds (21), but also spiders, mantids, lizards, and bats (e.g., 20,[22][23][24][25][26] attacked large adult leaf insects. Leaf-mimicking orthopteroid insects like phaneropterines and pseudophyllinae (Orthoptera: Tettigoniidae) can constitute a large component of the diet of small neotropical monkeys (28).…”

Insect Herbivory in Tropical Forests

“…This fluctuation rcsults from a legion of systcm variables including weather (Readshaw 1965), disturbances (Smith 19821, historical processes (Southwood 1961), topography (\.Ward 19791, tree density ( Morrow and Fox 1980), plant structure (Lawton and Schroder 1977), plant secondary compounds (AIacauley and Fox 1980), or even random processes (Clark 1962). Stand growth and dynamics Inay ultimatcly be affected by hcrbivory and by thc susceptibility of a specics to grazing (rc~icwed in Schowaltcr et al 1986).…”

The worldwide status of phasmids (Insecta: Phasmida) as pests of agriculture and forestry, with a generalised theory of phasmid outbreaks

“…This observation was confirmed by Paine [26]: "Outbreaks seem to occur on certain islands, in certain situations, and not always very frequently, so that while over the whole range of this insect the loss of copra [the dried kernel of the coconut used to extract oil] resulting from its ravages is inconsiderable, losses to individual planters and village communities can be serious". This slow spread is also reported for Didymuria violescens outbreaks in Australia [27].…”

“…Other eucalypts are less favoured but also used as hosts, including the economically valuable alliances where E. delegatensis (alpine ash) or E. regnans (mountain ash) dominate [27]. The species can be univoltine or semi-voltine depending on environmental conditions, achieved through one of two stages of diapause in the egg stage [76,77].…”

“…Australia: New South Wales, Victoria [7,27] Podacanthus wilkinsoni Australia [7] Psidium guajava Trachyaretaon echinatus Philippines [106] Rhamphosipyloidea philippa Philippines [106] Pharnacia magdiwang Philippines [106] Pharnacia ponderosa Philippines [106] localized area". This observation was confirmed by Paine [26]: "Outbreaks seem to occur on certain islands, in certain situations, and not always very frequently, so that while over the whole range of this insect the loss of copra [the dried kernel of the coconut used to extract oil] resulting from its ravages is inconsiderable, losses to individual planters and village communities can be serious".…”