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Summary1. The spatial and temporal distribution of eggs laid by herbivorous insects is a crucial component of herbivore population stability, as it in¯uences overall mortality within the population. Thus an ecologist studying populations of an endangered butter¯y can do little to increase its numbers through habitat management without knowledge of its egg-laying patterns across individual host-plants under dierent habitat management regimes. At the other end of the spectrum, a knowledge of egg-laying behaviour can do much to control pest outbreaks by disrupting egg distributions that lead to rapid population growth. 2. The distribution of egg batches of the processionary caterpillar Ochrogaster lunifer on acacia trees was monitored in 21 habitats during 2 years in coastal Australia. The presence of egg batches on acacias was aected by host-tree`quality' (tree size and foliar chemistry that led to increased caterpillar survival) and hosttree`apparency' (the amount of vegetation surrounding host-trees).3. In open homogeneous habitats, more egg batches were laid on high-quality trees, increasing potential population growth. In diverse mixed-species habitats, more egg batches were laid on low-quality highly apparent trees, reducing population growth and so reducing the potential for unstable population dynamics. The aggregation of batches on small apparent trees in diverse habitats led to outbreaks on these trees year after year, even when population levels were low, while sitewide outbreaks were rare. 4. These results predict that diverse habitats with mixed plant species should increase insect aggregation and increase population stability. In contrast, in open disturbed habitats or in regular plantations, where egg batches are more evenly distributed across high-quality hosts, populations should be more unstable, with sitewide outbreaks and extinctions being more common. 5. Mixed planting should be used on habitat regeneration sites to increase the population stability of immigrating or reintroduced insect species. Mixed planting also increases the diversity of resources, leading to higher herbivore species richness. With regard to the conservation of single species, dierent practices of habitat management will need to be employed depending on whether a project is concerned with methods of rapidly increasing the abundance of an endangered insect or concerned with the maintenance of a stable, established insect population that is perhaps endemic to an area. Suggestions for habitat management in these dierent cases are discussed. 6. Finally, intercropping can be highly eective in reducing pest outbreaks, although the economic gains of reduced pest attack may be outweighed by reduced crop yields in mixed-crop systems.

Populations of Ochrogaster lunifer Herrich-Schaffer display different larval behaviours (ground-nesting and canopy-nesting), suggesting the existence of two or more species. Here, the behaviour and biology of ground-nesting populations are described from extensive surveys in southeastern Queensland. Females oviposit at the base of the host tree, covering the eggs with scales from the anal tuft. The larvae are processionary and pass through eight instars. First instars do not feed, remaining at the base of the tree trunk within the scale mass. Second instars ascend the tree in single file in the morning to feed in the canopy each day, returning to the scale mass after several hours. Later instars feed only at night. Six new host plants are recorded: five species of Acacia, and one species of Casuarina. Predators and parasitoids recorded on 0. lunifer included chalcidoids, dermestids, tachinids and a predatory pyrrhocorid bug. The results of the study highlight a number of major differences between ground-nesting and canopy-nesting populations. Differences include habitat and geographical distribution; host plant range; number of instars; size and colour of final instar; size of adult female; oviposition site and larval behaviour. These differences provide further evidence for the existence of two species.

A major ongoing debate in population ecology has surrounded the causative factors underlying the abundance of phytophagous insects and whether or not these factors limit or regulate herbivore populations. However, it is often difficult to identify mortality agents in census data, and their distribution and relative importance across large spatial scales are rarely understood. Here, we present life tables for egg batches and larval cohorts of the processionary caterpillar Ochrogaster lunifer Herrich-Schäffer, using intensive local sampling combined with extensive regional monitoring to ascertain the relative importance of different mortality factors at different localities. Extinction of entire cohorts (representing the entire reproductive output of one female) at natural localities was high, with 82% of the initial 492 cohorts going extinct. Mortality was highest in the egg and early instar stages due to predation from dermestid beetles, and while different mortality factors (e.g. hatching failure, egg parasitism and failure to establish on the host) were present at many localities, dermestid predation, either directly observed or inferred from indirect evidence, was the dominant mortality factor at 89% of localities surveyed. Predation was significantly higher in plantations than in natural habitats. The second most important mortality factor was resource depletion, with 14 cohorts defoliating their hosts. Egg and larval parasitism were not major mortality agents. A combination of predation and resource depletion consistently accounted for the majority of mortality across localities, suggesting that both factors are important in limiting population abundance. This evidence shows that O. lunifer is not regulated by natural enemies alone, but that resource patches (Acacia trees) ultimately, and frequently, act together to limit population growth.