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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.

1. At the end of November in subtropical areas of Australia, second‐instar larvae of the processionary caterpillar Ochrogaster lunifer (Lepidoptera: Thaumetopoeidae) initiate feeding on the peripheral shoots of acacias (first‐instar larvae do not feed). Field surveys at ten localities in south‐east Queensland showed that larval survival was highly variable both among and within localities. Within‐locality variation in larval growth was low compared with variation among localities. Larval growth and survival rates were higher at coastal and island localities, where November rainfall was high, than at drier inland localities. 2. Potted Acacia concurrens were grown in the greenhouse under high and low watering regimes, with and without nitrogen‐rich fertilizer. Plant vigour (height, foliar water content and quantity of flush growth) was significantly greater in high‐water treatments than in low‐water treatments. Watering also affected foliar nitrogen, with plants in the high‐water/no fertilizer treatment having similar nitrogen levels to those in fertilized treatments. Fertilizer increased foliar nitrogen levels of plants in low‐water treatments and increased the number of shoots in high‐water treatments. Different treatments had no effect on leaf toughness. After the first 3 weeks of feeding, size and survival of larvae were significantly reduced on the small, less vigorous plants in low water treatments. These results do not support the plant stress hypothesis. 3. Early‐instar larvae (instar II–IV) developed more quickly and grew larger when reared on flush leaves than when reared on senescent leaves of A. concurrens. As water uptake affects the quantity of flush growth available to early stage larvae as well as foliar quality, rainfall and water availability may have important consequences for the distribution and population dynamics of the moth at local and regional scales.

Various insect species display a uniform geometric increase in size during the larval stage (that is they follow the Brooks-Dyar rule). Here, results of larval development are presented for the bunny-tailed moth, Ochrogaster lunifer. The processionary larvae of this species live in a communal cohort, and moult en masse in the silken nest spun at the base of their host tree (usually a phyllodinous acacia). The exuviae, which remain buried in the accumulated silk and frass of the nest, provide a life history record of the larval cohort. Larval exuviae were collected from 773 cohorts at 37 localities in southeastern Queensland between November 1993 and May 1994. The 6,948 exuviae examined were from cohorts feeding on Acacia concurrens Pedley. Head-capsules showed a strongly uniform geometric increase in size through eight larval instars, supporting the Brooks-Dyar rule. The number of instars did not vary between trees or localities. A bimodal distribution of final instar head-capsule widths was shown to be a sexual dimorphism, and similar bimodal distributions were found for instars V-VII. Pupal size was also sexually dimorphic. The geometric size increase from one larval instar to the next holds for both males and the larger females. The geometric rule was tested using larval cohorts reared on A . concurrens in the greenhouse through instars I-IV; development was remarkably similar to that in the field. Larval growth patterns of 0. lunifer are very different from the structurally similar bag-shelter moth. The ability to distinguish different instars of 0. lunifer with a high degree of precision from field-collected exuviae will allow accurate comparisons of development, survival and dispersal of larvae in different group sizes, on different trees and in different localities.

In the past century, the debate over whether or not density‐dependent factors regulate populations has generally focused on changes in mean population density, ignoring the spatial variance around the mean as unimportant noise. In an attempt to provide a different framework for understanding population dynamics based on individual fitness, this paper discusses the crucial role of spatial variability itself on the stability of insect populations. The advantages of this method are the following: (1) it is founded on evolutionary principles rather than post hoc assumptions; (2) it erects hypotheses that can be tested; and (3) it links disparate ecological schools, including spatial dynamics, behavioral ecology, preference–performance, and plant apparency into an overall framework. At the core of this framework, habitat complexity governs insect spatial variance, which in turn determines population stability. First, the “minimum risk distribution” (MRD) is defined as the spatial distribution of individuals that results in the minimum number of premature deaths in a population given the distribution of mortality risk in the habitat (and, therefore, leading to maximized population growth). The greater the divergence of actual spatial patterns of individuals from the MRD, the greater the reduction of population growth and size from high, unstable levels. Then, based on extensive data from 29 populations of the processionary caterpillar, Ochrogaster lunifer, four steps are used to test the effect of habitat interference on population growth rates. (1) The costs (increasing the risk of scramble competition) and benefits (decreasing the risk of inverse density‐dependent predation) of egg and larval aggregation are quantified. (2) These costs and benefits, along with the distribution of resources, are used to construct the MRD for each habitat. (3) The MRD is used as a benchmark against which the actual spatial pattern of individuals is compared. The degree of divergence of the actual spatial pattern from the MRD is quantified for each of the 29 habitats. (4) Finally, indices of habitat complexity are used to provide highly accurate predictions of spatial divergence from the MRD, showing that habitat interference reduces population growth rates from high, unstable levels. The reason for the divergence appears to be that high levels of background vegetation (vegetation other than host plants) interfere with female host‐searching behavior. This leads to a spatial distribution of egg batches with high mortality risk, and therefore lower population growth. Knowledge of the MRD in other species should be a highly effective means of predicting trends in population dynamics. Species with high divergence between their actual spatial distribution and their MRD may display relatively stable dynamics at low population levels. In contrast, species with low divergence should experience high levels of intragenerational population growth leading to frequent habitat‐wide outbreaks and unstable dynamics in the long term. Six hypotheses, erec...

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