Population regulation is a fundamental process related to most phenomena in ecology, including evolutionary ecology. I review the conceptual basis for defining reg- ulation as bounded fluctuations in abundance, in contrast to the unbounded fluctuations of random-walk populations. Regulation arises as a result of potentially stabilizing density- dependent processes, even when brought about by "non-equilibrium" mechanisms. Al- though the phenomenon is unambiguous in theory, detecting regulation by finding evidence for density dependence in a series of population estimates faces unsolved statistical prob- lems. So, while there is growing evidence for widespread regulation, severe detection prob- lems remain. I illustrate these with data from bird populations. Whether regulation is typically achieved by local stabilizing mechanisms or via metapopulation dynamics remains to be determined. I summarize recent studies on a particularly well-regulated system - red scale (Aonidiella aurantii) and its controlling parasitoid, Aphytis melinus. We tested and failed to find evidence for eight hypotheses that might account for the system's stability, including spatial heterogeneity in attack rates, a refuge, and metapopulation dynamics. We also failed to find evidence for density-dependent parasitism, but such density dependence might be still size-structured~rthu host.eipen

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Population regulation in theory and practice

1. Ecologists concerned with life-history strategies of parasitoid wasps have recently focused on interspecific variation in the fraction of the maximum potential lifetime egg complement that is mature when the female emerges into the environment. Species that have all of this complement mature upon emergence are termed 'pro-ovigenic', while those that do not are termed 'synovigenic'. We document and quantify the diversity of egg maturation patterns among 638 species of parasitoid wasps from 28 families. 2. We test a series of hypotheses concerning variation in 'ovigeny' and likely life- history correlates by devising a quantitative index - the proportion of the maximum potential lifetime complement that is mature upon female emergence. 3. Synovigeny, which we define as emerging with at least some immature eggs, was found to be by far the predominant egg maturation pattern (98.12% of species). Even allowing for some taxonomic bias in our sample of species, pro-ovigeny is rare among parasitoid wasps. 4. There is strong evidence for a predicted continuum in ovigeny index among parasitoid wasps, from pro-ovigenic (ovigeny index = 1) to extremely synovigenic species (ovigeny index = 0). 5. As predicted, synovigenic species are longer-lived than pro- ovigenic ones, and ovigeny index and life span are negatively correlated across parasitoid taxa, suggesting a life span cost of concentrating reproductive effort early in adult life. 6. There is equivocal evidence that host feeding (i.e, consumption of host haemolymph and/or tissues by adult wasps) is confined to synovigenic parasitoid wasps. It is also not certain from our analyses whether host feeding is associated with a relatively low ovigeny index. 7. As predicted, egg resorption capability is concentrated among producers of yolk-rich eggs. Also, the hypothesis that it is associated with a tendency towards a low ovigeny index is supported. Parasitoid species that produce yolk-rich eggs also exhibit a lower ovigeny index than species that produce yolk-deficient eggs. 8. Ovigeny index appears to be linked to parasitoid development mode (koinobiosis-idiobiosis). 9. We conclude that 'ovigeny' is a concept applicable to insects generally. [KEYWORDS: feeding strategies; pro-ovigeny; synovigeny; timing of reproduction Leptopilina-boulardi hymenoptera; host-feeding strategies; egg limitation; reproductive-biology; clutch size; independent contrasts; functional-response; asobara-tabida; drosophila-melanogaster; evolutionary argument]

https://besjournals.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2656.2001.00507.x

Life-history strategies in parasitoid wasps: a comparative analysis of ‘ovigeny’

The inflorescences of 53 plant species in a variety of habitats were examined for the presence of adult hymenopteran parasitoids (‘Parasitica’ only), and the wasps observed for feeding behaviour. At inflorescences of 32 of the plant species examined, over 1000 parasitoid individuals were observed displaying feeding behaviour (either feeding at nectaries or displaying behaviour associated with searching for food). Over 900 individuals were collected. Around 250 species belonging to 15 families of Hymenoptera were represented. The taxonomic, behavioural and ecological significance of known parasitoid-food plant associations are discussed, and the literature reviewed.

Flower-visiting by hymenopteran parasitoids

Assessing the suitability of flowering herbs as parasitoid food sources: flower attractiveness and nectar accessibility

The evolution of host-feeding behaviour in insect parasitoids

The effects of host-feeding behaviour on the dynamics of parasitoid-host interactions, and the implications for biological control

Until now, mathematical models of parasitoid-host interactions have not incorporated the tendency for destructively host-feeding parasitoids to partition their feeding and oviposition behaviour in relation to different host stages. A literature survey reveals a trend for female parasitoids to feed preferentially or exclusively on earlier host stages and to oviposit preferentially or exclusively in/or later ones. We explore the relative advantages to host-feeding parasitoids of a number of possible host stage selection strategies. We develop hypotheses, formalizing and testing them using modifications to our earlier simulation model of host-feeding strategies (Jervis and Kidd, 1986). We conclude from our modelling that the advantage to be gained from feeding on early host stages and ovipositing in late ones is likely to be associated with: 1) reduced handling times when feeding on early stage hosts; 2) reduced wastage of progeny from mortality factors other than host-feeding by the parent parasitoid, achieved by confining oviposition to late host stages; and 3) reduced probability of progeny mortality resulting from the parent's host-feeding activities.

Host-feeding and oviposition strategies of parasitoids in relation to host stage

The large pine aphid,Cinara pinea lives exclusively onPinus species, where it feeds on the foliated shoots of the current and previous year. The paper describes the development of a computer model designed to simulate the aphid’s population dynamics on saplings in the controlled environment of the laboratory, i.e. in the absence of natural enemies. The model was able to account for about 80% of the variation in aphid numbers within and between trees over a three month period. Sensitivity analysis revealed that the number of pine aphids is limited primarily by nymphal emigration, the operation of which is sensitive both to density and to plant quality as reflected in aphid growth rates. Of secondary importance are changes in reproduction acting through increased reproductive delay, again a result of altered growth rates and adult size. Development, too, has an important secondary influence. Contrary to expectation and conventional belief, however, alate production proved to be of negligible importance, either in limiting or regulating population numbers. Alatae are produced in too few numbers and for too short a period to significantly alter the pattern of population change.

N. A. C. Kidd

Papers

The effects of host-feeding behaviour on the dynamics of parasitoid-host interactions, and the implications for biological control

Host-feeding and oviposition strategies of parasitoids in relation to host stage

The population dynamics of the large pine aphid, Cinara pinea (Mordv.) I. Simulation of laboratory populations.