The basic feaures of an intensive study on the various stages o f a secondary succession, from fallow lield to birch woodland, are described. The a+ diversities of the green plants, and two orders of insects, Heteroptera and adult Coleoptera, are described. For the vegetation, in addition to taxonoinic diversity, structural diversity, with both spatial and architectural components, was recognized. It was found that up to a successional age of 16 months, the taxonomic diversities of plants and insects rose; therealter the diversity o l t h e plant species declined far more than the insect species diversity. I t was concluded that in the later successional stages the maintenance ofa high level 01' taxonomic diversity of these orders of insects is correlated with the rising structural diversity of the green plants, which virtually compensates for their falling taxonomic diversity. The larger fungi appear t o sliow a similar trend to the insects.

Observations on leaves from plants with a wide range of life-forms, ruderals to trees, indicate that palatability to insect herbivores is strongly correlated with the life-expectancy of the leaves. The amount of damage suffered in the field is however inversely correlated with palatability; although the rate of damage is less on unpalatable leaves, their longer life means that they accumulate damage over a longer period. It is only with extremely well-defended evergreen leaves, that the total damage is less than that experienced by less palatable (but short-lived) leaves. These observations are related to the current theories on relative palatability (the apparency theory and the resource availability theory), within the framework of the habitat templet.

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The palatability of common plants from three stages of secondary sere have been assessed by a bioassay method. Foliage was exposed to five taxonomically very different herbivores (Agriolimax, Schistocera, Spodoptera, Myzus and Trialeurodes). Palatability was measured by foliage consumption or, for sap suckers, by survival and reporduction. The results of these experiments supported the predictions of Feeny's 'Apparency theory', but indicated considerable variations in palatability within plant communities and between herbivores.

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Exposing diapausingDelia radicum (L.) to temperatures from-10.2 to-14.8° C during a 80-day experimental period reduced survival to eclosion and the rate of postdiapause development. No such effects occurred among those exposed to-7.1° C for up to 80 days. A continuous exposure to-10.2 or-14.8° C reduced survival of overwinteringD. radicum by the same amount as an equal duration of exposure to these temperatures interrupted by 14 days at 2° C, indicating that the effects of cold exposure were additive and that repair of cold-induced injuries did not occur at 2° C. The decreased survival with increasing duration of exposure to-10.2 or-14.8° C were described by the equations:[Formula: see text] Wherep=the proportion surviving to eclosion andt=the number of days of exposure to the temperature given by the subcript. An increased incidence of malformed adults was associated with low survival. Adult emergence occurred within two periods. Early-emerging flies required up to 230° D and late-emerging flies more than 330° D above 5.6° C. Among early-emerging flies, a reduced rate of postdiapause development was associated with low survival but the development of late-emerging flies was not affected.OverwinteringD. radicum parasitized byTrybliographa rapae (Westw.) or byAleochara bilineata Gyll. responded similarly to unparasitizedD. radicum although those parasitized byT. rapae were less resistant to cold injury when exposed to-14.8° C. D. radicum and its parasitoids are capable of surviving temperatures more severe than those normally occurring in England and Wales; and, therefore, cold injury is unlikely to affect their population dynamics.

The Canadian population of Delia radicum is more cold hardy than the English population in that the supercooling point is slightly lower and overwintering pupae are less sensitive to lower temperatures and longer exposure times. However, the Canadian population is slightly less cold hardy at higher temperatures within the cold injury zone. Female D. radicum were more susceptible to cold-induced mortality than males among the Canadian population, but this sex difference was not significant in the English population. The proportion of malformed adults and the rate of postdiapause development were not related to cold injury in either population. The bounds of the cold injury zone for species or populations of freezing susceptible, diapausing insects plus the sensitivity of the species to cold injury within this zone can provide an ecologically sound method of describing cold hardiness. Sensitivity, measured by the slope of a regression describing the relationship between survival and the duration of exposure at a low temperature, can also be used to calculate the rate at which cold injury occurs at any temperature within the cold injury zone. This slope may reflect the overwintering conditions of a species or population because Canadian populations of both D. radicum and Mamestra configurata (Noctuidae) have similar coefficients, but these are very different from the coefficient of the English population of D. radicum. The supercooling point did not provide a useful indication of the susceptibility of these cold-hardy diapausing insects to cold injury.

Abstract.  1. This study explored the temporal and spatial aspects of coexistence over many generations in a multispecies host–parasitoid assemblage. 2. The long‐term interaction between the cabbage root fly, Delia radicum (Diptera: Anthomyiidae), and two of its natural enemies, Trybliographa rapae (Hymenoptera: Fitigidae) and Aleochara bilineata (Coleoptera: Staphylinidae), in a cultivated field at Silwood Park over 19 years was explored. 3. Although time series showed that the populations were regulated, the impact of the natural enemies was highly variable. Within‐year determinants showed that the spatial response of the specialist parasitoid, T. rapae, was predominantly independent of host density while A. bilineata acted simply as a randomly foraging generalist parasitoid. 4. These findings are compared and contrasted with an earlier investigation of the same system when only the first 9 years of the time series were available. This study demonstrated the potential of long‐term field studies for exploring hypotheses on population regulation, persistence, and coexistence.