Summary 1. Nesting behaviour and interactions between the bee Chelostoma florisomne (L.) (Megachilidae) and its nest parasite Sapyga clavicornis (L.) (Sapygidae) were studied through continual observations of individuals and dissections of bee nests. Protection of bee offspring is based on (1) the bee’s discovery and removal of parasite eggs deposited prior to the construction of a cell closure, (2) minimising the time when fully provisioned cells might be parasitised successfully, and (3) the construction of empty cells in front of brood cells. 2. An empty cell was found in front of 64.4% of all brood cells and, if the outermost brood cell in a nest was excluded, in front of 74.3% of inner brood cells. A vestibule closure is most often constructed in front of the outermost brood cell. 3. Following oviposition, the bee made only five flights, which together lasted 6–13 min, to construct a cell closure. A cell closure does not prevent the nest parasite from oviposition inside the brood cell, however, and parasite eggs deposited through the cell closure are not detected and removed by the bee. Only an additional cell closure, i.e. the formation of an empty cell, may protect a brood cell when the bee is not in the nest. The nest parasite often oviposited through the additional cell closure but its offspring were then trapped in the empty cell and starved to death. 4. Only 5.4% of the inner brood cells that were protected by an empty cell were parasitised, compared with 28.9% of those without an anterior, empty cell; 27.4% of the empty cells contained dead parasite offspring (eggs and larvae). Thus, the empty cells provided significant protection and, combined with additional means of protection of brood cells, led to a low degree of parasitism. More than 77% of the wasp offspring died at an early stage due to intraspecific interference competition within brood cells and as result of the wasps’ oviposition into empty cells.

Mü nster-Swendsen, M. and Berryman, A. 2005. Detecting the causes of population cycles by analysis of R-functions: the spruce needle-miner, Epinotia tedella , and its parasitoids in Danish spruce plantations. Á/ Oikos 108: 495 Á/502.Explaining the causes of regular multi annual oscillations (cycles) in animal populations has been a major problem for ecology, partly due to a lack of methodological rigor. In this paper we show how the analysis of R-functions, the functional relationship between the per capita rate of change of a species and components of its environment, can be used to detect the causes of population cycles. Analysis of the R-functions enables one to separate cycles due to negative feedback between species (endogenous causes) from those forced by one-way effects (exogenous causes). We illustrate the approach by reference to the spruce needle-miner inhabiting Danish spruce plantations, and conclude that population cycles in this insect are probably caused by interactions with two species of parasitic hymenoptera. ).Mikael Mü nster-Swendsen and I were working on this paper at the time of his untimely death in 2003. The purpose of the paper was to show how my approach to population analysis (Berryman 1990(Berryman , 1999(Berryman , 2001 could be used to detect the causes of population cycles in his data on the spruce needle-miner, Epinotia tedella (Cl.) (Mü nster-Swendsen 1979, 1982, 1985. I should point out here that, at the time we first met, Mikael was not too impressed with my approach, being a proponent of the life table Á/key factor approach (Varley et al. 1975). In time, however, he was to change his mind, and the reason for that is one of the lessons of this paper. I, on the other hand, was very impressed with his data. For 19 years he measured, with considerable precision (Mü nster-Swendsen 1985), the density of spruce needle-miners and their primary insect parasitoids emerging annually from the litter of a Danish spruce plantation. He then constructed detailed life tables, including all suspected mortality factors, for 9 of those years. Finally, he repeated the sampling for various shorter periods of time at 10 additional locations. Others may have longer time series, more detailed life tables, or more spatial replication, but none to my knowledge have more of all three. Mikael recently reviewed his analysis in chapter 2 of a book that I edited (Mü nster-Swendsen 2002), so there is no need for me to repeat it here. Suffice to say that the needle-miner population was found to exhibit spatially synchronous 6 Á/7 year cycles of abundance, and that the key factor affecting the population change was reduced fecundity. He also briefly mentions how my approach led to a different conclusion, that insect parasitoids were mainly responsible for the cyclic dynamics, and that the discovery of pseudoparasitism reconciled this conflict. However, he did not cover details of our analysis, referring instead to the present paper, which was

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Abstract. A hitherto unknown factor causing cyclic reduction in population fertility of Epirzotia tedella (Cl.) was identified as sublethal infections of the neogregarine Mattesia sp. Infection probably takes place when larvae enter hibernation, but does not develop into spore production until the formation of pupae and adult moths. At the individual level, the sublethal neogregarine infection causes a slight delay in adult emergence, a decreased adult lifespan, a suppression of egg development, and thus a reduction in fertility. At the population level, fertility reduction has a delayed density dependent component and, through correlation, Mattesia is assumed to be causative. Since fertility reduction is a key‐factor in E.tedella, Mattesia may even be a dominant factor in the dynamics of this species. Mattesia infections were found in no other dominant insect species at the locations, and consequently the interaction between E.tedella and Mattesia sp. seems to be specific. Such delayed density dependent interaction can create host oscillations, as seen in many forest insects. However, host‐parasitoid interaction alone leads to oscillations with the observed period of 6–8 years length, and the neogregarine infections apparently act in parallel with the parasitoids and enhance the amplitude of the oscillations.

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