Although the use of aggregation pheromones has been reported for hundreds of nonsocial arthropod species, the evolutionary ecological aspects of this behavior have received little attention. Despite the elaborate literature on mechanisms, robust data on costs and benefits of aggregation pheromones are scant. Existing literature indicates that, in contrast to the diversity of mechanisms, the ecological conditions in which aggregation pheromones are used are more alike. This points to a few general categories for costs and benefits of aggregation pheromones, and these are discussed. We subsequently review interspecific interactions that may be affected by the use of aggregation pheromones. We encounter a strikingly frequent association of aggregation pheromones with fungi and microorganisms and address cross-attraction by competitor species and exploitation by natural enemies. We show that aggregative behavior by individuals through the use of pheromones can profoundly affect ecological interactions and advocate further evolutionary and ecological investigations of pheromone-mediated aggregation.

Measurement of food intake in the fruit fly Drosophila melanogaster is often necessary for studies of behaviour, nutrition and drug administration. There is no reliable and agreed method for measuring food intake of flies in undisturbed, steady state, and normal culture conditions. We report such a method, based on measurement of feeding frequency by proboscis-extension, validated by short-term measurements of food dye intake. We used the method to demonstrate that (a) female flies feed more frequently than males, (b) flies feed more often when housed in larger groups and (c) fly feeding varies at different times of the day. We also show that alterations in food intake are not induced by dietary restriction or by a null mutation of the fly insulin receptor substrate chico. In contrast, mutation of takeout increases food intake by increasing feeding frequency while mutation of ovoD increases food intake by increasing the volume of food consumed per proboscis-extension. This approach provides a practical and reliable method for quantification of food intake in Drosophila under normal, undisturbed culture conditions.

Abstract. 1. Aggregation pheromones can evolve when individuals benefit from clustering. Such a situation can arise with an Allee effect, i.e. a positive relationship between individual fitness and density of conspecifics. Aggregation pheromone in Drosophila induces aggregated oviposition. The aim of the work reported here was to identify an Allee effect in the larval resource exploitation by Drosophila melanogaster, which could explain the evolution of aggregation pheromone in this species.2. It is hypothesised that an Allee effect in D. melanogaster larvae arises from an increased efficiency of a group of larvae to temper fungal growth on their feeding substrate. To test this hypothesis, standard apple substrates were infested with specified numbers of larvae, and their survival and development were monitored. A potential beneficial effect of the presence of adult flies was also investigated by incubating a varying number of adults on the substrate before introducing the larvae. Adults inoculate substrates with yeast, on which the larvae feed.3. Fungal growth was related negatively to larval survival and the size of the emerging flies. Although the fungal growth on the substrate was largely reduced at increased larval densities, the measurements of fitness components indicated no Allee effect between larval densities and larval fitness, but rather indicated larval competition.4. In contrast, increased adult densities on the substrates prior to larval development yielded higher survival of the larvae, larger emerging flies, and also reduced fungal growth on the substrates. Hence, adults enhanced the quality of the larval substrate and significant benefits of aggregated oviposition in fruit flies were shown. Experiments with synthetic pheromone indicated that the aggregation pheromone itself did not contribute directly to the quality of the larval resource.5. The interaction among adults, micro-organisms, and larval growth is discussed in relation to the consequences for total fitness.

We assessed the extent to which traits related to ejaculate investment have evolved in lines of Drosophila melanogaster that had an evolutionary history of maintenance at biased sex ratios. Measures of ejaculate investment were made in males that had been maintained at male-biased (MB) and female-biased (FB) adult sex ratios, in which levels of sperm competition were high and low, respectively. Theory predicts that when the risk of sperm competition is high and mating opportunities are rare (as they are for males in the MB populations), males should increase investment in their few matings. We therefore predicted that males from the MB lines would (1) exhibit increased investment in their first mating opportunities and (2) deplete their ejaculates at a faster rate when mating multiply, in comparison to FB males. To investigate these predictions we measured the single mating productivity of males from three replicates each of MB and FB lines mated to five wild-type virgin females in succession. In contrast to the first prediction, there was no evidence for differences in productivity between MB and FB line males in their first matings. The second prediction was upheld: mates of MB and FB males suffered increasingly reduced productivity with successive matings, but the decline was significantly more pronounced for MB than for FB males. There was a significant reduction in the size of the accessory glands and testes of males from the MB and FB regimes after five successive matings. However, the accessory glands, but not testes, of MB males became depleted at a significantly faster rate than those of FB males. The results show that male reproductive traits evolved in response to the level of sperm competition and suggest that the ability to maintain fertility over successive matings is associated with the rate of ejaculate, and particularly accessory gland, depletion.

Summary1. Previous work has suggested that species diversity in resource-limited insect communities on patchy resources is maintained by spatial aggregation, not by resource partitioning. The most comprehensive test of this claim to date was by Shorrocks & Sevenster (1995), but some of their datasets included only a few resource types, which reduces the likelihood of ®nding a strong eect of resource partitioning. Also, methods of analysis have since been re®ned, e.g. to account for patch size. 2. We collected 733 mushroom samples belonging to 66 taxa in a Dutch woodland area. From these mushrooms, 38,891 insects were reared, belonging to 60 taxa of Diptera and Hymenoptera. Drosophilid species and their parasitoids were identi®ed to the species level; other taxa to the family level. We argue that the community is resource limited. 3. Generally, the insects have fairly narrow diets, including only a few of the available mushroom species. The degree of niche overlap varies widely in this community. 4. Within single resource types, co-existence can be explained by intra-speci®c aggregation over patches alone, in accordance with previous studies. 5. This conclusion remains unchanged for the mycophagous community as a whole: intra-speci®c aggregation of competitors is a sucient and necessary mechanism for co-existence in this diverse community, while resource partitioning does not contribute detectably to species diversity. This is the ®rst time that this pattern has been demonstrated in a dataset involving such a large number of resource types. 6. Our conclusions are strongly supported by data manipulations in which we removed or intensi®ed the eect of resource partitioning and spatial aggregation. 7. We argue that this community may be close to saturation, but we emphasize that saturation is a gradual phenomenon in patchy systems. 8. Since dierential use of resource types does not reduce competitive interactions among the insects, it seems unlikely that inter-speci®c competition constitutes the selective pressure favouring specialization.

Fly immune response to parasitoids

Expression profiling of the transcriptional response at 9 time points of Drosophila larvae attacked by insect parasites revealed 159 genes that were differentially expressed between parasitized and control larvae. Most genes with altered expression following parasitoid attack had not previously been associated with immune defense.

The development of biological control methods for exotic invasive pest species has become more challenging during the last decade. Compared to indigenous natural enemies, species from the pest area of origin are often more efficient due to their long coevolutionary history with the pest. The import of these well‐adapted exotic species, however, has become restricted under the Nagoya Protocol on Access and Benefit Sharing, reducing the number of available biocontrol candidates. Finding new agents and ways to improve important traits for control agents (“biocontrol traits”) is therefore of crucial importance. Here, we demonstrate the potential of a surprisingly under‐rated method for improvement of biocontrol: the exploitation of intraspecific variation in biocontrol traits, for example, by selective breeding. We propose a four‐step approach to investigate the potential of this method: investigation of the amount of (a) inter‐ and (b) intraspecific variation for biocontrol traits, (c) determination of the environmental and genetic factors shaping this variation, and (d) exploitation of this variation in breeding programs. We illustrate this approach with a case study on parasitoids of Drosophila suzukii, a highly invasive pest species in Europe and North America. We review all known parasitoids of D. suzukii and find large variation among and within species in their ability to kill this fly. We then consider which genetic and environmental factors shape the interaction between D. suzukii and its parasitoids to explain this variation. Insight into the causes of variation informs us on how and to what extent candidate agents can be improved. Moreover, it aids in predicting the effectiveness of the agent upon release and provides insight into the selective forces that are limiting the adaptation of indigenous species to the new pest. We use this knowledge to give future research directions for the development of selective breeding methods for biocontrol agents.

1. The effects of an aggregation pheromone on individual behaviour and food web interactions were investigated in two ecological communities, using Drosophila melanogaster and D. simulans as focal species.2. Fruit substrates with aggregation pheromone were significantly more attractive to adult D. melanogaster and D. simulans than control fruit substrates, and the response was positively dose dependent. Competing species and natural enemies were also significantly attracted to substrates with the aggregation pheromone of D. melanogaster and D. simulans.3. Significantly more eggs were deposited on pheromone-treated fruits than on control fruits, and the microdistribution of eggs within fruits was correlated to the microdistribution of the pheromone. The aggregation pheromone induced more females to share the breeding site.4. The extremely high densities of fruit flies in the large aggregations appeared to reduce the oviposition rate of females. Physical interactions with conspecific and heterospecifics were frequently observed in the aggregations, and often led to patch leaving of the fruit flies. 5. Competition for food among larvae occurred at high densities and parasitism was density dependent. Aggregation pheromones can be directly responsible for these patterns through their effects on the con-and heterospecific behaviour.6. The combined results show that aggregation pheromones affect a multitude of aspects in the ecology of interacting animals. The importance of incorporating the communication signals in ecological theory of aggregations is discussed.