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The study of the role of chemical information in species interactions has been mostly restricted to studies at the level of individual organisms. The central question in this thesis is how intraspecific chemical information conveyance and exploitation thereof by a natural enemy affects the spatial population dynamics of a species. To answer this question, I developed a spatio-temporal model where both host and parasitoid can respond to infochemicals. Our model system consists of the fruit fly Drosophila melanogaster, and its natural enemy, the larval parasitoid Leptopilina heterotoma. D. melanogaster uses its aggregation pheromone in combination with odours from fermenting fruits to localise suitable resources for reproduction. L. heterotoma uses these same odours to localise its host. For D. melanogaster, aggregation on a resource can be beneficial when a population is small and has to overcome negative effects associated with low population densities. Such negative effects, known as the Allee effect, can for instance be caused by difficulties in resource exploitation or in finding a mate. Aggregation also involves costs. Individuals within an aggregation frequently experience more severe competition for food, space and mates than they would experience when being on their own. Furthermore, I investigated which behavioural decisions enhance the ability to find - and distinguish between – odour sources that differ in their suitability for reproduction. On the individual level, this research showed that, like real fruit flies, the modelled fruit flies need to have a preference for the presence of both aggregation pheromone and food odours, over food odours alone, to be able to distinguish between the two types of odour sources. The results show that this stronger preference does not have to be innate. As long as fruit flies are able to remember and adjust their current preference based on the odour concentrations that they perceive, more fruit flies find the more attractive odour source. On a population level, this thesis shows that the use of chemical information by D. melanogaster affects its population dynamics. In the absence of its natural enemy, and when the Drosphila population is small, the use of food odours and aggregation pheromone has a positive effect on population growth and enhances the fruit fly’s colonization ability. When the population becomes larger, however, the negative effects of larval competition are stronger than the positive effects of reduced mortality due to the Allee effect. The use of chemical information was crucial to colonize an area from the boundaries. A fruit fly population that was unable to use chemical information could not colonize the area and went extinct. When parasitoids can use chemical information, parasitism rates are higher, resulting in a slower population growth of their host. No difference was recorded in fruit fly population size and in larval mortality due to parasitism, when parasitoids exploited the aggregation pheromone of the fruit fly adults as compared with the simulations where the parasitoids could only respond to chemicals emitted by the host habitat. In contrast, the use of chemical information by the host enhanced its population growth and enabled it to survive, even at higher parasitoid densities. This research showed that mortality when the population was small had a greater impact on population size than mortality due to competition or parsitism. Food patches are not always abundant in nature. Thus, the reproductive success of fruit flies is mainly determined by their opportunities of producing clutches (i.e. locating patches) rather than by preventing over-aggregation or parasitism. As a result, the use of chemical information has a net positive effect on fruit fly population dynamics, despite the fact that L. heterotoma is able to exploit it.

The study of the role of chemical information in species interactions has been mostly restricted to studies at the level of individual organisms. The central question in this thesis is how intraspecific chemical information conveyance and exploitation thereof by a natural enemy affects the spatial population dynamics of a species. To answer this question, I developed a spatio-temporal model where both host and parasitoid can respond to infochemicals. Our model system consists of the fruit fly Drosophila melanogaster, and its natural enemy, the larval parasitoid Leptopilina heterotoma. D. melanogaster uses its aggregation pheromone in combination with odours from fermenting fruits to localise suitable resources for reproduction. L. heterotoma uses these same odours to localise its host. For D. melanogaster, aggregation on a resource can be beneficial when a population is small and has to overcome negative effects associated with low population densities. Such negative effects, known as the Allee effect, can for instance be caused by difficulties in resource exploitation or in finding a mate. Aggregation also involves costs. Individuals within an aggregation frequently experience more severe competition for food, space and mates than they would experience when being on their own. Furthermore, I investigated which behavioural decisions enhance the ability to find - and distinguish between – odour sources that differ in their suitability for reproduction. On the individual level, this research showed that, like real fruit flies, the modelled fruit flies need to have a preference for the presence of both aggregation pheromone and food odours, over food odours alone, to be able to distinguish between the two types of odour sources. The results show that this stronger preference does not have to be innate. As long as fruit flies are able to remember and adjust their current preference based on the odour concentrations that they perceive, more fruit flies find the more attractive odour source. On a population level, this thesis shows that the use of chemical information by D. melanogaster affects its population dynamics. In the absence of its natural enemy, and when the Drosphila population is small, the use of food odours and aggregation pheromone has a positive effect on population growth and enhances the fruit fly’s colonization ability. When the population becomes larger, however, the negative effects of larval competition are stronger than the positive effects of reduced mortality due to the Allee effect. The use of chemical information was crucial to colonize an area from the boundaries. A fruit fly population that was unable to use chemical information could not colonize the area and went extinct. When parasitoids can use chemical information, parasitism rates are higher, resulting in a slower population growth of their host. No difference was recorded in fruit fly population size and in larval mortality due to parasitism, when parasitoids exploited the aggregation pheromone of the fruit fly adults as compared with the simulations where the parasitoids could only respond to chemicals emitted by the host habitat. In contrast, the use of chemical information by the host enhanced its population growth and enabled it to survive, even at higher parasitoid densities. This research showed that mortality when the population was small had a greater impact on population size than mortality due to competition or parsitism. Food patches are not always abundant in nature. Thus, the reproductive success of fruit flies is mainly determined by their opportunities of producing clutches (i.e. locating patches) rather than by preventing over-aggregation or parasitism. As a result, the use of chemical information has a net positive effect on fruit fly population dynamics, despite the fact that L. heterotoma is able to exploit it.

Sinds 1967 wordt in Nederland onderzoek gedaan naar de mogelijke rol van het gebruik van seksferomonen in de geïntegreerde bestrijding van schadelijke insecten in de land- en tuinbouw. Seksferomoonvallen worden al vele jaren op grote schaal in de praktijk toegepast als een gemakkelijke en betrouwbare techniek om de aanwezigheid van deze insecten vast te stellen. Naast de mogelijkheid om insecten waar te nemen zijn in de loop der tijd diverse methoden ontwikkeld waarbij seksferomonen worden toegepast als een directe bestrijdingstechniek. De meest voor de hand liggende methode is het wegvangen van grote aantallen insecten m.b.v. grote aantallen feromoonvallen. Een andere methode, de zogenaamde verstoringstechniek is gebaseerd op het principe dat mannetjesinsecten de vrouwtjes niet meer kunnen opsporen wanneer het kunstmatige seksferomoon in hoge concentraties in de leefatmosfeer aanwezig is. Een tweede vorm van verstoring is om veel puntbronnen met een lage dosering kunstmatig seksferomoon aan te brengen zodat de mannetjes op de verkeerde poot worden gezet. Een techniek waarbij de lokkende werking van het seksferomoon gecombineerd wordt met de dodende werking van een insecticide is de zgn. 'Lure&Kill' (NL: Lokken en doden) techniek. Het grote voordeel van deze techniek is dat de insecten naar het insecticide worden gebracht i.p.v. het insecticide naar het insect zoals bij een gangbare bespuiting. Daardoor kan van veel minder insecticide gebruik gemaakt worden en worden zowel de toepasser als het gewas niet blootgesteld aan drift. Bovendien wordt nevenwerking op nuttige organismen geminimaliseerd omdat seksferomonen dermate soortspecifiek zijn dat buiten het doelinsect geen andere insectensoorten worden beïnvloed.

Sinds 1967 wordt in Nederland onderzoek gedaan naar de mogelijke rol van het gebruik van seksferomonen in de geïntegreerde bestrijding van schadelijke insecten in de land- en tuinbouw. Seksferomoonvallen worden al vele jaren op grote schaal in de praktijk toegepast als een gemakkelijke en betrouwbare techniek om de aanwezigheid van deze insecten vast te stellen. Naast de mogelijkheid om insecten waar te nemen zijn in de loop der tijd diverse methoden ontwikkeld waarbij seksferomonen worden toegepast als een directe bestrijdingstechniek. De meest voor de hand liggende methode is het wegvangen van grote aantallen insecten m.b.v. grote aantallen feromoonvallen. Een andere methode, de zogenaamde verstoringstechniek is gebaseerd op het principe dat mannetjesinsecten de vrouwtjes niet meer kunnen opsporen wanneer het kunstmatige seksferomoon in hoge concentraties in de leefatmosfeer aanwezig is. Een tweede vorm van verstoring is om veel puntbronnen met een lage dosering kunstmatig seksferomoon aan te brengen zodat de mannetjes op de verkeerde poot worden gezet. Een techniek waarbij de lokkende werking van het seksferomoon gecombineerd wordt met de dodende werking van een insecticide is de zgn. 'Lure&Kill' (NL: Lokken en doden) techniek. Het grote voordeel van deze techniek is dat de insecten naar het insecticide worden gebracht i.p.v. het insecticide naar het insect zoals bij een gangbare bespuiting. Daardoor kan van veel minder insecticide gebruik gemaakt worden en worden zowel de toepasser als het gewas niet blootgesteld aan drift. Bovendien wordt nevenwerking op nuttige organismen geminimaliseerd omdat seksferomonen dermate soortspecifiek zijn dat buiten het doelinsect geen andere insectensoorten worden beïnvloed.

Many insect species use an aggregation pheromone to form groups with conspecifics in certain localities of the environment. This type of behaviour has a variety of implications for ecological interactions, both directly through the effect of the pheromone on the behaviour of con- and heterospecifics, and indirectly through the consequential aggregative distributions that may affect species interactions. The evolutionary ecology of the use of aggregation pheromone has received only little attention. Yet, these pheromones may play an intricate role in food web interactions by providing an accompanying information web.The aim of this thesis is to further our understanding on the ecological and evolutionary aspects of the use of aggregation pheromone in insects. By unravelling costs and benefits that arise from the use of aggregation pheromone in our ecological model organism, Drosophila melanogaster , we strive to answer why they use an aggregation pheromone and elucidate the ecological consequences of an aggregation pheromone in a food web context.In laboratory and field studies, we identified behaviours and interactions of the fruit fly D. melanogaster that were affected by its aggregation pheromone. The pheromone affected the distribution of adults, their eggs, competitor species and parasitoids. Moreover, a number of costs and benefits to the use of aggregation pheromone were indicated. In subsequent studies, the major hypotheses on costs and benefits were examined.A major benefit of using aggregation pheromone was shown to be aggregated oviposition. Aggregated oviposition enhanced the quality of the larval resource, as indicated by a higher survival of the larvae and larger size of the emerging flies. This Allee effect was characterised by a positive effect of adult density on larval fitness components, and may have arisen from the interaction between adult flies and micro-organisms (yeasts and fungi). Fungi antagonise yeast and larval development, while adults can inoculate yeast on a substrate and temper fungal growth. Larvae also tempered fungal growth, but an increased larval density did not result in an Allee effect but in competition instead.A major cost of using aggregation pheromone arose from an increased risk of parasitism. The parasitoid Leptopilina heterotoma uses the aggregation pheromone of adult fruit flies to localise the larval hosts, and based on this information this parasitoid can differentiate quantitatively at long range between substrates that differ in profitability. After arrival on a substrate, the pheromones no longer play a role in the host searching behaviour. A behaviour-based model was developed to predict the individual risk of parasitism for hosts in differently sized host aggregations. The functional and numerical responses of the parasitoids were combined with a flexible patch leaving decision rule for the parasitoid, to assess whether aggregation could also comprise a benefit to the hosts in terms of a diluted risk ( sensu Hamilton 1971). The model prediction reads that aggregation is not beneficial in the context of the Drosophila - Leptopilina interaction, and these predictions were supported by field data.In a simple spatio-temporal simulation model, the population dynamics arising from several modes of dispersal, food competition and an Allee effect were explored. The model is a first step towards a more extensive model that incorporates the responses of insects to spatially heterogeneous resources and chemical information (e.g., aggregation pheromone).The main conclusion from this thesis is that the aggregation pheromone of D. melanogaster plays an intricate role within a foodweb context, and that a variety of costs and benefits arise from multitrophic interactions. To understand the dynamic interactions in this and many other ecological systems, it is essential to gain more insight into the effect of aggregation pheromone on the behaviour of individuals.

Many insect species use an aggregation pheromone to form groups with conspecifics in certain localities of the environment. This type of behaviour has a variety of implications for ecological interactions, both directly through the effect of the pheromone on the behaviour of con- and heterospecifics, and indirectly through the consequential aggregative distributions that may affect species interactions. The evolutionary ecology of the use of aggregation pheromone has received only little attention. Yet, these pheromones may play an intricate role in food web interactions by providing an accompanying information web.The aim of this thesis is to further our understanding on the ecological and evolutionary aspects of the use of aggregation pheromone in insects. By unravelling costs and benefits that arise from the use of aggregation pheromone in our ecological model organism, Drosophila melanogaster , we strive to answer why they use an aggregation pheromone and elucidate the ecological consequences of an aggregation pheromone in a food web context.In laboratory and field studies, we identified behaviours and interactions of the fruit fly D. melanogaster that were affected by its aggregation pheromone. The pheromone affected the distribution of adults, their eggs, competitor species and parasitoids. Moreover, a number of costs and benefits to the use of aggregation pheromone were indicated. In subsequent studies, the major hypotheses on costs and benefits were examined.A major benefit of using aggregation pheromone was shown to be aggregated oviposition. Aggregated oviposition enhanced the quality of the larval resource, as indicated by a higher survival of the larvae and larger size of the emerging flies. This Allee effect was characterised by a positive effect of adult density on larval fitness components, and may have arisen from the interaction between adult flies and micro-organisms (yeasts and fungi). Fungi antagonise yeast and larval development, while adults can inoculate yeast on a substrate and temper fungal growth. Larvae also tempered fungal growth, but an increased larval density did not result in an Allee effect but in competition instead.A major cost of using aggregation pheromone arose from an increased risk of parasitism. The parasitoid Leptopilina heterotoma uses the aggregation pheromone of adult fruit flies to localise the larval hosts, and based on this information this parasitoid can differentiate quantitatively at long range between substrates that differ in profitability. After arrival on a substrate, the pheromones no longer play a role in the host searching behaviour. A behaviour-based model was developed to predict the individual risk of parasitism for hosts in differently sized host aggregations. The functional and numerical responses of the parasitoids were combined with a flexible patch leaving decision rule for the parasitoid, to assess whether aggregation could also comprise a benefit to the hosts in terms of a diluted risk ( sensu Hamilton 1971). The model prediction reads that aggregation is not beneficial in the context of the Drosophila - Leptopilina interaction, and these predictions were supported by field data.In a simple spatio-temporal simulation model, the population dynamics arising from several modes of dispersal, food competition and an Allee effect were explored. The model is a first step towards a more extensive model that incorporates the responses of insects to spatially heterogeneous resources and chemical information (e.g., aggregation pheromone).The main conclusion from this thesis is that the aggregation pheromone of D. melanogaster plays an intricate role within a foodweb context, and that a variety of costs and benefits arise from multitrophic interactions. To understand the dynamic interactions in this and many other ecological systems, it is essential to gain more insight into the effect of aggregation pheromone on the behaviour of individuals.