| OBJECTIVES: Extrafloral nectar (efn) and honeydew are both collected and aggressively defended by ants. In their interaction with ants these sugar sources are opposed in their putative functions, i.e. defending the plant against herbivory and enhancing herbivore survival, respectively. Given this contrariety, there is an obvious conflict in situations in which sap-feeders and extrafloral nectar co-occur. To the plant this conflict increases with increasing cost of sap-feeder infestation. While some sap-feeders (such as aphids) constitute no or only minor fitness costs to the plant, others are known to induce strong direct toxic effects, or entail indirect costs as vectors of viruses. To the plant the extent of a possible conflict depends on the behavioural response of the ants. Plants could manipulate this aphid-ant mutualism, when extrafloral nectar secretion induces ants to change from being aphid mutualists to becoming aphid predators. Plants could outcompete aphids by providing a higher quantity of food, or by providing food of higher quality. We hypothesise that the latter can be achieved when secondary plant compounds (spc) are primarily expressed in honeydew and to a lesser extend in extrafloral nectar. APPROACH: To substantiate the hypothesis a model will be developed that connects ant optimal foraging with aphid population dynamics. Assuming that proteins and carbohydrates are both essential food components for ants, and that each ant species has an optimal intake ratio, the consumption of these food sources can be linked with a fitness measure for the ant colony. Different levels of predation of ants on aphids translate into different aphids population developments, and consequently the (future) availability of prey and honeydew. By calculating the consequences of different predation strategies the optimal strategy (in terms of ant fitness) can be identified. Subsequently the impact of an additional sugar source (efn), and the quality and quantity of the honeydew can be investigated. The possible confounding impact of environmental factors such as additional protein sources and competitors will be considered. Cotton (Gossypium sp.) is a good hostplant to experimentally test the hypothesis. First, these plants have different types of extrafloral nectaries (foliar and bractal nectaries). Second, both the direct and indirect defences (terpenoid and extrafloral nectar production) are well studied and are known to be inducible in these plants. Third, wild cotton species vary strongly in the levels of (induced) terpenoid and efn. production. And fourth, several aphid species (such as Aphis gossypii) are know to cause serious damage to these plants, and are known to be tended by ants (such as Lasius niger). To test our hypothesis, the following aspects will be studied: 1. The level of spc in both honeydew and extrafloral nectar should be compared within the same plant, both before and after induction. The initial focus could be on cotton species that have both a high nectar production and a high terpenoid level in the leaves (such as G. arboreum). 2. To study if spc indeed affect ants' foraging behaviour, the ants can be provided a choice between (natural or artificial) sugar sources with different levels of terpenoids. 3. When honeydew and extrafloral nectar show different terpenoid levels, and ants respond accordingly, ant behaviour in the full aphid/efn/ant systems should be investigated. By comparing host plants that differ in terpenoid content, and by manipulating the availability and quality of efn, the effect of these plant traits on the aphid-ant interactions can be addressed. In this experiment the monitoring should focus on aphid predation by the ants. |
