DEFENSE REACTIONS IN APHIDS - THE INFLUENCE OF STATE AND FUTURE REPRODUCTIVE SUCCESS

1. This paper investigates the influence of life-history traits on the defence reactions shown by aphids. In particular, we combine experimental data on the reproductive biology of the monophagous aphid Uroleucon jaceae (L.) feeding on Centaurea jacea L. with theoretical considerations to predict the behaviour of this aphid when attacked by a predator. 2. In laboratory experiments we first determine the reproductive behaviour of this aphid on different plant developmental stages and plant qualities. In food deprivation experiments we examine the effect of different patterns of resource allocation to soma and gonads on the ability to survive without food. Aphids feeding on the shooting stage of high-quality plants had high reproductive rates and survived longest when deprived of food. In contrast, aphids from dying plant stages did worst. Aphids feeding on the 4-leaf stage developed high reproductive rates, but were less resistant to starvation. 3. Using these laboratory results in addition to field data on plant phenology and predator impact, a model based on the premise of maximizing the expected total reproductive success (ETRS) is developed to make qualitative predictions about the behaviour of an aphid when being attacked by a predator. 4. In the model, dropping off the host-plant, in response to a predator attack, is shown more frequently by aphids feeding on the tillering and shooting plant stages in comparison to aphids feeding on the 4-leaf stage. If the number of offspring produced is weighted (discounted), behavioural differences between aphids feeding on high and on poor quality host-plants were more pronounced. 5. Finally, empirical evidence on the dropping behaviour is presented for aphids feeding on different plant phenological stages. Dropping reactions were only shown on high quality host-plants, but not on the 4-leaf stage. Aphids which were just moulting into the adult stage presented this type of reaction more frequently than older aphids. 6. Both the model and the experiments demonstrate the importance of reproductive and behavioural features in the life-history strategy of aphids. The adaptive significance of a physiological and behavioural polyphenism is discussed.