Seasonal changes in leaf nutritional quality influence grass miner performance

1. The plant phenological age hypothesis predicts that phytophagous insects should prefer and perform better on phenologically young plants than on old plants because plant nutritional quality decreases with plant phenological age. This hypothesis was tested under field and laboratory conditions with the grass miner Chromatomyia milii on the free-growth species Holcus lanatus. 2. The field experiment was conducted at four sample sites in Belgium where nutritional quality of H. lanatus leaves and performance of C. milii were monitored throughout the growth season. Foliar nutritional quality was highest early in the season due to high levels of proteins and soluble carbohydrates and low levels of lignin. Offspring performance (pupal size) decreased with plant phenological age, due at least partially to the decreasing foliar nutritional quality. 3. In the laboratory experiment. preference and performance of C. milii were determined on three different age classes of H. lanatus. Multiple-choice experiments demonstrated that oviposition preference did not differ among age classes. Offspring survival decreased with plant phenological age. while pupal size did not differ among a e classes. The relationship between foliar nutritional quality and plant phenological age was equivocal and did not correspond to the predictions of the plant phenological age hypothesis. 4. The results of the field experiment were consistent with the idea that the phenological age hypothesis holds in free-growth species. The laboratory experiment gave only little support to the plant phenological age hypothesis. Possible causes for the differences between field and laboratory experiments are discussed.