Nutritional benefits of the leaf-mining behaviour of two grass miners: a test of the selective feeding hypothesis

1. The selective feeding hypothesis predicts that miners prefer tissues high in nutrient content and low in chemical and structural defence content. This hypothesis was tested for two dipteran grass miners, Chromatomyia milii and C. nigra, feeding on Holcus lanatus. 2. Leaf sections revealed that both miners preferred to feed on the mesophyll layer, minimised the intake of veinal tissues, and never fed on the epidermal layer. Nutritional quality analyses showed that the mesophyll had a higher nutrient content and a lower structural defence content than the epidermal and veinal tissues. 3. The nutritional quality of mined leaf areas was lower than that of unmined leaf areas, suggesting that the mining larvae remove the most nutritious tissues; however this commonly used approach for studying the selective feeding hypothesis is questioned in this study. 4. Larval feeding rates were higher on the mesophyll than on the veinal tissues, probably due to a higher structural defence content of the veins. Feeding rates were higher on the small veins than on the larger and more sclerified, strengthened veins. 5. This study supports the selective feeding hypothesis as both species concentrated their feeding on the mesophyll, which is the most nutritious and least tough tissue within a H. lanatus leaf.