Leaves as Islands: Interactions of Scolioneura betuleti (Hymenoptera) Miners in Birch Leaves

The intraspecific interactions of hymenopteran miners in birch leaves originating from the same tree were studied. The following relationships were found: (1) The number of larvae and total larval biomass per leaf correlated positively with the size of the host leaf measured as area. (2) The mean weight of individual larvae correlated positively with leaf area per larva and showed a non-linear inverse relation to the number of larvae occupying the same leaf. (3) When two larvae occupied the same mine, the difference in weight increased as total larval biomass per leaf area increased. (4) When two species of leaf miners were compared, species feeding on old leaves had more excreta per larval biomass than species feeding on young birch leaves. The theoretical implications of the empirical study were analyzed and the following relationships were suggested: (1) The number of larvae per leaf is determined by the balance between the oviposition rate and the mortality rate of larvae, both of which are functions of leaf area. (2) The mean weight of larvae has a linear relation to leaf area per larva and a non-linear inverse relation to the number of larvae when leaf area determines the maximum larval biomass that a leaf is able to support. (3) Competitive interference and variation in timing of larval growth are able to cause differences in weight between larvae occupying the same leaf. (4) Seasonal changes in leaf quality can cause differences in larval growth. It is emphasised that both the size and the quality of leaves are important for insect herbivores.