Insects as leaf engineers : can leaf-miners alter leaf structure for birch aphids?

This study examined the indirect impacts of leaf-mining insects, Eriocrania spp. Zeller (Lepidoptera: Eriocraniidae) on a phloem-feeding insect, Euceraphis betulae Koch (Homoptera: Drepanosiphinae). While many insect herbivores affect one another through changes to host plant chemical composition, Eriocrania also has the potential to affect E. betulae through structural modification of a shared leaf. Euceraphis betulae mortality was higher when caged on leaves with Eriocrania leaf-miners. Mortality was not affected by the amount of leaf mined or elevated phenolic compound concentrations in mined leaves, but leaf-miner induced damage to the midrib was strongly correlated with poor aphid survival. In field surveys, E. betulae was significantly less abundant on mined leaves with midrib damage than on mined leaves with just lamina damage, or mine-free leaves. Experiments simulating leaf-miner damage on B. pendula leaves pinpointed midrib damage as being associated with higher E. betulae mortality, whereas lamina damage had no effect on aphid mortality. Disruption of phloem hydraulics is proposed as the mechanism underpinning the negative impacts on the aphid. Eriocrania larvae mining leaves with manually damaged midribs weighed more than those in which the midrib was intact. There was also a trend towards higher nitrogen concentrations in leaves in which Eriocrania had damaged the midrib. There could therefore be a selective advantage to leaf-miners that damage the midrib if severance improves leaf nutritional quality, in addition to rendering the leaves unsuitable to potential competitors.