THE IMPORTANCE OF LEAF STRUCTURE IN OVIPOSITION BY LEAF-MINING MICROLEPIDOPTERA

Broad patterns exist in the site of oviposition for British leaf-mining Lepidoptera. Of 227 species, 21% lay only on the upper surface of leaves, 73% only on the lower surface and 6% on both. We examine possible explanations for this pattern based on the requirements of the egg, larva and ovipositing adult, and on the surface and internal structures of the leaves on which the larvae feed. A significantly greater proportion of tree-feeding than herb-feeding species lays on the lower surface of leaves, suggesting that surface microclimate could be important. Species laying on both surfaces tend to lay on plants which differ less in pubescence between the two surfaces. There are insufficient data on relative thicknesses of the upper and lower leaf epidermides to suggest whether or not penetration by the newly-eclosed larva is important. Several food plant species diverge from the usual dorsiventral organisation of palisade parenchyma and spongy mesophyll within the leaf, affecting the location of larval feeding and, apparently for Populus spp, oviposition also. We give several reasons why we expect species to lay on the lower surface, yet 20% selectively lay on the upper surface. We suggest this is because oviposition is time-limited and laying on the upper surface is quicker and simpler.