Leaf-mining insects, belonging to Lepidoptera, Diptera, Coleoptera, or Hymenoptera, have larvae that live and forage inside leaves. The mines remain visible for a long time and the life time of most leafmining larvae is spent inside the mine. Ecologists and even paleontologists can without much difficulty reconstruct the living conditions of these insects and evaluate their influences on host plants. Most leafminers are monophagous or oligophagous and many leaf mines are species-specific. Therefore, leafminers are often used as model organisms to study plant-insect-natural enemy interactions and co-evolution. Although leafminer outbreaks do not occur often in natural habitats, some leafminers are prevalent pests in agriculture and forestry. Due to their small body size, short life history, high reproductive ability and endophagous habit, leafmining pests are not easily discovered during their early stages of development. Once they break out, they damage the leaves and thus degrade photosynthesis and production. Compared to ectophagous insects, endophagous leafminers should depend more on leaf characteristics. There are many reports on relationship between leaf miners and chemical composition of host plant leaves, whereas few reviews on physical leaf features. Thence we summarize the effects of physical leaf properties on the feeding, survival, distribution, reproduction and behavior of leafminers. For example, the location, directional poisition, size, color and trichome of leaves have direct impacts on feeding behavior and oviposition site selection of adult leafminers; and therefore also on where larva live and quality of their food. Some physical features of the leaves also act directly on the feeding behavior, parasitism and development of leafmining larvae.
(1) Leaf location and direction: The oviposition and feeding of adults are generally located at the middle to lower parts of plant individuals and thus the eggs and larvae often occur in the same parts. The light conditions of leaves in different positions vary and this will indirectly affect the development, survival and parasitism of leafmining larvae via leaf features, nutrient contents, secondary compounds and mine temperatures. However, some leafmining larvae can avoid adverse light effects through selective foraging.
(2) Leaf size: Leaf size has a positive relationship with optimal oviposition choices for adults and the developmental success of larvae.
(3) Leaf thickness: There is no distinct relationship between leaf thickness and leaf mining.
(4) Leaf color: The influence of leaf color on leafmining varies with insect species.
(5) Leaf trichomes: Leaf trichomes can disturb the oviposition and feeding of leafminers, but some leafminers select leaves with more densed trichomes since trichomes can also disturb the searching behavior of paristic wasps and reduce the feeding competition of ectophagous insects.
(6) Leaf veins: Many leafminers avoid indigestible vascular bundles while some specialized leafminers prefer leaf veins because leaf veins can provide shelter against natural enemies.
Combining both physical and chemical information of leaves leafminers can form identifying patterns on host plants. The oviposition choices of leafminer adults determine larvae location and thus have a large influence on leafminer population dynamics. However, if polyphagous leafminers oviposite eggs on non-host plants and if they manage to complete their life history their host range can be extended. Studies on the defenses of leaf physical structure and the adaptations of leafminers to these structural defenses are helpful in understanding the co-evolution between leafminers and their host plants. Physical characteristics of plant leaves can be altered by environmental factors and genetic breeding. In this way, leaves can become resistant and the sustainable ecological control of leafmining pests may be achieved.