Forest Pests: Insects, Diseases & Other Damage Agents

Csóka, György (2003): Levélaknák és levélaknázók - Leaf mines and leaf miners. Hungarian Forest Research Institute. Erdészeti Turományos Intézet, Agroinform Kiadó, Budapest, 192 pp.

Leaf mining is a close physical connection with the host plant. It regularly also means fine adaptations and narrow host range. Majority of the leaf miners are specialist, mining one, or only a few closely related hosts. There are some evident exceptions from this pattern. A few species are among tortricids, which are extremely polyphagous (Cnephasia species). Mines of these species can be found on nearly any flowering plants with exception of conifers. Several hundreds plant species belonging to 50 families are know as their host plants. They mine only when they young, later live among leaves spun together. There are several agromyzid flies (i.e. Liriomyza species) mining on many, not closely related species. Contrary to these examples most leaf miners distinguish even the closely related hosts. Most of them are genus-monophages, living on members of one plant genus. Many examples are known for even higher specialization. There are several species (i.e. Stigmella szoecsiella, Ectoedemia cerris) are among pigmy moths (Nepticulidae) which mines only on species belonging to Cerris section of Quercus genus (i.e. Quercus cerris, Q. trojana). Other species of the same family (i.e. Stigmella basiguttella) live only on oaks belonging to Quercus section. Similarly high level of specialisation can also be observed among midget moths (Lithocolletidae) living on oaks.

Examples are among other leaf mining groups. The agromyzid Phytomyza senecionis occurs on most Senecio species. There are only one Senecio species (S. fuchsii) , that is mined by a different, highly specialist agromyzid (Phytomyza ovatus).

The number of leaf miners living on a host plant is determined by joint effect of several different factors. Important factors are among others the taxonomic isolation and the size of the area of the host. A host plant with larger area and many relatives will probably have more leaf miners, than a taxonomically isolated plant with small area. Betula, Quercus, Malus, Populus, Alnus, Prunus and Crataegus are the most species rich woody plant genera in Hungary.

Leaf miners and their hosts have a great variety of interactions. A few selected examples are given below.

Some leaf miners play important role in spreading of plant pathogens. Agromyzid flies of genus Liriomyza are know to be vectors of tobacco mosaic virus and other virus infections. The wounds made by Liriomyza trifolii serve as infection gate for Pseudomonas bacteria.

It has been observed on many different hosts that mined leaves fall earlier than the intact ones. This early leaf abscission can have a negative impact on the leaf miners themselves, because the leaves often fall down and dry before the larva completed its development. It can cause considerable mortality of larvae if happens early enough. This impact is negligible on some hosts, but significant on others. It has not been clarified, whether is it a host defence mechanism, or simply aftermath of the mining. Some lepidopteran leaf miner can compensate the negative effect of the early leaf fall. This phenomenon can be observed on the leaf mined by Stigmella tityrella (see page). Second generation larvae of this species mine until late autumn. Larvae continue feeding even on fallen, yellowing leaves keeping the tissues around the mine green. Most likely explanation for this is that the cytokinins secreted by the larva keeps the tissues alive, providing suitable food source until the end of the larval development. Similar interaction can be found among Phyllonorycter leaf miners on Fagus and Quercus hosts.

Most species of the diverse leaf miner fauna of oaks mines during summer and autumn in contrast to free feeders which peaking in both abundance and species richness in May. The likely explanation for this late appearance of miners, that they can avoid the direct (consumption of mined leaves) and indirect (host defence) negative impacts of free feeders by this timing.

The herbivore insects belonging to different feeding guilds react differently to the physiological conditions of and stress of host triggered by various environmental factors (drought, air pollution, etc.). It is a general view that leaf miners react positively to the moderate stress level of the host. Highest densities of Eriocrania leaf miners on birches living on polluted area were found at moderate level of pollution. According to studies made in The Netherlands high density of Phyllonorycter mines on oak or beech indicates stressed status of the tree. So the mine density can be considered as a health indicator of trees.

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