Leaf miners cause damage to plants both directly and indirectly. The most direct damage is caused by the larvae mining the leaf tissue, leading to desiccation, premature leaf-fall and cosmetic damage. In tropical and subtropical areas this can lead to burning in fruit such as tomato and melon. Loss of leaves also reduces yield. In full-grown plants of fruiting vegetable crops, however, a considerable quantity of foliage can get damaged before the harvest is affected.
The older larvae make wider tunnels. Feeding spots made by adult females can also reduce yield, although except with ornamental crops, this is usually of less significance. Seedlings and young plants can be completely destroyed as a result of the direct damage caused by leaf miners.
In gerbera, the larva of the American serpentine leaf miner (Liriomyza trifolii) eats its way outwards from its egg, so that its mines join to form small plates. In various other crops one finds intermediate forms of tunnelling between these ‘plate mines’ and normal mines, making it an unreliable criterion for the identification of the species.
Indirect damage arises when disease causing fungi or bacteria enter the plant tissue via the feeding spots.