Because of its origins in South America, the tomato plant has very few natural pests here in Great Britain although it will suffer the scourge of common glasshouse pests if they are grown under protection. In fact, you are far more likely to experience problems when growing under glass or in a grow bag than you will ever get when growing directly into well prepared soil.
The leaf miner adult - and more specifically the larvae - is one of those pests that can be quite difficult to control on tomatoes irrespective of whether they are grown under protection or not and its all down to their unusual life-cycle.
Symptoms: Typically the first signs you will come across are white ‘wiggle’ marks in the leaves which is the major form of damage by the larvae, and will result in the destruction of the internal leaf mesophyll. The mine becomes noticeable after about three or four days after oviposition and becomes larger in size as the larva matures. Both leaf mining and the stippling caused by the female adult can greatly reduce the leafs ability to photosynthesise. Extensive mining can also cause premature leaf drop.
Control: Leaf miner are difficult to control using a contact insecticides as they are pretty much out of reach, protected by the leaf membrane. They can be controlled though by a systemic insecticide but then you probably wouldn’t want to risk eating the crop afterwards. This is one of those cases where it may be best to leave them alone - other than picking off and destroying the worst affected parts of the plant. In fact, your tomato plants can have as much as 60% of its foliage affected with leaf miner without affecting the fruit or its growth. After harvesting your crop, you can try double digging the soil where the tomatoes grew as the adult leaf miners experience difficulty in emerging if they are buried. You can try repeating this several times over the winter before re-planting your crops in the late spring.