The native chrysanthemum leafminer (Chromatomyia syngenesiae, formerly Phytomyza syngenesiae) can be distinguished from Liriomyza leafminers because it produces a relatively clean, white coloured mine, which is more linear and often follows the leaf margin. Leaf miners cause damage by puncturing the leaf surface to lay eggs and to feed on the leaf tissue. 

When the eggs hatch, the larvae tunnel within the leaf tissue forming damaging and disfiguring mines. The adults that emerge in the spring lay eggs which result in the summer larval attack; these will produce the next generation of adults in July. 

The peg-shaped maggots of chrysanthemum leaf miner attack a range of cultivated and weed hosts outdoors but with little lasting harm. Damage is particularly severe on composites including various chrysanthemums, cineraria, calendula, gazania and helianthus. There can be several generations in glasshouses as the lifecycle is often less than four weeks in duration, but outdoors there are only two. The mines which are most obvious on the upper leaf surface are very sinuous and appear white to brown and can both disfigure and ultimately shrivel so many leaves that young plants die and the quality of the flowers deteriorates in older ones. The marks left by the females after they lay their eggs also blemish the plants.

Leaf miner attack on leaf tissue

Leaf Miner adult

Chrysanthemum leaf miner flies often invade glasshouses as the weather outside begins to cool down. Some fly larvae cause blotch mines on the leaves of a very wide range of herbaceous, climbers and woody plants. Among these are the carnation fly (Delia brunnescens), the chrysanthemum blotch miner (Euribia zoe), and the holly leaf miner (Phytomyza ilicis).

Two generations of the 6mm long sturdy maggots of the chrysanthemum blotch miner cause linear mines along the leaf veins of Chrysanthemum maximum and autumn-flowering chrysanthemums which later become blotches.

Controls
The only insecticides in the UK pesticide guide recommended for leafminer on ornamentals are Deltamethrin & Nicotine. Neither are available to amateurs,  neither are worth buying just for that one use only.  

It is understood that products containing imidacloprid will cure it even though it is not approved,  so using imidacloprid to control aphids should also deliver control of leaf miner at the same time. 

This site in the USA http://edis.ifas.ufl.edu/mg006 shows imidacloprid - Admire.  Bifenthrin – Talstar   Spinosad – Conserve and also Deltamethrin.

Link to another useful leafminer page   www.defra.gov.uk/planth/pestnote/2007/liriomyza.pdf

Leaf Miner grub inside leaf tissue