The Gardener's Almanac

Sounds like 'Carnation fly'

 

The maggots of this fly tunnel into the leaves and stems of carnations and pinks, causing shoots to wilt and die.

 

They are not easily eradicated once they have tunnelled into the plant tissues, you could try dusting or spraying with a suitable insecticide in April and early May this might give some control.

Remove the worst affected leaves and then spray three times at fortnightly intervals with a fungicide.

 

Keep your plants well spaced out to reduce humidity around the foliage.