Leek disease or pest

My leeks have diseased or pest damage but I can't identify the cause. There is insect damage in places on the higher parts of the outer leaves with an obvious insect trail within the leaf.Th bulb part of the leek breaks off when pulled and ends up shredded instead of the expected intact bulb. No obvious insects visible. The upper parts of some leaves are slightly yellowed and twisted. The plants are pretty unusable. What's the cause? Unfortunately I can't attach photos to help in ID