With more and more LEEKS being transplanted, you might want to take note of this news from the Cornell Vegetable Program: Transplanted Onions More Susceptible to Onion Maggot than Normal.
With planting being about 1 month behind [in upstate NY, Cornell Vegetable Agent Christy Hoepting] predict[s] that transplanted onions [including leeks] will be more susceptible to Onion Maggot (OM) damage than normal. Gravid OM females prefer onions with a stem diameter of 2.25 to 4.25 mm (1.5 to 2 leaf stage) to lay her eggs. During peak flight …, newly planted transplants will be more attractive than direct seeded onions that will be in the loop to flag leaf stage. … Make sure that you treat your transplants with an insecticide for OM control.
Growers with small acreage and/or no known history of OM most likely do not need to be concerned with OM. However, seed maggots (SM) can also attack onions and cause identical damage as OM. Onions planted into decaying residue in a cool wet spring are at high risk for SM.
(Cornell University Cooperative Extension VegEdge Weekly Vol. 7 (8) May 18, 2011)