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The other main problems are Pea Moth, Pea Weevil or Pea Thrip. The solution for all is to spray with derris immediately any problem is noticed, plus a precautionary spraying about a week before the plants flower - spray well at this stage to especially deter Pea Moth.
I understand that peas are "self-pollinating" so the covers can remain in place. Plants are now a good 9" tall and looking good. From past experience the pea moth often arrives too late for early peas, so they are mostly maggot-free. It is the main crop (summer) peas which usually attract the fly.Good luck