Its not cut and dried, I had some caterpillars on module plants only once in the last three years (Sep 13) after a hot summer when there were many around. But I raise plants in a greenhouse with the door mostly shut, perhaps its side and roof windows offer less point of entry.
Perhaps it would be worth a mesh where doors are open, but not windows I think.
Also if the weather stays wet, young brassicas grow away better without mesh and I have mine uncovered at the moment, until mid July probably.
