Shading prevents bulbs beeing infested with bulbfly, the greater and the lesser! Here in Germany light shading should used from April on until the end of the dormant period. Bulb flys only lay their eggs at places with full sun to be sure that their is enough summer heat for the development of their larvae in the ground. If you have bulbs planted out in the garden I have best results were deciduous trees and shrubs form naturally some shade in april. Micael