Photo of Western flower thrip damage to leaves
Thrips rasp (by puncturing individual surface cells and sucking cellular contents) the surface of fresh young plant tissues as they feed. Their feeding can weaken and deform flowers and cause a white specking or silvery discoloration on the underside of leaves. Under severe infestations, the specking can be seen as yellowing and fading of healthy green leaf color on the upper leaf surface as well.

Photo Source: Michael Bush, WSU Extension, Yakima, WA