Ask Mr. Smarty Plants

Shasta daisies (Leucanthemum x superbum) are great perennial plants and according to the Missouri Botanical Garden's Plant Finder website, they were originally developed by Luther Burbank in the 1890s near the snow covered Mt. Shasta in northern California. They aren't a native plant but are hybrid crosses made from four plants: Leucanthemum vulgare (European oxeye daisy), Leucanthemum lacustre (Portuguese field daisy), Leucanthemum maximum (Pyrenees chrysanthemum) and Nipponanthemum nipponicum (Japanese field daisy).

Anyway, since there are no signs of insects (although they could have been active at night) and the cultural conditions are adequate for blooms, the plant may be showing signs of Aster Yellows. One symptom of Aster Yellows is deformed flowers without petals (phyllody, which is the development of leaf-like flower petals). This phytoplasma disease is difficult to control and there is no cure. Infected plants should be removed and destroyed if you suspect this is the reason for your petalless flowers.