Karen Strickler provides a list gleaned from the literature about mason bees (see below), unfortunately many plants have genus only listed and some probably won't do well in PA. J. Bosch & W. Kemp have a comprehensive list in their book: How to Manage the Blue Orchard Bee.

I would look at spring-blooming plants that are good nectar/pollen sources generally, and especially those in the rose family (strawberry, apple, cultivated cherry, peach, plum, apricot, nectarine, quince, serviceberry, raspberry, cotoneaster) that will bloom in the late March to early June interval. I'm not too sure about whether pear or some wild cherries (e.g Prunus serotina) are attractive. Oregon grape (Mahonia aquifolium), eastern redbud (Cercis canadensis), willows (e.g. pussy willow, Salix discolor), spring-blooming viburnums (e.g. Viburnum carlesii, V. trilobum), currants/gooseberries (Ribes spp.), bush-type honeysuckles (Lonicera spp.), and buckthorns (Rhamnus spp.) should be good tree/shrub resources for these bees that will do well in PA. Caution: non-native honeysuckles and buckthorns can be highly invasive and detrimental to conservation areas. Ideally, you want a mixture of blooming plants that will span the activity period of the adult bees. There should also be moist soil available nearby the nest site.

I would think that spring-blooming heaths (e.g. Erica darleyensis), bog rosemany (Pieris pallida), and blueberries (Vaccinium spp.)would be at least as attractive as Japanese andromeda, which Brian Griffin is so fond of.

Herbaceous plants that I have observed mason bees foraging are meadow sage (Salvia pratensis), other early-blooming salvias (e.g. Salvia x superba 'May Night', Salvia officinalis), cultivated and wild early-blooming mustards, cresses, and radishes (e.g. arugala, Brassica spp., Sisymbrium spp.), dames rocket (Hesperis matronalis), and dandelion (Taraxacum officinale).

Here is a link that might be useful: bee garden for mason bees

I'd like to add Cornellian cherry dogwood, Cornus mas, as a large shrub that should be a good pollen-nectar source for mason bees. In northeast Ohio it blooms in March or April, depending on the weather and microclimate. It's a nice ornamental that also provides edible fruit for people or wildlife.

The scientific name of bog rosemany is Andromeda polifolia or Pieris polifolia, not Pieris pallida, as I said in a previous posting.