For veileds I dust the insects with a phosphorous-free calcium powder at almost every feeding and twice a month with a phosphorous-free calcium/D3 powder and twice a month with a vitamin powder.
I feed/gutload crickets, superworms, etc with a wide assortment of greens and veggies and a bit of fruit.....dandelion greens, kale, endive, escarole, carrots, sweet potato, sweet red pepper, squash, zucchini, berries, apples, pears, melon, etc.
Appropriate temperatures play a part to because they need the heat to digest the food properly.