The 'eggs' on the bee are called Phoretic Mites, almost certainly the Wool Carder Bee Mite (Sennertionyx manicati). They gather in the narrow junction between the thorax and abdomen which is a prime location because the bee cannot reach back far enough with its legs to groom or brush them off. Many solitary bees have developed folds in their exoskeleton designed to specifically carry these mites to a new home. Once the female bee finishes constructing her nest, the mites hop off into the chamber where they feed on molds, fungi, or excess debris, which keeps the developing bee larvae clean and healthy.