Pollen mites

While not of concern to honey bees, pollen mite (Chaetodactyllus) infestations can be of concern to other bee species, particularly blue orchard bees (Osmia lignaria) and other Osmia species (mason bees, Japanese hornfaced bee, etc.) of the family Megachilidae.  The Krombein's hairy-footed mite (Chaetodactyllus krombeini) is a pollen mite native to North America.  Additional species of Chaetodactyllus occur on other parts of the globe, more commonly in humid rather than arid regions.

When they infest brood cells, they will kill and then feed on bee eggs and larvae.  They will also feed on pollen stores, hence the common name.  Reproduction in the cell can produce a shocking mass of mites: 

USDA/ARS image of C. krombeini mites in a blue orchard bee brood cell.

Emerging adult bees from other cells in the nest can become covered with the mites, and give them a lift to new frontiers.  Some mites may also stay behind in the nest and parasitize a new female bee looking into using the abandoned nest.

Here is a link to a good description of the pollen mite, Chaetodactylus krombeini, from the North American Bee Mite Project at the University of Michigan: Species_Accounts/Chaetodactylus_krombeini.htm.  The second image is from the USDA/ARS.

A recent paper "Distribution of Chaetodactyllus krombeini (Acari: Chaetodactylidae) within Osmia cornifrons (Hymenoptera: Megachilidae) nests: implications for population management" by Matthew McKinney and Yong-Lak Park is here:  http://link.springer.com/article/10.1007/s10493-012-9629-7

Abstract

Chaetodactylus krombeini (Baker) (Acari: Chaetodactylidae) is a cleptoparasitic mite that negatively affects propagation of Osmia spp. (Hymenoptera: Megachilidae) for orchard pollination in the USA. This study was conducted to determine the effect of C. krombeini on mortality of male and female Osmia cornifrons, the Japanese hornfaced bee. A total of 107 O. cornifrons nests were examined to determine within-nest distribution of C. krombeini with regression analyses. A total of 30 mite-free O. cornifrons nests were observed and within-nest distribution of male and female O. cornifrons was determined with non-linear regression analyses. In addition, cocoons from 20 mite-infested O. cornifrons cells were examined to determine whether C. krombeini could be found inside cocoons of O. cornifrons. The results of this study showed that female O. cornifrons and C. krombeini were found more frequently in the inner part of the nest, and male O. cornifrons were found mostly in the center of the nest. No C. krombeini were found inside O. cornifrons cocoons. These results indicate that C. krombeini have a greater negative impact on mortality in the egg and larval stages of female O. cornifrons than in male O. cornifrons. Implications for management of C. krombeini and O. cornifrons populations for orchard pollination are discussed in this article.