Biological observations on Horstia virginica associated with Xylocopa virginica krombeini are summarized below from Krombein, 1962b; Krombein, 1967:
All examined nest cells infested with Horstia contained dead bee larvae and some infested cells contained dead eggs. Presumably the mites caused the death of the bees, but the exact mechanism is unknown. Occasionally bees have been able to develop successfully in mite-infested cells, but these bee larvae may have reached maturity before their cells were invaded by mites. An alternate explanation for these observations is that mites may disperse relatively easily through a nesting aggregation, which may allow mites to exploit those cells where bees have not developed due to other mortality factors and still allow deutonymphs to locate and attach to live emerging bees.
Rearing experiments demonstrated that mites fed on nectar (and possibly also the upper layer of pollen grains) and flourished on this food source without the presence of either live or dead bees.
Presumably bees from non-infested cells would become infested as they passed through an infested cell during their emergence from the nest. Or, it may be that live phoretic deutonymphs remain in the old nest and infest new cells when another female uses the old nest.
Phoretic deutonymphs have been found clustering on the female bee pupa on the pupa's thoracic sternum, middle of mesonotum, and wings.
Our observations indicate that phoretic deutonymphs of Horstia are very common in intersegmantal spaces on the sides of first metasomal sternite (hidden by overlapping tergite 1) of large carpenter bees Xylocopa in the Neotropics (Figs. 18, 19).