probably harmful; mites presumably kill bee larvae and eggs in infested cells

Name and classification

Horstia Oudemans, 1900

Superorder Acariformes » Order Sarcoptiformes » Suborder Oribatida » Infraorder Desmonomata » Hyporder Astigmata » Family Acaridae » Genus Horstia

Type species
Trichotarsus ornatus Oudemans, 1900

Common synonyms
Ceroglyphus Vitzthum, 1920 [1918] (described from males with enlarged chelicerae); Petzschia Oudemans, 1923


Phoretic deutonymph: Coxal setae 1a, 3a, 4a filiform (not conoidal) (Figs. 3, 4). Posterior coxal apodeme IV absent (Figs. 3, 4). Supracoxal setae scx may be bifurcate (Figs. 5,11) or trifurcate. Claws I-IV almost straight (not distinctly hooked) (Figs. 3, 4, 5, 6). Tarsi I-II without setae ba I-II (Fig. 5). Tarsus I without seta aa (Fig. 5). Tarsus IV with 3 long setae (Figs. 3, 4, 6). Tibiae I-II with 1 seta each (gT I-II present, hT I-II absent) (Figs. 3, 4, 5). Tibiae IV without ventral seta (kT IV absent) (Fig. 6). Dorsal setae long (subgenus Amhorstia, Figs. 1, 2) or very short (subgenus Horstia, Figs. 7, 8). Coxal fields II closed (subgenus Amhorstia, Figs. 3, 4) or open (subgenus Horstia, Figs. 9,10).

Adult: In female, setae ve situated posterior to setae vi, approximately halfway between vi and se & si, distinctly off prodorsal sclerite (Fig. 15). Supracoxal setae scx may be bifurcate (Fig. 15). Tarsus I without setae aa (Fig. 16). Tibiae I-II with 1 ventral seta (gT I-II present, hT I-II absent) (Fig. 16). Tibiae IV without ventral seta (kT IV absent) (Fig. 17). Male sometimes with enlarged chelicerae and palps (Figs. 13, 14).

Species identification

A dichotomous key to phoretic deutonymphs is available in Fain, 1984. Two species, Horstia major and H. minor, were described later and should be identified using their original description (Alzuet and Abrahamovich, 1987).

Adults are known for H. virginica, H. longa, H. monstruosa, and H. malaysiensis (probably a junior synonym of H. helenae), and these should be identified using the original descriptions.


SE Palaearctic (Japan), Nearctic, Neotropical, Afrotropical, Oriental, and Australian regions. The subgenus Amhorstia is distributed in The New World, while the subgenus Horstia is distributed in the Old World.

Bee hosts

Associated with large carpenter bees (Xylocopa). Occasional or accidental records include small carpenter bees (Ceratina), bumble bees (Bombus), and leafcutting and resin bees (Megachile).

Host association level


associated exclusively with bees or their close relative, wasps; cannot live without these hosts


some life stages are associated with bees, while others are not

Facultative or opportunistic

can complete entire life cycle without bees or their close relative, wasps


Host associations, feeding, and dispersal

  • All stages live in nests of large carpenter bees (Xylocopa spp.), where they may kill bee eggs and larvae and/or feed on nectar and pollen.
  • Phoretic deutonymphs disperse on bee hosts. Mites are usually situated in intersegmental space, on the sides of first metasomal sternite (hidden by overlapping tergite 1) on bees from the Neotropical region (Figs. 18, 19).


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).