The fern mite, Hemitarsonemus tepidariorum (Warburton), has been collected in association with a number of different fern species, including Athyrium procumbens, Asplenium bulbiferum, Diplazium procorum, Rumohra adiantiformis (=Arachniodes adiantiformis, =Polystichum adiantiforme), as well as with members of the genera Polystichum, and Pteris. These mites feed on the leaf tissue of their host plants, stunting leaf growth and deforming the leaflets (Beer 1954, Lindquist 1986). Severely affected leaflets exhibit reduced size and rounding of their borders, and older leaflets show a bronze or rusty coloring on the ventral surface. On larger leaflets, the mite population tends to be concentrated around the main vein. Female mites deposit their eggs between the vein and the lamina. The larval stage of this species feeds very aggressively on the fern leaflets, and environmental cues play a role in sex determination once mites reach the adult instar.

H. tepidariorum has been collected in Costa Rica, England, and the U.S. primarily from ferns grown in commercial greenhouses. The mite was collected in 1903 from ferns in England and was first described by Warburton in 1904 as Tarsonemus tepidariorum. Cameron collected additional specimens from English greenhouses in 1924 and provided a detailed account of the mite's life history traits (Cameron 1925). Ewing (1939) described the genus Hemitarsonemus to recognize similarities between T. tepidariorum and another Tarsonemus species, T. latus Banks. Ewing based his description of H. tepidariorum on both Warburton's original description of T. tepidariorum and additional specimens collected from "hollyfern" (Polystichum sp.) in Minnesota. However, further studies revealed distinct morphological features separating H. tepidariorum and H. latus, and H. latus was subsequently used as the type species for the genus Polyphagotarsonemus (Beer and Nucifora 1965), leaving Hemitarsonemus as a monotypic genus containing only the morphologically distinct fern mite, H. tepidariorum (Warburton).

Additional references to this mite include Pritchard (1951) who noted its occurrence on ferns in commercial greenhouses in the San Francisco Bay area, and Lindquist (1986) who studied material collected in 1955 from host plants (Athyrium procumbens from Malaya and Diplazium procorum from Sri Lanka) housed in the propagating houses of the Royal Botanical Gardens at Kew, England. The most detailed illustrations currently available for both the male and female of H. tepidariorum can be found in Lindquist (1986).

The images below depict Hemitarsonemus
tepidariorum (Warburton) damage on leatherleaf fern

Loss of serrated borders of leaflets is
typical of H. tepidoriarum activity

 

The mite population on an infested fern
will be concentrated on the new growth

 

Yellowing of the undersurface of leaflets is often
seen on mature ferns that have experienced
H. tepidariorum infestation

Images provided by: Dr. Helga Blanco, Insect Museum,
University of Costa Rica

 

SELECTED REFERENCES

Beer, R.E. 1954. A revision of the Tarsonemidae of the Western Hemisphere (Order Acarina). University of Kansas Science Bulletin. 36(16): 1091-1387.

Beer, R.E. and A. Nucifora. 1965. Revisione dei generi della famiglia Tarsonemidae (Acarina). Bollettino di Zoologia agraria e di Bachicoltura, Serie II. 7: 21-43.

Cameron, W.P.L. 1925. The fern mite, Tarsonemus tepidariorum Warburton. Annals of Applied Biology. 12(1): 93-112.

Ewing, H.E. 1939. A revision of the mites of the mites of the subfamily Tarsoneminae of North America, the West Indies and Hawaiian Islands. Technical Bulletin U. S. Department of Agriculture. 653: 63 pp.

Lindquist, E.E. 1986. The world genera of Tarsonemidae (Acarina: Heterostigmata): A morphological, phylogenetic and systematic revision with a reclassification of family-group taxa in the Heterostigmata. Memoirs of the Entomological Society of Canada. 136: 517 pp.

Pritchard, A.E. 1951. The fern mite: a newly recognized pest on California ferns readily controlled by treatment with proper chemicals. California Agriculture. 5(7):10.

Warburton, C. 1904. Mites of the genus Tarsonemus with a description of two new species. Journal of the Royal Agricultural Society, England. 65: 273-287.