[Edited_by_A._Ciancio,_C.N.R.,_Bari,_Italy_and_K.(Bookos.org)

MITE PATHOGENS IN IPM
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4.2.1. Hirsutella Infections in Mites
A well-known genus in the Hyphomycetes is Hirsutella. Approximately 80 species
have been described in this genus; most species are pathogenic to tropical
invertebrates, some are pathogens of Acari, in particular of eriophyids. Teleomorphs
of Hirsutella have been reported as Cordyceps and Torrubiella spp. (cf. Chandler
et al., 2000). Hirsutella species observed in mites are given in Table 3.
The first published report of a Hirsutella infection in an eriophyid goes back to
1924: Speare and Yothers (1924) observed a sudden decimation of a large
population of the citrus red mite (CRM) (Phyllocoptruta oleivora) on grapefruit in
Florida. The population density of CRM was in June sometimes extremely high with
5,000 mites on a single grapefruit, but soon after the maximum density had been
reached, populations went down to almost zero. The authors were able to discern
fungal hyphae in mite cadavers and also noted that disease incidence was much
lower after application of a fungicidal copper spray. Fisher (1950) described this
fungus later as Hirsutella thompsonii (Fig. 3).
Figure 3. Epitremerus goniathrix (Eriophyidae), infected by Hirsutella sp.
Photograph: Leo P.S. van der Geest.
The conidia are the infective propagules of the fungus. They are formed on
phialides (flask-like conidiophores) that taper into a long narrow neck, and produce
usually only 1–3 conidia in a dense terminal sphere of slime. This gives the
impression that only one spore on each phialide is produced (Fig. 4). The mucous
coat facilitates adhesion of the spores to the host cuticle. Penetration of the fungus is
usually through all parts of the host’s body, but in case of spider mites, it is usually
through the legs. The fungi enter the host through the cuticle which is composed of
about 30% chitin embedded in a protein matrix. Entomogenous fungi often produce
proteases that may play a role in the early stages of penetration by exposing the
chitin fibrils. Subsequently, chitinolytic enzymes are excreted that causes
degradation of the fibrils into chains of glucose-N-acetyl of variable length.