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75% after 2 months. The colony was completely eradicated after a further 2 weeks.
Several other authors have described microporidia from various species of mites.
For more details refer to Table 4.
In the late 1980s poor performance was noted of phytoseiids used as biological
control agent for thrips and spider mites in greenhouse crops and this observation
has led to studies on the possible involvement of pathogens in predator colonies.
Beerling and Van der Geest (1991a, 1991b) studied a microsporidosis in mass
cultures of the predators Amblyseius barkeri and Neoseiulus cucumeris
(Phytoseiidae) that are being used for the control of the thrips pests Frankliniella
occidentalis and Thrips tabaci (Thripidae) on vegetable and ornamental crops in
greenhouses. Diseased predators show a low reproduction and predation capacity of
the mites unsatisfactory. Predatory mites were sluggish in their movement and had a
swollen and whitish appearance (Beerling & Van der Geest, 1991a).
The presence of numerous spores could be detected in squash preparations of
the mites and it was assumed, that the pathogen involved belonged to the
Pleistophoridae (Microspora). Also infected stored product mites were observed.
Further work showed that three different spore types are found in the predator
cultures. It is difficult to say that three species of Microsporidia are involved, since
some species have several spore types during their life cycle. Recently, a new
species of a microsporidium parasite, Intexta acarivora, was observed in the gut
epithelium of the forage mite T. putrescentiae (Larsson et al., 1997), obtained from a
commercial culture in The Netherlands. Mites of this culture are used as prey for a
commercial rearing of N. cucumeris.
Beerling and Van der Geest (1991a, 1991b) also studied infected mite strains
from a commercial rearing in The Netherlands, but it is not known whether this has
the same origin as the samples studied by Larsson et al. (1997). The spore size
values of this microsporidium do not correspond with any of the values of the spores
in Beerling, Rouppe van der Voort, and Kwakman (1993).
Bjørnson et al. (1996) studied colonies of P. persimilis that were obtained from
suppliers of biological control agents. On the basis of spore morphology, three
distinct microsporidia could be observed in strains of P. persimilis, obtained from
three different suppliers. The ultrastructure of the pathogen and the course of the
disease of a colony obtained from Europe were studied in more detail. Schizonts
were observed inside the nuclei of the digestive cells of the ventriculus and within
the protoplasm of cells that line the caecal wall and the muscle tissue underlying it.
The properties of the pathogen made it difficult to assign it to an existing genus. For
that reason, it was placed in the collective group Microsporidium. Vertical
transmission for this microsporidium was proven, as mature spores were observed in
developing eggs inside gravid females (Fig. 11). The performance of an infected
colony was greatly affected (Bjørnson & Keddie, 1999): mean fecundity and prey
consumption of infected mites were significantly reduced. Short-term survivability
was variable and was not a good measure of predator quality. However, uninfected
females lived longer than infected females.