Few bacteria have been reported as pathogens of the Acari but in recent years research has
been concentrated on intracellular organisms such as Wolbachia that may cause distorted sex
ratios in offspring and incompatibility between populations. The role of these organisms in
natural populations of spider mites is in particular discussed. The effect of Bacillus thuringiensis
on mites is also treated in this review, although its mode of action in arthropods is mainly
due to the presence of toxins and it is, therefore, not considered to be a pathogen in the true
sense of the word.