AbstractThe cassava green mite,Mononychellus tanajoa(Bondar), is an exotic pest in Africa and is the target of a classical biological control programme. Field data from the Neotropics, where it is indigenous, are presented for the first time, charting the variation in abundance ofM. tanajoaover several seasons. This was highly variable, with a characteristic trough mid-year and a peak at the turn of the year. This pattern corresponded positively with rainfall levels, appearing to fit a phenology also characteristic of African studies, where rainfall at the start of the wet season promotes a leaf flush and so growth inM. tanajoapopulations. Analyses implied some impact of leaf-inhabiting predatory mites (predominantlyNeoseiulus idaeusDenmark & Muma) and a considerable impact of the fungal pathogenNeozygites floridanaFisher onM. tanajoapopulations. This pathogen was not observed in the host population for several (generally dry) periods implying survival outside the host, perhaps as resting spores. This is a particularly desirable characteristic of a biological control agent. It is therefore proposed thatN. floridanamight be of particular use in drier cassava-growing areas where rainfall at the outset of the wet season is not sufficiently intense to cause heavyM. tanajoamortality but may be sufficient to stimulate epizootics of the fungal pathogen, protecting the flush of new cassava growth.