MODELING FUNGAL (NEOZYGITES CF FLORIDANA) EPIZOOTICS IN LOCAL-POPULATIONS OF CASSAVA GREEN MITES (MONONYCHELLUS-TANAJOA)

The fungus, Neozygitis cf. floridana is parasitic on the cassava green mite, Mononychellus tanajoa (Bondar) (Acari: Tetranychidae) in South America and may be considered for classical biological control of cass ava green mites in Africa, where cassava is an important subsistence c rop, cassava green mites are an imported pest and specific natural ene mies are lacking. Spider mites generally have a viscous structure of l ocal populations, a trait that would normally hamper the spread of a f ungus that is transmitted by the contact of susceptible hosts with the halo of capilliconidia surrounding an infectious host. However, if in fected mites search and settle to produce capilliconidia on sites wher e they are surrounded by susceptible mites before becoming infectious, then the conditions for maximal transmission in a viscous host popula tion are met. Because the ratio between spider mites and the leaf area they occupy is constant, parasite-induced host searching behaviour le ads to a constant per capita transmission rate. Hence, the transmissio n rate only depends on the number of infectious hosts. These assumptio ns on parasite-induced host search and constant host density lead to a simple, analytically tractable model that can be used to estimate the maximal capacity of the fungus to decimate local populations of the c assava green mite. By estimating the parameters of this model (host de nsity, per capita transmission rate and duration of infected and infec tious state) it was shown that the fungal pathogen can reduce the popu lation growth of M. tanajoa, but cannot drive local mite populations t o extinction. Only when the initial ratio of infectious to susceptible mites exceeds unity or the effective growth rate of the mite populati on is sufficiently reduced by other factors than the fungus (e.g. lowe r food quality of the host plant, dislodgement and death by rain and w ind and predation), will the fungal pathogen be capable of decimating the cassava green mite population. Under realistic field conditions, w here all of these growth-reducing factors are likely to operate, there may well be room for effective control by the parasitic fungus.