Modelling Fungal (Neozygites cf. Floridana) Epizootics in Local Populations of Cassava Green Mites (Mononychellus Tanajoa)

Abstract

General rightsIt is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. ABSTRACTThe 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 cassava green mites in Africa, where cassava is an important subsistence crop, cassava green mites are an imported pest and specific natural enemies are lacking. Spider mites generally have a viscous structure of local populations, a trait that would normally hamper the spread of a fungus that is transmitted by the contact of susceptible hosts with the halo of capilliconidia surrounding an infectious host. However, if infected mites search and settle to produce capilliconidia on sites where they are surrounded by susceptible mites before becoming infectious, then the conditions for maximal transmission in a viscous host population are met. Because the ratio between spider mites and the leaf area they occupy is constant, parasite-induced host searching behaviour leads to a constant per capita transmission rate. Hence, the transmission rate only depends on the number of infectious hosts. These assumptions 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 cassava green mite. By estimating the parameters of this model (host density, per capita transmission rate and duration of infected and infectious state) it was shown that the fungal pathogen can reduce the population growth of M. tanajoa, but cannot drive local mite populations to extinction. Only when the initial ratio of infectious to susceptible mites exceeds unity or the effective growth rate of the mite population is sufficiently reduced by other factors than the fungus (e.g. lower food quality of the host plant, dislodgement and death by rain and wind and predation), will the fungal pathogen be capable of decimating the cassava green mite population. Under realistic field conditions, where all of these growth-reducing factors are likely to operate, there may well be room for effective control by the parasitic fungus. 0168-8162

Failure of the mite-pathogenic fungus Neozygites tanajoae and the predatory mite Neoseiulus idaeus to control a population of the cassava green mite, Mononychellus tanajoa

“…Densities of active mites peaked at 1,600 individuals per 100 cm 2 of leaf so it is unsurprising that leaf loss ensued. This density is far above those used by Oduor et al (1997) to model epizootics in this system (they took 440 mites per 100 cm 2 as a characteristic density, probably a quite realistic figure during population growth and prior to overexploitation). The crash in the prey population cannot be attributed to the predators, however, as the cassava leaves were entirely chlorotic and massive leaf loss was observed, such that it even became difficult to find sample material.…”

“…Although the potential for natural epizootics of N. tanajoae to control populations of M. tanajoa has been demonstrated, the variability of this control has also been noted (Oduor et al 1997;Elliot et al 2000). Attempting either to harness this in new areas via the establishment of exotic strains or to improve upon it by the artificial introduction of inoculum or conservation is dependent upon the conditions being adequate for cycling of the fungus, both in terms of the abiotic conditions and the availability of potential hosts (Oduor et al 1997;Elliot et al 2002a).…”

“…Attempting either to harness this in new areas via the establishment of exotic strains or to improve upon it by the artificial introduction of inoculum or conservation is dependent upon the conditions being adequate for cycling of the fungus, both in terms of the abiotic conditions and the availability of potential hosts (Oduor et al 1997;Elliot et al 2002a). The sensitivity of the pathogen to abiotic conditions, particularly saturation deficits, has been demonstrated in laboratory (Oduor et al 1995a(Oduor et al , b, 1996a(Oduor et al , b, 1997 and field studies (Elliot et al 2002a).…”

Modelling cassava production and pest management under biotic and abiotic constraints

“…floridan. into Africa from South America as a form of biocontrol, Oduor et al (1997) developed a susceptibleinfected-contagious compartmental model between the CGM and this fungus maintaining a constant fungal percapita transmission rate. The authors showed that the fungal pathogen can reduce the population growth of CGM when other factors such as low temperature, low food quality and other environmental variables are right for fungal development.…”

Evaluation of Growth, Yield and Susceptibility of Five Cassava Cultivars in Ilorin, Southern Guinea Savanna, Nigeria

“…The fungus, Neozygitis cf. floridana, is parasitic on CGM in South America and may be considered for classical biological control of the pest in Africa (Oduor et al, 1997). The most reliable and easy-to-adopt method of controlling these biotic stresses is the cultivation of resistant cassava cultivars (Ogbe et al, 2003).…”