Effects of intercropping cassava and maize and the release of the exotic predator, Typhlodromalus limonicus (Garman & McGregor) (Acarina: Phytoseiidae) on the dynamics of the cassava green mite, Mononychellus tanajoa (Bondar) (Acarina: Tetranychidae)
Field and laboratory studies were conducted in 1990-1991 to investigate the effects of intercropping cassava and an exotic predator Typhlodromalus limonicus Garman & McGregor (Acari: Phytoseiidae) on the population dynamics of the cassava green mite, Mononychellus tanajoa (Bondar) (Acarina: Tetranychidae). Additional studies were conducted on the predator's survival at low prey density as well as the effects of soil nitrogen on the densities of M. tanajoa. Cassava and maize were planted in May 1990 at two locations: at the International Institute of Tropical Agriculture (IITA) and Dogbo in Benin, West Africa. The effects of intercropping, predator release and varieties on CGM densities were measured. Results indicated that intercropping cassava and maize had no effects on CGM densities because maize was harvested 3-4 months before the increase of CGM in the dry season. The release of the exotic predator, T. limonicus reduced CGM densities in both the "release" plots and "no release" plots suggesting a high dispersive ability by the predator. Differences in CGM densities between sites were not related to leaf nitrogen concentrations of plants growing on different soil types. There were differences in CGM densities associated with the level of susceptibility to CGM attacks in the varieties used. "Agric" sustained higher CGM densities throughout the sampling period than TMS 30572. Nitrogen effects on CGM population dynamics were further investigated in greenhouse and laboratory studies. There was no consistent relationship between leaf nitrogen and CGM development and reproduction. The correlations between CGM and leaf N in different varieties were variable. Laboratory experiments on the effects of cassava exudate on T. limonicus indicated that cassava exudate is an important food that can allow T. limonicus to survive periods of low CGM availability. Although cassava exudate allows only survival, but not reproduction, reproduction resumed when predators were fed prey, following a diet of exudate. Intercropping cassava with shorter duration crops at the beginning of the rainy season can not be relied on as a control measure for CGM. Releasing tolerant varieties to CGM and effective predators such as T. limonicus are probably the most recommendable ways to reduce CGM densities.
O'Neil, Purdue University.
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