Mononychellus tanajoa (cassava green mite).

The cassava green mite, M. tanajoa, is of Neotropical origin but was accidentally introduced to Africa in 1971 (Nyiira, 1972). By 1985, the pest had spread throughout the cassava belt of Africa (Yaninek and Heren, 1988). M. tanajoa affects the important annual crop cassava (Manihot esculenta) and can cause a reduction of about 50% in leaf weight, and up to 80% tuber yield loss (Shukla, 1976; Gutierrez et al., 1988; Pallangyo et al., 2004). M. tanajoa is mainly dispersed by human activity, whereby infested plant materials and contaminated media are transported over long distances. Natural dispersion by wind and water may also spread the cassava green mite. In areas where both the pest and host plant are exotic, there is no evidence that indigenous natural enemies are significant factors in limiting the mite population growth rates. M. tanajoa can feed and reproduce on other plant species (Moraes et al., 1995) and is reported as a quarantine pest (Delalibera et al., 1992; EPPO, 2009).

Resistance levels to the cassava green mite, Mononychellus tanajoa, in cassava germplasm (Manihot esculenta)

The cassava green mite (CGM), Mononychellus tanajoa (Acari: Tetranychidae), is one of the main pests of cassava, causing direct damage by sucking the plant’s sap. Although the mite has a wide distribution in Latin America and Africa and a high potential to expand to Asia, limited information is available on M. tanajoa biology and life history parameters on its primary host. In this study, we quantified the levels of resistance of 10 cassava genotypes (i.e., NAT-31, ALT-12, ALT-6, COL-1505, ECU-72, ECU-160, PER-182, PER-335, 60444, CMC-40) based on the mite’s oviposition preference and development time in no-choice and choice bioassays. The genotype NAT-31 significantly differed from other genotypes for M. tanajoa development time and oviposition rate: each stage of the CGM life cycle appeared to be delayed in NAT-31, suggesting that NAT-31 resistance is mediated through a general reduction of CGM fitness on this genotype. Resistance in the remaining genotypes was variable in comparison to a susceptible (control) genotype. ECU-72, a parental line of NAT-31, present a difference related to oviposition preference, development time and sex ratio. These parameters allow the identification of different levels of resistance (antixenotic and antibiosis) when compared to the susceptible genotype. CGM displayed significantly different oviposition preference from the susceptible genotypes. Identification and characterization of resistance to CGM in cassava germplasm might be key to further advance knowledge about natural resistance mechanisms and develop strategies to introgress resistance to CGM in farmer- and industry-preferred cassava varieties.