Sweet orange (Citrus sinensis) value-chain analysis and fruit-fly identification in Rusitu Valley, Zimbabwe

Musasa, Stephen, Tawanda (2017)

Thesis

Citrus sinensis is one of the most economically important plant species of the subfamily Aurantioideae. Citrus sinensis is believed to have originated in the south-eastern region of Asia, in an area that includes China, India and the Indochinese peninsula and nearby archipelagos. In Rusitu Valley (Zimbabwe) communal farmers benefited from the 1982 European Union’s Lome Convention funding programme that supported Manicaland Smallholder Coffee and Fruit projects. These projects transformed a number of “backyard” orange orchards into viable commercial orchards. However, orange production in Rusitu Valley declined from 2007 due to postharvest losses. The study addressed five important issues in Rusitu Valley’s sweet orange production chain. These are; 1) mapping the sweet orange value-chain in Rusitu Valley, 2) establishing the postharvest losses and their major causes, 3) identifying the fruit fly species that have invaded Rusitu Valley, 4) determining the possible invasion sources and pathway of fruit flies, and 5) establishing the population dynamics and fruit damage during seasons of sweet oranges in Rusitu Valley. The study of sweet orange value-chain was largely pragmatic choice, since oranges are the most threatened source of livelihoods in Rusitu Valley, Zimbabwe. Therefore, an in depth analysis of the sweet orange production value chain in Rusitu Valley was done and fruit flies (Diptera: Tephritidae) were singled-out as the major cause of losses. The damaging activity of fruit flies is mainly due to adult female which uses its ovipositor to lay eggs in clutches under the skin of the fruit. By this process, spoilage microbes are introduced into the fruit causing the fruit to break down and rot. The annual postharvest losses in 2014 for Rusitu Valley were estimated to be 3%, 36% and 42% occurring; during transportation, in the field and at the market, respectively. These amounted to a total of 81% annual postharvest losses with an estimated monetary value of US$ 11 003 126.40. There was a significant positive association (Pearson r =0.29, p<0.05) between the farmers’ score of pests incidence in their sweet orange field and the reported postharvest losses. Having singled out that fruit flies were the major cause of losses in the valley, the study further investigated on phenotypic and phylogenetic relationships of fruit fly species present in Rusitu Valley; leading to the appropriate identification of reported fruit flies as Bactrocera dorsalis (Hendel). The haplotype network analysis revealed that fruit fly DNA sequences obtained from Rusitu Valley were similar to many other B. dorsalis sequences from all over Africa. While performing the phenotypic characterization 16% of the sampled fruit flies had a broken dorsal stripe which may resemble B. kandiensis, suggesting introgression within the Rusitu Valley populations. Nevertheless, the Haplotype network analysis performed clearly confirmed that this biased sample is not very different from other specimens without a broken dorsal stripe. Therefore, there is no evidence for introgression in Rusitu Valley’s fruit fly populations. The findings of the study clarified the process by which the physical flow of oranges move within the value-chain, the marketing alternatives to farmers, and constraints faced by primary actors in Rusitu Valley’s production chain. The large population size, polyphagous nature of the species, and continuous availability of suitable fruits along the year complicates the eradication of this species. The positive morphological and molecular identification of Zimbabwe flies as B. dorsalis allows for management measures to be designed at regional level. Thus, the findings of this study will help in the more effective regional approach in the control and/or eradication of the B. dorsalis (Hendel).

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