Control Of Cassava Green Mite Research Articles

English

The aim of the study was to evaluate the economic impact of biological control of cassava green mite (CGM) in Ghana by using the natural enemy Typhlodromalus manihoti. Both primary and secondary data were used for analysis. In total, 714 cassava producing households were drawn from 17 districts in seven regions of Ghana by employing a combination of proportional, purposive, simple random, and systematic random sampling techniques. Formal interviews with the use of standardized structured questionnaire were combined with field/farm visits to elicit information for the study. The ‘with and without’ comparison and trend analysis of secondary data were performed to evaluate project impact. The economic surplus model was used to estimate net benefits of biological control of CGM in Ghana in terms of Benefit-Cost Ratio (BCR), Net Present Value (NPV) and Internal Rate of Return (IRR). Following the release of about 2,344,817 CGM predators (T. manihoti) in seven regions of Ghana between the year 2007 and 2012, the study found evidence of T. manihoti presence in beneficiary communities. However, the level of establishment and extent of spread were markedly different across regions and districts in Ghana. The CGM biological control project in Ghana was found to have been successful in reducing CGM populations on cassava fields, reducing cassava root losses, improving productivity and profitability under varying climatic conditions and soil types. Biological control of CGM leads to higher economic gain to farmers and the country as a whole. Biological control of CGM was found to generate economic returns (NPV) of US$228.5 million, a BCR of 5,393.74 and an IRR of 3,424% at a discount rate of 20% for the period 2006 to 2046. Sensitivity analysis showed that the returns are robust even at higher discount rate (50%) and under pessimistic assumptions about yield gains. The overall project impact is expected to be higher when benefits to ecological and human health, which are usually difficult to quantify, are considered. Periodic inoculation in released communities and fresh releases in new communities are recommended if the country is to realize the full benefits of biological control of CGM.   Key word: Biological control, cassava green mite, economic gain, impact, Typhlodromalus manihoti.

Native phytoseiid mites as indicators of non-target effects of the introduction of Typhlodromalus aripo for the biological control of cassava green mite in Africa

Abstract The need to evaluate non-target effects of classical biological control of arthropod pests has received considerable attention in recent years. We determined with repeated field surveys the changes in abundance and distribution of the phytoseiid mite fauna in cassava fields resulting from the introduction of the neotropical phytoseiid Typhlodromalus aripo into two countries—Malawi and Mozambique—in southern Africa for the biological control of Mononychellus tanajoa . Typhlodromalus aripo abundance was similar, while the abundance of the target pest M. tanajoa declined progressively, during the 2 years after the introduction of T. aripo into the target countries. We did not detect any changes in the abundance of the most common native phytoseiids mites— Euseius baetae , Euseius bwende and Ueckermannseius saltus —on cassava in Mozambique. In contrast, the abundance of two of the most common native phytoseiids, Euseius fustis and Iphiseius degenerans , on cassava in Malawi were apparently facilitated—i.e., their abundance was enhanced by the introduction of T. aripo; while the abundance of a third species, U. saltus , was not affected. For only one species, E. baetae , within-plant distribution shifted to the lower parts of the cassava canopy as a result of the introduction of T. aripo , which resides in the upper parts of the cassava canopy, but without any measurable negative effects on biological control of M. tanajoa . While the overall abundance of phytoseiid mites found on non-cassava vegetation was not affected by T. aripo introduction, there were some changes in relative abundance of some species in Malawi. Possible mechanism for the increase in abundance of I. degenerans and E. fustis in Malawi, and changes in within-plant distribution of E. baetae and I. degenerans in Mozambique and Malawi, respectively, are discussed. The methodology developed for assessing potential non-target effects of T. aripo introduction into southern Africa has significantly advanced classical biological control efforts against an economically important cassava pest.

Biological control of cassava green mite with exotic and indigenous phytoseiid predators—Effects of intraguild predation and supplementary food

Abstract Intraguild predation, one aspect of predator–predator interaction, has received in recent years increasingly greater attention because of mounting evidence of the impact of predator–predator interactions on the structure of ecological communities and biological pest control. In laboratory experiments, we determined if intraguild predation occurs between Typhlodromalus manihoti and Euseius fustis , two phytoseiid mite predators of the cassava green mite Mononychellus tanajoa on cassava in Africa, and if the level of intraguild predation is affected by the availability of the primary prey, M. tanajoa , and maize pollen as alternative non-prey food. In the laboratory, the two predators attacked and killed heterospecific larvae but they did so only when prey mites and alternative food (maize pollen) were absent or scarce. On a diet of intraguild prey alone, the two predator species survived for several days but failed to reproduce. Addition of abundant levels of M. tanajoa and maize pollen increased survival and reproduction of the two predator species and reduced intraguild predation to very low levels. We then determined, on whole plants in pot experiments in a screenhouse, the effect of maize pollen (an alternative food) on the interactions between the two predator species and the impact of predator–predator interactions on suppression of M. tanajoa population densities. In single predator species treatments, both E. fustis and T. manihoti significantly reduced M. tanajoa densities; but surprisingly, E. fustis appeared to be superior to T. manihoti . The co-presence of the two predators on the same cassava plant resulted in similar suppression of M. tanajoa population densities, but greater suppression of M. tanajoa compared with T. manihoti alone, regardless of presence or absence of maize pollen. The presence of the two predator species together reduced their respective abundance compared with single predator species treatments. Addition of maize pollen, however, significantly increased densities of the generalist predator E. fustis , in both single and two predator species treatments; and tilted the balance of the interactions between the two predator species in favour of E. fustis . The findings of our research underscore the role of generalist predators like E. fustis in the suppression of M. tanajoa populations on cassava, and the potential role of alternative non-prey food in altering the interactions between co-occurring predators sharing the same primary prey (by favouring the generalist predators), and the effect of these interactions on suppression of population densities of the shared primary prey.

Cassava green mite in East Africa: A regional approach to research and control

A regional programme, involving seven national programmes and three international agencies, has been launched in East Africa to promote research on the biological control of cassava green mite, Mononychellus tanajoa (Bondar) sensu lato. The organization and objectives of the programme are described and outline results of the first 2 years of work are reported. Trials to assess losses due to mites were carried out at 10 sites, using acaricides to protect split plots of two to five varieties in each trial; at 6 sites, significant losses of tuber yield were recorded on one or more varieties. Surveys of indigenous natural enemies indicated that the predator complex is similar throughout the region and does not provide adequate green mite control. Two Neotropical phytoseiid predators have so far been introduced for local evaluation. One of these, Neoseiulus idaeus, has been released at 12 sites in three countries; recoveries were made at several sites, but no impact on the pest population was detectable. Plans for further collaborative work are discussed.

Prey preference and reproductive success of the predatory mite Neoseiulus idaeos On the prey species Mononychellus tanajoa and Tetranychus lombardinii

Cassava green mite (CGM), Mononychellus tanajoa s.l., originating from South America, has become a serious pest in Africa, where it often occurs in mixed infestations with indigenous spider mites like Tetranychus lombardinii. The neotropical phytoseiid Neoseiulus idaeus is a candidate predator for classical biological control of CGM. Its prey preference and reproductive success on CGM and T. lombardinii were studied in the laboratory. Foraging individually on leaf discs, N. idaeus consumed CGM and T. lombardinii adult females equally in single-species populations but showed a distinct preference for CGM when both species were presented together at the same density. The predators developed and reproduced equally well on CGM and T. lombardinii.