Browsing by Author "Mathulwe, Letodi Luki"

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    Control of the obscure mealybug, Pseudococcus viburni (Hemiptera: Pseudococcidae) using entomopathogenic fungi and nematodes
    (Stellenbosch : Stellenbosch University, 2021-12) Mathulwe, Letodi Luki; Stokwe, Nomakholwa F.; Malan, Antoinette P.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Pseudococcus viburni (Signoret) (Hemiptera: Pseudococcidae), the obscure mealybug, is a serious cosmopolitan, polyphagous agricultural insect pest of deciduous fruit crops. In South Africa, the mealybug negatively affects apple and pear production. Feeding of the mealybug on the arboreal parts of fruit trees results in different types of damage, such as early flower and fruit drop, leaf and fruit discoloration and, in severe cases, the premature death of the host plant shoot. On apples and pears, various life stages of the mealybug are often found in the fruit core and calyx, and on the pedicel. The mealybug extracts and ingests large amounts of phloem sap when feeding, then excretes the excess sap as a carbohydrate-rich sugary substance, which is known as honeydew, which falls onto the fruit, leaves and bark. This results in severe secondary damage, as it promotes the growth of sooty mould. Infestation of the fruit and the presence of sooty mould on the fruit have severe economic implications, as both stained and infested fruits are likely to be rejected or downgraded in pack- houses. Currently, the management of obscure mealybug in orchards relies on the use of both biological and chemical control. However, current management strategies have proven to be ineffective, as the mealybug has developed resistance to a range of chemical insecticides. Both entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN) are effective biological control agents of various agricultural insect pests, as they cause epizootics in various insect orders. The main aim of this study was to assess the potential of both local and commercial EPF species and of local in vivo- and in vitro-cultured species of EPN as possible biological control agents of P. viburni. The pathogenicity of the entomopathogens was assessed under laboratory conditions, with the EPF being tested at a conidial concentration of 1 × 10⁷ conidia/ml, and EPN at a concentration of 200 infective juveniles (IJs) per insect. From the study, two local EPF isolates, namely Metarhizium pinghaense (5HEID) and M. robertsii (6EIKEN) were identified as the most effective EPF isolates against P. viburni, resulting in >90% insect mortality, a week after exposure to the entomopathogens. The local in vivo-cultured Heterorhabditis indica (SGS) was found to be the most effective EPN isolate against P. viburni, causing 79% insect mortality, 48 h post inoculation. Further evaluation assessing the effect of combined application of M. pinghaense or M. robertsii with H. indica against P. viburni was conducted under laboratory conditions, using both sequential and simultaneous exposure methods. For the sequential method, the entomopathogens were each applied in turns, using first either the EPF or the EPN, followed by the other entomopathogen, at different time intervals. For the simultaneous method, the mealybugs were exposed to both EPN and EPF simultaneously. The combined application of H. indica with M. pinghaense or M. robertsii was observed to result in increased mortality of P. viburni, relative to when they were applied individually. The simultaneous application of H. indica and M. pinghaense was the most effective combination method of the two entomopathogens against P. viburni, resulting in 95% mortality five days post inoculation. The sequential method of application of H. indica and M. robertsii was the most effective, resulting in >85% mortality of P. viburni after four and three days of exposure. The interaction between the entomopathogens, when used in combination, was found to have an additive effect in all combination exposures. The infective conidia of M. pinghaense and M. robertsii were mass cultured, using agricultural grains as solid substrates. Rice grains, inoculated using blastospores, were the best for mass culture and production of conidia of both M. pinghaense and M. robertsii. Oil carriers (liquid paraffin, coconut oil, canola oil and olive) and a mineral carrier (diatomaceous earth) were investigated as potential formulations for mass-produced conidia of both M. pinghaense and M. robertsii. The viability of the formulated conidia was assessed over a period of eight weeks, and the efficacy of the formulation against P. viburni being assessed at the end of eight weeks. Oil carriers were found to be the most effective formulation for conidia of both the fungal isolates, maintaining conidial viability of >95% over time. The conidia in all the oil formulations also induced high mortality, ranging between 60% and 90% for M. pinghaense, and 70% and 90% for M. robertsii, when used against P. viburni. The current study provides insight into the efficacy of both EPF and EPN as possible biological control agents of P. viburni. Further long-term evaluation of the efficacy of the tested isolates of M. pinghaense, M. robertsii and H. indica, used individually or in combination, against P. viburni under field conditions, will provide insight into the potential success of using the entomopathogens for the management of the mealybug in orchards, as well as providing information regarding the possible integration of the entomopathogens as biocontrol agents in an integrated pest management system against mealybugs.
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    Control of the woolly apple aphid, Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae), using entomopathogenic fungi
    (Stellenbosch : Stellenbosch University, 2019-04) Mathulwe, Letodi Luki; Stokwe, Nomakholwa F.; Malan, Antoinette P.; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Eriosoma lanigerum (Hausmann) (Hemiptera: Aphididae), or woolly apple aphid (WAA), is a serious pest of apple trees (Malus domestica Borkhausen), attacking both the root system and the arboreal parts of the tree. Current management of WAA in apple orchards relies on the use of both biological and chemical control. However, biological control using the principal parasitoid of the WAA, Aphelinus mali (Heldemann) (Hymenoptera: Aphelinidae), was found to be ineffective. The use of chemical control has also proven to be negative, as biotypes of the WAA have developed resistance to the chemicals used. The use of entomopathogenic fungi (EPF) has been identified as promising biological control method against a wide array of insect pests. The main aim of the current study was to conduct a survey of EPF in the local apple orchards of the Western Cape province, and to screen their ability to control the root colonies of the WAA, under optimum laboratory conditions. The above was achieved by collection of soil and WAA-infested root samples from six apple farms. The EPF were baited from collected soil samples, using susceptible insect hosts, and directly from WAA females collected from the infested root samples. Successfully isolated fungi were grown on agar plates and screened for their pathogenicity against insects. The isolated EPF were identified both morphologically and molecularly, which include Beauveria bassiana, Isaria fumosorosea, Metarhizium brunneum, Metarhizium pinghaense, Metarhizium robertsii and Purpureocillium lilacinum. The second aim of the study was to screen the six isolated EPF for their virulence against the WAA under optimum laboratory conditions. The above was achieved by conducting screening, concentration-dose-response and exposure-time-response bioassays. Metarhizium pinghaense and M. brunneum proved to be the most effective species against the root colonies of WAA, indicating that the local isolates of EPF have potential for the biological control of the WAA. The final aim of the current study was to test for the persistence of M. pinghaense and M. brunneum on apple bark over a period of 3 weeks under laboratory conditions, and to determine whether the fungal conidia would attach to crawlers, or the fourth-stage nymphs, of the WAA as they move up tree trunks from the roots to the aerial parts of the apple trees. The above was done by means of spraying apple bark with a standard conidial concentration of 1.0 x 107 conidia/ml of both M. pinghaense and M. brunneum, respectively. The persistence of fungal conidia on the apple bark was measured using codling moth larvae as an indicator. The results indicated M. pinghaense to have better persistence on the apple bark over a period of 3 weeks than did M. brunneum. Further analysis of persistence, whereby the root colonies of the WAA were exposed to the dried conidia of M. pinghaense on apple bark, showed that M. pinghaense was capable of inducing mortality of about 39-82% in colonies of WAA under optimum laboratory conditions, when exposed for a period of 10 days. The current study has highlighted both the diversity of soilborne EPF in the local apple orchards of the Western Cape, and their potential to be successfully integrated in managing the presence of the WAA colonies in apple orchards. The local isolate of M. pinghaense has shown to be the best candidate for managing WAA. Therefore, future research should focus on testing the efficacy of the local isolate of M. pinghaense against the WAA, under both glasshouse and field conditions.