Control of Woolly apple aphid using entomopathogenic fungi

The main aim of the current study was to survey for EPF in local apple orchards and to screen them for their ability to control WAA. The first objective was to survey for the presence of EPF in local apple orchards, and to identify isolates to species level using molecular techniques, while the second objective was to screen selected identified EPF isolates for their virulence against WAA under laboratory conditions. The last objective was to apply the most virulent EPF isolate to apple bark, so as to be able to determine the degree of persistence and contact infection attained under laboratory conditions.

The field survey was conducted in the Grabouw and Vyeboom areas located in the Western Cape province, South Africa. A total of 48 soil samples, comprised of 240 soil subsamples, were collected across six apple farms. The pick mattock method was used to collect root samples and these infested roots were kept in plastic containers to maintain a steady supply of insects. In the laboratory soil samples were baited for EPF using wax moth and mealworm larvae. DNA was extracted from the 12 selected morphologically different EPF isolates. Six EPF isolates were evaluated for their potential as biological control agents for WAA in the laboratory, using a 24-well bioassay protocol screening method. The lethal dose and time of the most promising isolates were also tested in the laboratory.The persistence of fungal conidia on apple bark was also evaluated. The apple barks were sprayed with conidial suspensions and the persistence of the fungal conidia was determined. The natural infections and the mortality of WAA, following exposure to the apple tree bark treated with conidial suspensions over time were also determined.

Six EPF were isolated and identified both morphologically and molecularly. Screening bioassays identified Metarhizium pinghaense (70.83 % ± 5.14 %) and M. brunneum (65.00 % ± 5.71 %) as the most virulent against WAA. The LT50 for M. pinghaense and M. brunneum were found to be 4.77 and 9.43 days, respectively. Metarhizium pinghaense was shown to have better persistence on the apple bark over a period of 3 weeks than M. brunneum. Further analysis of persistence 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 concludes that EPF offer a good chance of managing WAA in apple orchards. The study has indicated that M. pinghaense is the best local fungal candidate for the integrated management of WAA. Future studies should test M. pinghaense against WAA under glasshouse or field conditions with the fungus either being applied directly to the root system of apple trees to control the root colonies, or applied to the arboreal parts (e.g. the tree trunks) of the apple trees. Another option requiring investigation is the use of conidia-infected barriers, to target WAA crawlers as they move up the tree trunk to the arboreal parts of the tree.