Entomopathogenic fungi mortality on grapevine mealybug

by | Sep 1, 2021 | Viticulture research, Winetech Technical

Global demand for environmentally friendly grapevine cultivation and pest control has necessitated an improved understanding of naturally occurring antagonists such as entomopathogenic fungi (EPF).

 

Introduction

Conventional control of grapevine mealybug in agricultural production has depended on synthetic chemicals, which causes negative effects such as environmental degradation, concerns over workers well-being, declining biodiversity in agro-ecosystem, development of insecticide resistance, and resurgence of pests, among others. This has necessitated the quest for alternative and more environmentally benign biological control agents such as entomopathogenic fungi (EPF). EPF are pervasive, however, the type of farming and farming system could influence fungal species distribution. More isolates of the entomopthogenic fungus Beauveria bassiana (Hypocreales: Clavicipitaceae) occur organically than conventionally farmed soils. Farming activities such as grapevine cultivation disrupt natural biotic and abiotic conditions, causing ecological disequilibrium, which can affect insect pest occurrences and their natural enemies, including EPF.

 

Grapevine mealybug.

 

Serious infestation of vine mealybug on grapes showing large quantities of honeydew.

 

The objectives of this study were to:

  1. survey the occurrence of EPF in vineyard soils in the Cape Winelands, and
  2. ascertain their pathogenicity against the grapevine mealybug.

 

Soil samples were collected from wine production areas, namely Stellenbosch, Constantia, Franschhoek, Paarl and Worcester (Table 1). At each vineyard, three blocks of grapevine were randomly selected, 200 m apart. Soil samples were collected with a garden spade at a depth of 15 – 20 cm. Soil samples, 300 g and 1 kg in weight, were put into plastic containers and paper bags, respectively. The 1 kg bags of soil were sent to Bemlab (Pty) Ltd (Somerset West, Western Cape) for soil analyses and the 300 g soil samples were transported to the laboratory, air-dried and sieved using a 2-mm sieve, and then used in the process of fungal isolation within 24 hours of collection. Fungi were isolated from the soil samples by baiting with fifth instar larvae of Cydia pomonella (Lepidoptera: Tortricidae). Conidia from the cultures of each fungal isolate were harvested. Twenty-three fungal cultures were transferred to the Molecular Biology Laboratory in the Department of Microbiology, Stellenbosch University for a more precise identification. Prior to assessing pathogenicity of chosen EPF on the grapevine mealybug, Planococcus ficus, the viability of conidia was determined to be 90%. An immersion bioassay was used to assess the virulence of four species of EPF isolates on adult females of P. ficus. The fungal isolates were tested at a standardized conidial concentration of 1×108 conidia mL-1. The control solution had sterile 0.05% Tween 80. Insects were dipped individually into 3 mL conidial suspensions or control solution for 30 seconds. For each treatment, 10 adult female insects were placed in a 9 cm diameter Petri-dish containing a disk (6 cm in diameter) of sterilized leaves of V. vinifera ‘Cabernet Sauvignon’, replicated five times. Mortality data was recorded on the 4th day and mycosis confirmed under a dissecting microscope.

 

 

Discussion and conclusion

No mycosis was observed on the control insects. Mycosis among C. rosea f. catenulata and B. beauveria isolates was > 90%, while isolates M. roberstii was 50%. Beauveria bassiana was the most virulent exerting insect mortality of 77.0 – 87.0% (Table 1). The fungal isolates collected from the sampled vineyards, 26.0% belonged to the species C. rosea f. catenulata, closely followed by M. robertsii (22.0%). On the basis of this study, it appears that C. rosea f. catenulata is the predominant Metarhizium species in the Cape Winelands region. It was also determined that M. robertsii and C. rosea proliferate better in disturbed and conventionally cultivated habitats, whilst B. bassiana flourishes better in uncultivated habitats. Consequently, this study suggested that the isolate B. bassiana, could be considered for further development as microbial control agent of the vine mealybug, P. ficus.

Abstract

Soil samples collected from vineyards in the Western Cape were examined for the presence of EPF prevalence. Fungi were isolated with methods of insect baiting and selective media. In addition, fungal isolates were tested against a key grapevine pest, Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae). Pathogenicity of fungal strains against P. ficus was assessed with an immersion bioassay at a concentration of 19 108 conidia mL–1. Strains of Beauvaria bassiana (Hypocreales: Cordycipitaceae) caused the highest mortalities.

 

– For more information, contact Kwaku Achiano at AchianoK@arc.agric.za.

 

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