Sampling and evaluation of entomopathogenic fungi for control of Bathycoelia distincta (Hemiptera : Pentatomidae) in South Africa

Abstract

The two-spotted stink bug, Bathycoelia distincta (Distant) (Hemiptera: Pentatomidae), is the most abundant stink bug species in macadamia orchards in South Africa. This pest is associated with serve macadamia nut damage and is mainly controlled with insecticides, placing selection pressure on the pest, which can result in resistance development. Chemical control also poses potential health and environmental risks, and alternative pest management tactics are therefore required. The use of effective entomopathogenic fungi (EPF), which are naturally occurring and environmentally friendly biological control agents with insecticidal action, may reduce these negative effects. The first aim of this study was to isolate and identify indigenous EPF isolates from B. distincta bugs in macadamia orchards. A survey was conducted in an unsprayed macadamia orchard, as well as on research and commercial farms in the Mpumalanga province in South Africa. Beauveria bassiana and Purpureocillium lilacinum were the only EPF isolated from naturally occurring stink bugs. Other insect-associated fungi genera isolated, were Aspergillus, Chaetomium, Lasiodiplodia, Penicillium, Rhizopus, Trichoderma, Myriodontium, Fusarium, Epioccum, Talaromyces, Bionectria, Gelasinospora, Nigrospora and Pestalotiopsis. The second aim, to assess the effect of temperature on growth of 11 of these B. bassiana isolates obtained, showed the optimal temperature for radial growth to be between 25-30 °C. Seven of these isolates did not differ significantly in terms of radial growth. To select the most virulent isolate for further evaluation under semi-field conditions, five B. bassiana isolates (PPRI 26695, 26696, 2667, 26700 and 26704) were selected and tested under laboratory conditions for their ability to infect and kill B. distincta adults. All five of these isolates were pathogenic towards B. distincta. The level of pathogenicity varied between the isolates and the mortalities caused by B. bassiana increased with time after application. Beauveria bassiana isolate (PPRI 26695) was selected for further assessment, based on 100 ± 8.61% efficacy of B. distincta control, mycosis of 86.67%, a LT50 of 6.99 and LT90 of 12.85 days. In the semi-field trial, the commercial B. bassiana product (Eco-Bb®) (not registered for control of stink bugs in South Africa), the experimental B. bassiana isolate, PPRI 26695 and insecticide, cypermethrin were compared for control of B. distincta adults. All three of these products provided effective control, with PPRI 26695, Eco-Bb® and cypermethrin that caused mortalities of 100, 90 and 80%, respectively, 18 days after treatment. The potential of B. bassiana for control of B. distincta The two-spotted stink bug, Bathycoelia distincta (Distant) (Hemiptera: Pentatomidae), is the most abundant stink bug species in macadamia orchards in South Africa. This pest is associated with serve macadamia nut damage and is mainly controlled with insecticides, placing selection pressure on the pest, which can result in resistance development. Chemical control also poses potential health and environmental risks, and alternative pest management tactics are therefore required. The use of effective entomopathogenic fungi (EPF), which are naturally occurring and environmentally friendly biological control agents with insecticidal action, may reduce these negative effects. The first aim of this study was to isolate and identify indigenous EPF isolates from B. distincta bugs in macadamia orchards. A survey was conducted in an unsprayed macadamia orchard, as well as on research and commercial farms in the Mpumalanga province in South Africa. Beauveria bassiana and Purpureocillium lilacinum were the only EPF isolated from naturally occurring stink bugs. Other insect-associated fungi genera isolated, were Aspergillus, Chaetomium, Lasiodiplodia, Penicillium, Rhizopus, Trichoderma, Myriodontium, Fusarium, Epioccum, Talaromyces, Bionectria, Gelasinospora, Nigrospora and Pestalotiopsis. The second aim, to assess the effect of temperature on growth of 11 of these B. bassiana isolates obtained, showed the optimal temperature for radial growth to be between 25-30 °C. Seven of these isolates did not differ significantly in terms of radial growth. To select the most virulent isolate for further evaluation under semi-field conditions, five B. bassiana isolates (PPRI 26695, 26696, 2667, 26700 and 26704) were selected and tested under laboratory conditions for their ability to infect and kill B. distincta adults. All five of these isolates were pathogenic towards B. distincta. The level of pathogenicity varied between the isolates and the mortalities caused by B. bassiana increased with time after application. Beauveria bassiana isolate (PPRI 26695) was selected for further assessment, based on 100 ± 8.61% efficacy of B. distincta control, mycosis of 86.67%, a LT50 of 6.99 and LT90 of 12.85 days. In the semi-field trial, the commercial B. bassiana product (Eco-Bb®) (not registered for control of stink bugs in South Africa), the experimental B. bassiana isolate, PPRI 26695 and insecticide, cypermethrin were compared for control of B. distincta adults. All three of these products provided effective control, with PPRI 26695, Eco-Bb® and cypermethrin that caused mortalities of 100, 90 and 80%, respectively, 18 days after treatment. The potential of B. bassiana for control of B. distincta was demonstrated in this study and further research in this regard is justified.