The efficacy of selected indigenous entomopathogenic fungal strains against the grapevine mealybug, Planococcus ficus

Abstract

Soil-borne micro-organisms are crucial in maintaining ecological functionality and stability of soils. More recently, a group of fungi, entomopathogenic fungi (EPF), are gaining prominence due to their versatility – they are rhizopheric, endophytic and arthropod pathogenic. EPF are being used for pest management in many countries as a biological control and plant growth promoting agents. Despite a plethora of evidences of bio-efficacy of EPF against insects under greenhouse and field conditions, inconsistent efficacies in field studies and stringent regulatory requirements on the use of imported EPF strains are hampering their widespread use. To overcome these setbacks, it is necessary to isolate and identify virulent indigenous strains of EPF, understand how ecological factors factors influence the occurrence, persistence, and virulence of EPF, and develop more effective conidial formulations. However, few studies have isolated and investigated the relationship between EPF and the ecological factors in Africa in general and the Cape Peninsula region in particular. This study explored the relationships between the occurrence of indigenous EPF and land use and season of the Cape Peninsula region. The study also examines the enhancing effect of insect oil in conidial formulations on the bioactivity activity of EPF against P. ficus in vitro. To investigate the ecological relationships between entomopathogenic fungal occurrence in the Cape Peninsula region and season and land use, soils were collected from randomly selected sampling sites within randomly selected nature reserves (undisturbed sites) and agricultural farms (disturbed sites) in the Cape Peninsula region. Soil samples from the same sites were collected in different seasons (winter and summer). The isolates were identified morphologically and molecularly. In total, 96 EPF strains were identified. The associations among land use, season and fungal occurrence were determined by correspondence analysis (CA). The number of fungal strains occurring in the sites, the differences in the number of fungal species and strains found in the two seasons and in the farms (disturbed sites) and reserves (undisturbed sites) were compared using the Fischer Chi-square test. The results revealed that season positively influenced EPF occurrence, with significantly (χ2 = 10.286; p < 0.01) higher number of EPF isolates occurring in winter than in summer, as well as in farms than in reserves. Furthermore, a higher EPF species diversity occurred in winter compared to summer — up to 20 EPF species were isolated in winter compared with only eight species in summer. Metarhizium spp. were more frequently isolated from agricultural farms than in reserves. In addition, this study aimed to screen indigenous fungal strains against the grapevine mealybug, Planococcus ficus and assess the prospect of using insect-based oil for formulating EPF against the mealybug in vitro. The study focused on screening the 96 fungal isolates collected from soils in the Cape Peninsula region against grapevine mealybug using a dipping method. Insect mortalities caused by the fungal strains, varied significantly (χ2 = 204.8, p < 0.01, dF = 380). Metarhizium isolates induced the highest mortalities (ranging from 50 % - 80 % kill rate) within three days. Based on high insect mortality, two fungal isolates (Isolate 71 and 74) were selected and formulated in oil (Ento-oil®) obtained from black soldier fly at varying conidial concentrations of 1×107, 1×105, and 1×103 conidia mL-1. Metarhizium brunneum isolate (Isolate 74), tested at 1 × 107 conidia/mL and formulated in 20% Ento-oil (20% oil and 0.5% aqueous Tween 80 mixture) yielded the highest mealybug mortality (83%) within 3 days. The addition of oil obtained from black soldier fly larvae to the formulation increased the virulence of the M. brunneum isolate. This study presents interesting insights into the influence of seasonal change on the occurrence of EPF under different land use conditions in the Cape Peninsula region. Furthermore, the formulation of selected entomopathogenic fungal conidia with insect oil and water emulsion enhances insecticidal activity against P. ficus. The findings, suggest that indigenous EPF could play a major role in the future of biological control and sustainable farming practices.