UV sensitivity of Beauveria bassiana and Metarhizium anisopliae isolates under investigation as potential biological control agents in South African citrus orchards

Abstract

Seven indigenous entomopathogenic fungal isolates were identified as promising biocontrol agents of key citrus pests including false codling moth, Thaumatotibia leucotreta Meyrick (Lepidoptera: Tortricidae), citrus thrips, Scirtothrips aurantii Faure (Thysanoptera: Thripidae) and citrus mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae) under laboratory conditions. Even though field trials using the two most virulent isolates (Beauveria bassiana G Ar 17 B3 and Metarhizium anisopliae FCM Ar 23 B3) against soil-dwelling life stages of T. leucotreta were positive, foliar application against citrus mealybugs and thrips, has been disappointing. Thus, the UV sensitivity of the seven initial promising isolates (four B. bassiana and three M. anisopliae) in comparison with two commercial isolates (M. anisopliae ICIPE 69 and B. bassiana PPRI 5339) and their formulated products were investigated in this study. All isolates investigated were highly sensitive to UV radiation, and a 2 h exposure to simulated full-spectrum solar radiation at 0.3 W/m² killed conidia of all tested isolates. Nonetheless, variability in susceptibility was found amongst isolates after exposure for 1 h. The most virulent M. anisopliae isolate, FCM Ar 23 B3, was the most susceptible to UV radiation with <3 % relative germination, 48-51 h post-exposure. Whilst isolates of the two mycoinsecticides showed similar susceptibility to UV radiation, their formulated products (vegetable oil and emulsifiable concentrate) were tolerant, when tested for 1 h. These findings indicate that a suitable UV protectant formulation of these fungi or a different application strategy will be required for success against P. citri and S. aurantii.

Keywords: Entomopathogenic fungi; Microbial control; Simulated solar radiation; UV tolerance.