Formulation and bio-efficacy of different isolates of Beauveria bassiana against adults and third nymphal instar of desert locust (Schistocerca gregaria Forskål)

Abstract

Recently, the Horn of Africa witnessed a swift increase in the incidence of desert locust (Schistocerca gregaria) invasion. During outbreaks, pesticides are applied through aerial or ground spraying to kill the insects, and/or to prevent their spread to new grounds. However, after decades of extensive use, many drawbacks such as contamination of the environment, toxicity to non-target organisms, harmful residues on food, pest resistance, and bioaccumulation in the food chains emerged. Entomopathogenic fungi offer viable alternatives to chemical pesticides against many insect invasions, but few studies have tested their bio-efficacy in desert locusts. Therefore, the current study aimed at isolating, formulating local isolates 231, 334, 333, 341, 349, 351, 339 of Beauveria bassiana, and testing their bio-efficacy against larval and adult desert locusts. The 21-day experiment was conducted under controlled conditions in a greenhouse. Soil samples were collected from two agroecological zones in Isiolo and Laikipia Counties in Kenya. B. bassiana was isolated from the soil samples using the Galleria bait method and cultivated in Sabouraud Dextrose Agar Yeast (SDAY). The isolates were identified based on molecular techniques (DNA and PCR amplification). The conidia of the isolates were screened and bioassays on 30 locusts was conducted for 14 days. The best isolates eliciting over 90 % mortality during screening were used for formulations using three carrier materials (liquid paraffin, Diatomaceous Earth, and whey) which were again tested against adult and 3rd nymphal instars of the locusts. The stability of the formulations was also tested after 1 and 2 months. All the tested isolates of B. bassiana significantly outperformed the control and thus pathogenic to the adults and 3rd nymphal instars of S. gregaria under laboratory conditions. They caused mortality ranging from 57.8–100 % after 14 days post-incubation. The isolates 341, 231, and, 334 elicited 50 % mortality responses at concentrations 1.1 × 10⁵ conidia/ml, 2.5 × 10⁵ conidia/ml and 1.7 × 10⁶ conidia/ml respectively in adults and 1.1 × 10⁵ conidia/ml, 2.5 × 10⁵ conidia/ml and, 1.7 × 10⁶ conidia/ml respectively in 3rd nymphal instars. Formulations with 341–1, 341–2, and, 334–1 had the highest efficacy (>99 %) against the adult locusts. There was a significant 3-way interaction (P < 0.05) of isolate for the formulation, carrier material and, time in determining the Cfu of the B. bassiana formulations. After 1 month, the best Cfu occurred in formulation with isolates 231 and 341 formulated using Diatomaceous Earth, while the highest Cfu was observed in formulation with isolate 334 formulated with either liquid paraffin or whey. After 2 months, the highest Cfu occurred in formulation with isolates 231, 341, and, 334 formulated using liquid paraffin. This study provides encouraging data for the development of potential biopesticides formulated with different carrier materials against desert locusts.