Larvicidal activities of local Bacillus thuringiensis isolates and toxins from nematode bacterial symbionts against the Rift Valley fever vector, Aedes caspius (Diptera: Culicidae)

Abstract

Mosquito resistance to insecticides has recently increased the burden of the majority of mosquito-borne diseases, and controlling the spread of these diseases should mainly rely on vector control measures. The present study aimed to investigate the toxicity of locally isolated mosquitocidal Bacillus thuringiensis (Bt) isolates and toxin complexes (TCs) from the two nematode bacterial symbionts Photorhabdus luminescens akhurstii (HRM1) and Photorhabdus luminescens akhurstii (HS1) on Aedes caspius. Out of sixty-eight local Bt isolates, four showed LC₅₀ values ranging from 13.8–15.6 ppm, similar to LC₅₀ recorded against the reference Bacillus thuringiensis israelensis (Bti-H14) (13.0 ppm). Seventeen isolates were more toxic than the Bti-H14 by 1.3–11.1 fold (LC₅₀ values 1.17–9.9 ppm
vs 13.0 ppm). Two isolates were less toxic than Bti-H14, showing LC₅₀ values of 39.4 and 24.1 ppm, respectively. Additionally, the TCs from HRM1 and HS1 showed promising larvicidal toxicity, with HS1 being 2.74 fold more toxic than HRM1. Larval morphological features and body size were altered upon HS1 toxin-treatment. This study suggests that there is a new effective local Bt strain, and TCs toxin complexes could be used as eco-friendly biocontrol agents that might contribute to controlling mosquito-borne diseases. The synergistic effect of the Bt-TCs combination on mosquito larvae should be further investigated.

Keywords: biocontrol, larvicides, mosquitoes, nematodes