Isolation and toxicity evaluation of bacterial entomopathogens against phytophagous white grubs (Coleoptera: Scarabaeidae) in Indian Himalayan hills

Abstract

The north-western Indian Himalayan hill region contains around 40 phytophagous species of white grub (Coleoptera: Scarabaeidae). Anomala dimidiata and Holotrichia seticollis are the predominant species. Under natural conditions, up to one-fifth of the population of the grubs was found to exhibit symptoms of bacterial infection. Therefore, we attempted to isolate and characterize the most potent bacterium for the management of white grubs. Of 27 bacterial isolates tested against A. dimidiata, WGPSB-2 was found to be highly toxic. Based on morphology, physiology, biochemistry and sequencing of the 16S rRNA gene, the isolate WGPSB-2 was identified as Bacillus cereus. A dose determination bioassay indicated that first instar grubs of A. dimidiata and H. seticollis were more susceptible than second instars. Although the LT₉₀ values for second instar grubs exceeded 8 weeks, the grubs stopped feeding after ingestion of spores, mostly after the third week. In outdoor microplots, a dose of 1.7 × 10¹⁰ spores/m² applied to the root zone was found to be potentially satisfactory for control of the pests. Bacillus cereus isolate WGPSB-2 appears to be a potentially effective bacterial pathogen for the management of white grubs.

Keywords:

• White grub
• Anomala dimidiate
• Holotrichia seticollis
• bacterial entomopathogens
• Bacillus cereus
• toxicity

Acknowledgements

The authors are grateful to Indian Council of Agricultural Research for funding the study. Complete sequencing of the 16S rDNA was carried out at the Institute of Microbial Technology (IMTECH), Chandigarh, India. The technical help provided by Mr Keshar Singh Rawat is duly acknowledged.