Plant Protection (Scientific Journal of Agriculture)

Plant Protection (Scientific Journal of Agriculture)

Study of the culturable bacterial community in the digestive tract of Helicoverpa armigera larvae and the effect of the fungus Beauveria bassiana on the abundance of digestive tract bacteria

Document Type : Research paper-Persian

Authors
M. Amiri 1
A. Khani 2
N. Sahebzadeh 2
M. Najimi 3
1 Ph.D. student, Department of Plant Protection, Faculty of Agriculture, University of Zabol, Zabol, Iran
2 Associate Professor, Department of Plant Protection, Faculty of Agriculture, University of Zabol, Zabol, Iran
3 Associate Professor, Department of Pathobiology, Faculty of Veterinary Medicine, University of Zabol, Zabol, Iran
10.22055/ppr.2026.50800.1851
Abstract
Background and Objectives
Bacteria are an important group of microorganisms that live within the insect guts. Bacteria inhabiting the gut of insects provide many benefits to their hosts, such as aiding in digestion, reproduction, immunity, tissue homeostasis, adaptation to environment and resistance to pathogen and pesticides. Insect microbial associates play important roles in the development of diseases caused by entomopathogens and toxicoses caused by insecticides. Different types of interactions may occur between bacterial symbionts and insect-pathogenic ascomycetes (e.g., B. bassiana). Helicoverpa armigera Hubner (Lepidoptera: Noctuidae) is a polyphagous and economically important pest of the order Lepidoptera, that infests important crops worldwide, including cotton, tomatoes, sunflowers, corn, peas, vegetables and agricultural crops. However, studies on the microbiota in the insect order are scarce. This study aimed to identify the symbiotic bacteria associated with H. armigera guts and to survey the effect of the pathogenic fungus, B. bassiana (concentration of 106 conidia/ml) on the abundance of bacterial flora.
Materials and Methods
Eggs and larvae were collected from the corn fields of Jiroft, Kerman, and reared in laboratory conditions at 27±2°C, 65±5% relative humidity and a 16L: 8D photoperiod. The fungus B. bassiana Isolate (IRAN 1395C) was obtained from the Iranian Plant Protection Research Institute and grown on Potato Dextrose Agar (PDA) at 25 °C until sporulation. Bacterial identification was performed based on phenotypic characteristics using standard bacteriological methods, as well as the 16S rRNA encoding gene sequence and its comparison with the NCBI database. The contents of the digestive tube of fifth instar larvae were cultured in enriched Nutrient Agar medium using the 16 linear methods. After colony formation and purification, Gram staining was performed, and individual isolates were transferred to specific culture media (blood agar for Gram-positive bacteria and Eosin Methylene Blue agar (EMB) and MacConkey agar for Gram-negative bacteria). After purification in specific culture media, biochemical and differential tests, including oxidase, urease, catalase, Simon citrate agar, mannitol fermentation, motility, methyl red, gas production in Triple sugar iron agar (TSI) and Wegas-Persquaer tests, were used to identify bacteria. The appearance of the colonies was examined based on certain characteristics, such as size, form, color, elevation, consistency and opacity. In addition, the study involved counting bacterial colonies in treated larvae inoculated with the fungus B. bassiana and in a control group, using a mixed-culture method.
Results
The results of biochemical and differential tests and phenotypic studies showed that the predominant commensal bacteria in the digestive tract of Helicoverpa armigera larvae were the Gram-positive bacteria genus, include Staphylococcus (Bacillales: Staphylococcaceae, are Gram-positive cocci 1μm in diameter that form clumps.), Streptococcus (Lactobacillales: Streptococcaceae, are Gram-positive, nonmotile, nonsporeforming, catalase-negative cocci that occur in pairs or chains.), Bacillus (Bacillales: Bacillaceae, are rod-shaped, endospore-forming, aerobic or facultative anaerobic), Enterococcus (Lactobacillales: Enterococcaceae, are Gram-positive cocci that often occur in pairs (diplococci) or short chains, and are difficult to distinguish from streptococci on physical characteristics alone) and Gram-negative bacteria genus contain Enterobacter (Enterobacterales: Enterobacteriaceae, is a genus of Gram-negative, facultatively anaerobic, rod-shaped or coliform bacilli, non-spore-forming bacteria) and Klebsiella (Enterobacterales: Enterobacteriaceae, is a genus of Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule). The accuracy of the results for biochemical and differential determination was verified by molecular analysis using DNA sequences of the 16S rRNA gene, compared with the NCBI sequences database. After identifying the culturable bacteria in biochemical media, the number of bacterial colonies in the digestive tract of larvae infected with the fungus B. bassiana in the bacterial culture medium was counted using a colony counter, and the numbers in the treated and control larvae were compared. The number of bacterial colonies in the growth medium decreased significantly in the digestive tracts of treated larvae infected with pathogenic fungi compared to control larvae.
Discussion
The findings deepened the understanding of the role of associated microbiota–pathogen interaction in maintaining microbiota stability. They may also contribute to the development of novel biological control agents and pest management strategies. However, this is an initial step on the study of gut microbe communities of cotton bollworm. Further functional analysis remains to be done, including isolation, identification, and metabolic analysis of gut microbial strains by culture-independent techniques, in addition to the impact of pathogens on the diversity and abundance of bacterial flora.
Keywords
Gut microbiota
Helicoverpa armigera
Pathogenic fungus
Pest management
Subjects
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Plant Protection (Scientific Journal of Agriculture)
Volume 49, Issue 1
Winter 2026
Pages 69-87