Background
Spodoptera litura (Fab.) (Lepidoptera: Noctuidae) commonly known as tobacco caterpillar is a polyphagous pest that causes significant damage to many agricultural crops. The extensive use of chemical insecticides against S. litura has resulted in development of resistance. In order to find potential biocontrol agents, gut microbes were investigated for insecticidal potential. These microbes live in a diverse relationship with insects that may vary from beneficial to pathogenic.
Results
Enterococcus casseliflavus, Enterococcus mundtii, Serratia marcescens, Klebsiella pneumoniae, Pseudomonas paralactis and Pantoea brenneri were isolated from adults of S. litura. Screening of these microbial isolates for insecticidal potential against S. litura showed higher larval mortality due to K. pneumoniae and P. paralactis. These bacteria also negatively affected the development of insect along with significant decline in relative growth and consumption rate as well as efficiency of conversion of ingested and digested food of insect. The bacteria significantly decreased the reproductive potential of insect. Perturbations in the composition of gut microbiome and damage to gut epithelium were also observed that might be associated with decreased survival of this insect.
Conclusions
Our study reveals the toxic effects of K. pneumoniae and P. paralactis on biology of S. litura. These bacteria may be used as potential candidates for developing ecofriendly strategies to manage this insect pest.
The symbiotic relationship between insects and gut microbes contributes to their fitness by serving immense range of functions viz. nutrition and digestion, detoxification, communication and reproduction etc. However, this relationship between insect and gut microbes varies from mutualistic to pathogenic. Gut microbes become pathogenic when the healthy normal microbial composition is perturbed leading to the death of insect host. Spodoptera litura (Fab.) is a polyphagous pest that causes significant damage to many agricultural crops. The management of this pest primarily depends upon chemical insecticides which have resulted in development of resistance. Thus in search for alternative strategies, culturable gut bacteria isolated from S. litura were screened for insecticidal potential. Among these Serratia marcescens and Enterococcus mundtii induced higher larval mortality in S. litura. The mortality rate increased from 32 to 58% due to S. marcescens at concentrations ranging from 2.6 × 108 to 5.2 × 109 cfu/ml and 26 to 52% in case of E. mundtii due to increase in concentration from 4.6 × 108 to 6.1 × 109 cfu/ml. Both the bacteria negatively affected the development, nutritional physiology and reproductive potential of insect. The results indicated a change in gut microbial composition as well as damage to the gut epithelial membrane. Invasion of gut bacteria into the haemocoel led to septicaemia and ultimately death of host insect. In conclusion both these gut bacteria may serve as potential biocontrol agents against S. litura.
Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) is a polyphagous pest causing economic losses to a number of agricultural crops. The present study was conducted to explore the insecticidal potential of some soil bacteria against S. litura. Among the tested bacterial isolates, Shewanella sp. (SS4) and Thauera sp. (M9) caused higher larval mortality. Concentration–response bioassays indicated a dose‐dependent increase in larval mortality with maximum mortality of 80 and 76%, respectively, due to Shewanella sp. and Thauera sp. Both the bacteria negatively affected the growth and development, reduced adult emergence and induced morphological deformities in adults. Nutritional parameters of S. litura i.e. relative growth and consumption rate, efficiency of conversion of ingested and digested food as well as approximate digestibility of food decreased with increasing bacterial concentrations. Histopathological studies revealed the disruption of peritrophic membrane and midgut epithelium of larvae. Profiles of digestive enzymes i.e. α‐amylases, α, β‐glucosidases and galactosidases, lipases and proteases were also significantly decreased with bacterial treatment. Comet assay test indicated DNA damage in larval haemocytes due to bacterial infection. These results imply that both these bacterial isolates have the potential as biological control agents and may prove as environmentally friendly alternative for pest management techniques.
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