Background Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) also known as tobacco caterpillar, is one of the most serious polyphagous pests that cause economic losses to a variety of commercially important agricultural crops. Over the past few years, many conventional insecticides have been used to control this pest. However, the indiscriminate use of these chemicals has led to development of insecticide resistant populations of S. litura in addition to harmful effects on environment. Due to these ill effects, the emphasis is being laid on alternative eco-friendly control measures. Microbial control is one of the important components of integrated pest management. Thus, in search for novel biocontrol agents, the current work was carried out with the aim to evaluate the insecticidal potential of soil bacteria against S. litura. Results Among the tested soil bacterial isolates (EN1, EN2, AA5, EN4 and R1), maximum mortality (74%) was exhibited by Pseudomonas sp. (EN4). The larval mortality rate increased in a dose-dependent manner. Bacterial infection also significantly delayed the larval development, reduced adult emergence, and induced morphological deformities in adults of S. litura. Adverse effects were also detected on various nutritional parameters. The infected larvae showed a significant decrease in relative growth and consumption rate as well as efficiency of conversion of ingested and digested food to biomass. Histopathological studies indicated damage to the midgut epithelial layer of larvae due to the consumption of bacteria treated diet. The infected larvae also showed a significantly decreased level of various digestive enzymes. Furthermore, exposure to Pseudomonas sp. also caused DNA damage in the hemocytes of S. litura larvae. Conclusion Adverse effects of Pseudomonas sp. EN4 on various biological parameters of S. litura indicate that this soil bacterial strain may be used as an effective biocontrol agent against insect pests.

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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