Field eficacy and yield protection of microbial and botanical insecticides against noctuid pests of vegetable and field crops: A review

Abstract:

How efective are microbial and botanical insecticides at protecting vegetable and field crop yields from noctuid moth damage under real-world farming conditions? Noctuid pests, including species of Spodoptera, Helicoverpa, and Agrotis, rank among the most damaging insect groups atacking crops worldwide. Conventional chemical insecticides have long been the default tool, but resistance build-up, environmental contamination, and residue concerns have pushed researchers and growers toward biological alternatives. This review assessed field-level evidence gathered between 2010 and 2024 on the performance of Bacillus thuringiensis (Bt) formulations, nucleopolyhedroviruses (NPVs), entomopathogenic fungi such as Beauveria bassiana and Metarhizium anisopliae, and botanical extracts derived from neem (Azadirachta indica) and pyrethrum. Published trials from vegetable crops (tomato, cabbage, broccoli, letuce, pepper) and field crops (maize, soybean, coton, sorghum) across tropical and subtropical regions were examined. Bt-based products consistently delivered 70-85% larval mortality in vegetable systems when applied at recommended doses and correct timing, though their eficacy dropped to 60-75% in field crops with dense canopies that limited spray coverage. NPV formulations targeting Helicoverpa and Spodoptera showed 65-80% control in vegetables, yet required careful cold-chain storage to maintain viability. Neem-based products reduced feeding damage by 45-70% and acted as growth disruptors, while pyrethrin extracts ofered rapid knockdown but shorter residual activity. Combined formulations mixing Bt with botanical synergists recorded the highest overall protection, reaching 80-90% yield preservation in tomato and cabbage trials. Yield losses in untreated plots ranged from 25% to 58%, confirming the economic necessity of intervention. Field persistence, spray timing relative to larval instar, and environmental conditions (temperature, rainfall, UV exposure) emerged as the main variables governing bio-insecticide success. The review highlights that integrating microbial and botanical agents within an IPM framework can match or closely approach the protection levels ofered by synthetic insecticides, provided application protocols are adapted to local cropping systems.