(a) Define microbial biopesticides. What are their types? With suitable examples, evaluate the potential of microbial…

Microbial Biopesticides: Types and Potential in Crop Protection

Microbial biopesticides are biological control agents derived from naturally occurring microorganisms—such as bacteria, fungi, viruses, and entomopathogenic nematodes—used to suppress pest populations, weed growth, and plant pathogens.

Types and Examples: Bacterial biopesticides include Bacillus thuringiensis var. kurstaki (producing Cry delta-endotoxins against lepidopteran pests like Helicoverpa armigera) and Bacillus sphaericus for dipteran larvae. Fungal biopesticides (entomopathogenic and antagonistic) include Beauveria bassiana and Metarhizium anisopliae targeting sap-sucking pests, alongside Trichoderma viride and T. harzianum, which suppress soil-borne fungal pathogens (Rhizoctonia, Fusarium). Viral biopesticides comprise Baculoviruses such as Helicoverpa armigera Nucleopolyhedrovirus (HaNPV) and Granuloviruses (GV) targeting borers. Entomopathogenic nematodes (EPNs) like Steinernema carpocapsae and Heterorhabditis spp. carry symbiotic bacteria to kill subterranean insect larvae.

Evaluation of Potential: Microbial biopesticides offer high host specificity, preserving beneficial pollinators, parasitoids, and predators. They leave zero toxic residues, decay rapidly without environmental persistence, and exhibit multi-site modes of action that delay pest resistance. In Indian agriculture, integrated pest management (IPM) strategies extensively deploy HaNPV against cotton bollworms and Trichoderma seed treatments against groundnut collar rot and rice sheath blight. However, their potential is constrained by slower knockdown action compared to synthetic chemicals, narrow environmental tolerance (susceptibility to solar UV and desiccation), limited shelf-life, and high formulation costs.

Viroids: Features, Viral Distinction, and Plant Diseases

Viroids are the smallest known infectious plant pathogens, discovered by T.O. Diener.

General Features: They consist exclusively of a single-stranded, covalently closed circular RNA molecule of low molecular weight (246–401 nucleotides) with extensive internal base-pairing resulting in a rod-like secondary structure. They lack a protein capsid or lipid envelope, possess no mRNA activity (do not encode any proteins), replicate autonomously through a rolling-circle mechanism mediated by host enzymes (RNA polymerase II), and some (family Avsunviroidae) exhibit self-cleaving hammerhead ribozyme activity.

Differences between Viroids and Viruses: Viroids possess naked circular ssRNA without a capsid or envelope, have a minute genome (246–401 nt), and do not encode proteins. In contrast, viruses possess a nucleic acid core (ds/ss DNA or RNA) enclosed within a protein coat (capsid) and sometimes a lipid envelope, have larger genomes (3–1000+ kb), and actively direct host machinery to synthesize viral proteins.

Diseases Caused by Viroids: Potato Spindle Tuber Disease (caused by PSTVd) results in severe stunting, erect foliage, and elongated, spindle-shaped, cracked tubers. Coconut Cadang-Cadang Disease (caused by CCCVd) produces progressive leaf chlorosis, non-bearing inflorescences, trunk tapering, and eventual crown death in Cocos nucifera.

Natural versus Phylogenetic Systems of Plant Classification

Natural System: Exemplified by George Bentham and Joseph Dalton Hooker (Genera Plantarum, 1862–1883), this system groups plants based on overall morphological similarities, character correlations, and shared gross diagnostic affinities.

• Merits: Highly practical for immediate identification; excellent keys for herbarium curation (utilized across Indian herbaria such as the Central National Herbarium, Howrah).
• Demerits: Evolutionarily blind; places Gymnospermae between Dicotyledonae and Monocotyledonae; artificially splits closely related taxa based on single artificial floral criteria (e.g., Monochlamydeae).

Phylogenetic System: Exemplified by Arthur Cronquist and the modern Angiosperm Phylogeny Group (APG IV, 2016), this system organizes taxa according to genealogical descent, ancestral lineages, and evolutionary divergence established through cladistic analysis of molecular (rbcL, matK, 18S rDNA) and morphological data.

• Merits: Reflects monophyletic lineages and natural evolutionary history; provides immense predictive value for biochemical and genetic traits.
• Demerits: System is dynamic and subject to frequent reclassifications with new molecular sequencing; less practical for quick field identification by morphology alone.

Verdict

Microbial biopesticides represent a biologically robust substitute for hazardous agrochemicals in sustainable farming, provided formulation stability is optimized. In plant taxonomy, while the natural system remains an enduring diagnostic tool for field herbaria, the phylogenetic system (APG IV) provides the true evolutionary framework, making it the indispensable scientific standard in modern botany.