Biological weed control

Definition; Biological weed control refers to the use of living organisms—such as insects, pathogens (fungi, bacteria, viruses), or grazing animals—to reduce the growth, reproduction, or spread of undesirable plants (weeds). Unlike chemical herbicides, it relies on natural interactions in the ecosystem to manage weeds sustainably.

Mechanisms of Biological Weed Control; The basic principle is that the control agent either directly damages the weed or competes with it, reducing its vigor. Main mechanisms include:

• Herbivory (Feeding on Weeds); Certain insects, snails, or grazing animals feed on specific weeds. Examples: Insects: Rhinocyllus conicus (weevil) feeding on musk thistle. Grazers: Goats feeding on bushy weeds like Lantana camara.
• Pathogens (Disease Agents); Fungi, bacteria, or viruses can infect and weaken weeds. Examples: Fungi: Puccinia chondrillina (rust fungus) attacking skeleton weed. Bacteria: Xanthomonas campestris used in some bioherbicides.
• Competition; Introducing fast-growing or allelopathic plants to suppress weed growth by occupying space, nutrients, and sunlight. Example: Cover crops like clover or rye suppressing annual weeds in fields.
• Allelopathy; Some plants release chemicals that inhibit weed germination or growth. Example: Black walnut producing juglone, which inhibits growth of some weeds.

Types of Biological Weed Control

• Classical Biological Control; Involves introducing a natural enemy from the weed’s native habitat to a new area. Goal: Establish a self-sustaining population that controls the weed. Example: Introduction of Cactoblastis cactorum (moth) to control prickly pear cactus in Australia.
• Augmentation; Mass-rearing and periodic release of natural enemies to suppress weeds. Example: Releasing large numbers of insects that feed on a specific weed.
• Conservation; Modifying the environment to favor existing natural enemies of weeds. Example: Providing habitats for predatory insects that attack weed pests.

Examples of Biological Agents

Bioherbicides

Definition:

• Bioherbicides are formulations containing living organisms or their products (like spores, toxins, or metabolites) that kill or suppress weeds. They are a subset of biological weed control but are applied more like traditional herbicides.
• Unlike chemical herbicides, bioherbicides are natural, often specific, and environmentally friendly.

Types of Bioherbicides; Bioherbicides are mainly based on microorganisms or plant-derived compounds:

• Fungal Bioherbicides; Most common type. Pathogenic fungi infect weeds and reduce their growth. Examples: Colletotrichum gloeosporioides – targets nutsedge (Cyperus rotundus). Phytophthora palmivora – used against milkweed (Parthenium hysterophorus).
• Bacterial Bioherbicides; Certain bacteria cause diseases in weeds. Example: Xanthomonas campestris pv. poae – controls annual bluegrass (Poa annua).
• Viral Bioherbicides; Viruses infect specific weeds and suppress them. Less commonly used due to complexity of production and host specificity.
• Plant-Derived Bioherbicides (Allelopathic); Contain natural plant extracts or phytotoxins that inhibit weed growth. Examples: Sorghum extracts containing sorgoleone. Walnut extracts producing juglone.

Mechanisms of Action; Bioherbicides suppress weeds through:

• Infection and Disease Fungi, bacteria, or viruses invade weed tissues → disrupt growth, photosynthesis, or vascular function → plant death.
• Toxin Production; Some microbes produce phytotoxins that inhibit seed germination or damage leaves.
• Allelopathy; Plant-derived compounds interfere with weed metabolism or seedling establishment.

Examples of Commercial Bioherbicides