Parasitic and aquatic weeds and their management in cropped and non-cropped lands:

Parasitic Weeds; Parasitic weeds depend on host plants for water and nutrients. Common examples include Striga, Cuscuta, and Orobanche.

Characteristics

• Attack crops like maize, sorghum, legumes, tomato, and sunflower.
• Cause severe yield losses (up to 100% in severe infestations).
• Difficult to control because they emerge from host roots or attach aboveground.

Management Strategies

• Cultural Methods
  • Crop rotation: Rotate with non-host crops to break parasite life cycles.
  • Intercropping: Plant resistant crops or trap crops that stimulate parasite germination without supporting growth.
  • Use of resistant/tolerant varieties: e.g., Striga-resistant maize or sorghum.
  • Fallowing: Leaving land fallow for one season may reduce seed bank.
• Mechanical Methods; Hand removal of young seedlings before they attach to the host. Deep plowing to bury seeds of root parasites like Striga.
• Biological Methods; Use of natural enemies like fungi (e.g., Fusarium spp.) to attack parasitic weed seeds.
• Chemical Control; Pre- and post-emergence herbicides that are selective and safe for crops. Seed treatments with herbicide-coated seeds to inhibit parasite germination.

2. Aquatic Weeds and Their Management

   • Introduction
   • Aquatic weeds are undesirable plants that grow in water bodies such as ponds, lakes, rivers, canals, tanks, and reservoirs.
   • They spend part or all of their life cycle in water, either floating, submerged, or emergent.
   • Though some aquatic plants have ecological value, excessive growth leads to weed problems.

   Problems Caused by Aquatic Weeds

   • Irrigation & Drainage – Block canals, reduce water flow.
   • Fisheries – Deplete oxygen, cause fish mortality.
   • Navigation & Recreation – Impede boat movement and tourism.
   • Public Health – Provide breeding grounds for mosquitoes (malaria, dengue) and snails (schistosomiasis).
   • Crop Cultivation – Compete with rice and aquatic crops.
   • Evaporation Losses – Increase water loss from reservoirs.

    

   2. Classification of Aquatic Weeds
   • Submerged Weeds – grow under water; roots may or may not be fixed. Hydrilla verticillata, Vallisneria spiralis, Potamogeton spp.
   • Floating Weeds – free-floating on water surface. Eichhornia crassipes (Water hyacinth), Salvinia molesta, Pistia stratiotes, Lemna minor.
   • Emergent Weeds – rooted in soil but leaves/flowers above water. Typha latifolia (Cattail), Cyperus rotundus, Sagittaria trifolia.

    

   • Methods of Aquatic Weed Control
   • Mechanical / Physical Methods

   Environmentally safe, but labor and cost intensive.

   • Dredging; Removes weeds along with mud/sediment. Effective in shallow ponds and canals.
   • Chaining; Dragging a heavy chain between two tractors across a canal/pond. Good for tough weeds like Hydrilla.
   • Draining; Lowering water level → exposing weeds → cut & sun-dry. Useful for perennial weeds.
   • Underwater Weed Cutters; Blades operated from boats cut weeds at different depths.
   • Netting; Nets (mesh ~3 cm) collect floating weeds like Pistia, Salvinia, Lemna.
   • Mowing; Weeds on banks/canal edges cut mechanically.
   • Burning; Emergent weeds burned after mowing/drying.

    

   • Chemical Control
   • Involves aquatic herbicides (spray or granular forms).
   • Should be used carefully due to risk of water pollution & fish mortality.

   Types of Herbicides

   1. Algaecides; Control algal blooms. Example: Copper sulfate, Endothall.
   2. Aquatic Herbicides; Selective & non-selective herbicides. Examples: 2,4-D – for water hyacinth, sedges. Glyphosate – for emergent & floating weeds. Diquat, Paraquat – fast-acting contact herbicides.

   Application Zones

   • Surface treatment – Spray herbicide on top 1/3 area.
   • Total water volume treatment – Applied throughout but limited to 1/3 volume at a time (to avoid oxygen depletion).
   • Bottom layer treatment – Herbicide injected in deeper lakes.
   • Soil surface treatment – Applied to drained ponds/channels.

   Advantages: Quick, effective, selective.
   Disadvantages: Toxicity to aquatic life, resistance, water contamination, legal restrictions.

    

   • Biological Control

   Eco-friendly, sustainable, but slower in action.

   1. Fish; Chinese Grass Carp (Ctenopharyngodon idella) – feeds on submerged weeds (Hydrilla, Potamogeton). About 75 fish can clear 1 ha of water.
   2. Insects Neochetina eichhorniae (weevil) → controls water hyacinth. Cyrtobagous salviniae → controls Salvinia. Flea beetles (Colasposoma spp.) damage Eichhornia.
   3. Fungi; Cercospora rodmanii – causes leaf blight in water hyacinth. Acremonium zonatum – pathogenic to Eichhornia.
   4. Mammals; Manatees (Trichechus spp.) → consume up to 20 kg/day aquatic vegetation.
   5. Snails; Limnaea acuminata → feed on submerged weeds and roots of floating plants like Salvinia, Eichhornia.
   • Advantages: Eco-friendly, self-sustaining, specific.
   • Disadvantages: Slow, species-specific, cannot eradicate weeds completely.

    

   • Integrated Aquatic Weed Management (IAWM)
   • Best approach = combination of methods. Example strategy for water hyacinth:
     • Mechanical removal of dense mats → release of Neochetina weevils → follow-up spot spraying with glyphosate.
   • Ensures long-term, cost-effective, and eco-friendly management.

    

   • Examples of Important Aquatic Weeds in India
   • Submerged: Hydrilla verticillata, Vallisneria spiralis, Potamogeton spp.
   • Floating: Eichhornia crassipes (water hyacinth), Salvinia molesta, Pistia stratiotes, Lemna minor.
   • Emergent: Typha latifolia, Sagittaria trifolia, Cyperus spp.