• Weed is a plant in the wrong place. On the other hand, weeds may be valuable plants in other situations. This fact is fundamental to a consideration of biological control. In fact the method of biological control of weed has been used when other methods found inadequate. To reduce the densities to non-economic levels by either direct or indirect action of natural enemies being used.

Fundamental of Biological Control of Weed

1. Concept of Natural Control as Related to Weed: 
   • The competitive weed should be fast-growing e.g. Stylo.
2. Kind of Natural Enemies: 
   • Species insect feeding made preferred roots, stems, seeds and flowers.
3. Nature of Controlling Action: 
   • Every weed cannot be controlled biological with insect.
   • a. Injury:
     • Direct feeding on vital parts like flowers, seeds or killing the plants. Indirect feeding injury increase susceptibility to diseases or secondary parasites.
   • b. Plant Parts Attacked:
     • Vital plant part should be attacked before seed formation.
4. Inter Relation of the Insect Agent and Food: 
   1. Struggle for existence, Insect attacked weed become weak and other species may become dominant. Disturbance to plant feeders by natural calamities.
5. Risk of Introduction and Host Specificity:
   • Great fear of become weed feeder as feeder of economic plants.
6. Specificity Tests, Starvation:
   • Leaf beetle, allied to host or even more on cabbage but found not multiplied in field.
7. Nature of Infection and Prospectus of Success:
   • Successful weed killer should have no pest of economic plants, should attack vital plant parts, should have least natural enemies and should have fast multiplication rate.

Bioagent	Weeds
Insect
Two beetles:   Octotoma scabripennis and Uroplata giraldi	Lantana camara
Scale insect Dactylopius tomentosus.	Prickly-pear weed – Opuntia
Flea beetle Agasicles hygrophyla	Alligator weed – Alternanthera philoxeroides
Fish
Common carp (Cyprimus carpio) Chinese carp	Aquatic weeds
Mammals Manetee or sea-cow	Water hyacinth
Snails     Marisa sp and other fresh water snails	Submerged weeds like coontail and algae
Fungi Rhizoctinia blight.	Hyacinth
Mites
Tetranychus sp	Prickly pear
Plants Cowpea as intercrop in sorghum	Effectively reduces the growth of weeds in sorghum