Use of living organism’s viz., insects, disease organisms, herbivorous fish, snails or even competitive plants for the control of weeds is called biological control.
In biological control method, it is not possible to eradicate weeds but weed populations can be reduced. This method is not useful to control all types of weeds. Introduced weeds are best targets for biological control.
Qualities of bio-agent
The bio-agent must feed or affect only one host and not other useful plants
It must be free of predators or parasites.
It must readily adapt to environment conditions.
The bio-agent must be capable of seeking out itself to the host.
It must be able to kill the weed or atleast prevent its reproduction in some direct or indirect way.
It must possess reproductive capacity sufficient to overtake the increase of its host species, without too much delay.
Least harm to the environment
No residual effect
Relatively cheaper and comparatively long lasting effect
Will not affect non-targeted plants and safer in usage
Multiplication is costlier
Control is very slow
Success of control is very limited
Very few host specific bio-agents are available at present
Mode of action
Differential growth habits, competitive ability of crops and varieties prevent weed establishment Eg. Groundnut, cowpea fast growing and so good weed suppresser.
Insects kill the plants by exhausting plant food reserves, defoliation, boring and weakening structure of the plant.
Pathogenic organisms damage the host plants through enzymatic degradation of cell constituents, production of toxins, disturbance of harmone systems, obstruction in the translocation of food materials and minerals and malfunctioning of physiological processes.
Outstanding and feasible examples of biological weed control
Larvae of Coctoblastis cactorum, a moth borer, control prickly pear Opuntia sp. The larvae tunnel through the plants and destroy it. In India it is controlled by cochinial insects Dactylopius indicus and D. tomentosus
Lantana camara is controlled by larvae of Crocidosema lantana, a moth bores into the flower, stems, eat flowers and fruits.
Cuscuta spp. is controlled by Melanagromyza cuscutae
Cyperus rotundus – Bactra verutana a moth borer
Ludiwigia parviflora is completely denuded by Altica cynanea (steel blue beetle)
Herbivorous fish Tilapia controls algae. Common carp, a non- herbivorous fish controls sub-mersed aquatic weeds. It is apparently due to uprooting of plants while in search of food. Snails prefer submersed weeds.
The use of plant pathogen which are expected to kill the targeted weeds. These are native pathogen, cultured artificially and sprayed just like post- emergence herbicides each season on target weed, particularly in crop areas. Fungal pathogens of weed have been used to a larger extent than bacterial, viral or nematode pathogens, because, bacteria and virus are unable to actively penetrate the host and require natural opening or vectors to initiate disease in plants.
Here the specific fungal spores or their fermentation product is sprayed against the target weed. Some registered mycoherbicides in western countries are tabulated below.