Weed Management in Horticultural Crops

BIOLOGICAL CONTROL

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 population 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.

Merits:

• 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

Demerits:

• 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 e.g. 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.

Bio-Herbicides/ Myco-herbicides:

The use of plant pathogens which are expected to kill the targeted weeds are known as bio-herbicides. 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 weeds 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 targeted weeds. Some registered myco-herbicides in western countries are tabulated below

No	Product	Content	Target weed
1	Devine	A liquid suspension of fungal spores of Phytophthora palmivora causes root rot.	Strangle vine (Morrenia odorata) in citrus
2	Collego	Wettable powder containing fungal spores of Colletotrichum gloeosporoides causes stem and leaf blight	Joint vetch (Aeschyomone virginica) in rice, soybean
3	Bipolaris	A suspension of fungal spores of Bipolaris sorghicola	Jhonson grass (Sorghum halepense)
4	Biolophos	A microbial toxin produced as fermentation product of Steptomyces hygroscopicus	Non-specific, general vegetation