Weeds compete with the crops for moisture, light, nutrients, and space, and cause reduction in crop yields. Effective weed control is necessary to boost crop production. This can be achieved through the proper adoption of suitable weed management strategies.
The information pertaining to nature and habitat of weeds, ontogeny, viability, dormancy, propagation and dispersal of weeds, response of weeds to the environment and edaphic factors in a given area should be known for designing any weed control strategy.
Biological control involves the usage of living agents as a method of control. These agents can be insects, bacteria, fungi, herbivorous fish, snails, competitive plants and animals. The effectiveness depends on the qualities of bio agent, weed infestation, environmental factors etc. In biological control, it is not possible to eradicate weeds but weed population can be reduced. This method is not useful to control all types of weeds.
Techniques of biological control
Classical biological control method: It is practiced when an invasive species has been established outside its native range (Julien, 1992). Natural enemies of the weeds are sought within their native range and are introduced and released in the countries, after evaluation of host specificity. This method has been the most successful and widely used.
Augmentation method: Laboratory bred naturally occurring population of the natural enemies are released to manage the target weeds when the existing natural enemies are ineffective for the control. Though augmentation approach is becoming popular in many countries, it is felt that this approach is not self sustaining, continued input is required and the technique is labour intensive (Batra, 1981).
Qualities of bio-agent
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The bio-agent must feed only one host and not other useful plants (Host specificity)
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Free from predators or parasites.
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Adapt to varied environmental conditions.
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High ability to disperse successfully and to locate its host plant
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Ability to kill weed or prevent its reproduction
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High reproductive capacity
Outstanding examples of biological weed control
|
Weed |
Bio agent |
|
Lantana ( Lantana camara )
|
Crocidosema lantanae (moth borer) Teleonemia scrupulosa (lace bug) Octotoma scabripennis (beetle) Uroplata giraldi (beetle) Agromyza lantanae( seed fly) |
|
Prickly pear (Opuntia dillenii)
|
Dactylopius tomentosus Dactylopius indicus (cochineal insect) Cactoblastis cactorum, (moth borer), |
|
Water hyacinth( Eichhornia crassipes )
|
Neochetina bruchi (weevil) Neochetina eichorniae (weevil) |
|
Salvinia (Salvinia molesta) |
Cryptobagus salviniae |
|
Dodder (Cuscuta)
|
Melanagromyza cuscutae Smicronyx cuscutae |
|
Nut grass( Cyperus rotundus ) |
Bactra verutana (moth borer) |
|
Ludiwigia parviflora |
Haltica cynanea (steel blue beetle) |
|
Thatch grass ( Imperata cylindrica) |
Orseoliella javanica |
|
Leptospermum scoparium |
Eriococcus orariensis (mealy bug) |
|
Eupatorium adenophorum |
Procecidochares utilis (gall fly) |
|
Hypericum perforatum (Johnswort) |
Chrysolina hyperici (leaf eating beetle) |
|
Alternanthera philoxeroides (Alligator weed) |
Agasiches connexa (flea beetle) |
|
Parthenium hysterophorus (Congress weed) |
Zygogramma bicolorata (beetle) |
|
Common carp and Snails |
Submersed aquatic weeds |
|
Fish - Tilapia |
Algae |
Bio Herbicides
Microscopic organisms such as bacteria, fungi, and viruses can cause diseases on many types of plants, including economically important crops and weeds. Under suitable environmental conditions, some plant pathogens can cause sufficient damage to susceptible weed hosts with the purpose of attaining weed population under control below an economic threshold level (Barreto and Evans, 1997). These are native pathogens, cultured artificially and sprayed like post-emergence herbicides each season on target weed, particularly in crop areas. Fungal pathogens are widely used for controlling weeds.
Outstanding examples of bio herbicides
|
Product |
Content |
Target weed |
|
Devine |
A liquid suspension of fungal spores of Phytophthora palmivora causes root rot. |
Strangler vine (Morrenia odorata) in citrus |
|
Collego |
Wettable powder containing fungal spores of Colletotrichum gloeosporoides causes stem and leaf blight |
Jointvetch (Aeschynomene virginica) |
|
Bipolaris |
A suspension of fungal spores of Bipolaris sorghicola |
Jhonson grass (Sorghum halepense) |
|
Biolophos |
A microbial toxin produced as fermentation product of Streptomyces hygroscopicus |
Non-specific,general vegetation |
|
LUBAO II |
C. gleosporoides f.sp. Cuscutae |
Dodder ( Cuscuta sp.) |
|
Caast |
Alternaria cassiae |
Cassia obtusifolia |
|
ABG 5003 |
Cercospora rodmanii |
Water hyacinth (Eichhornia crassipes) |
|
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Fusarium phallidorosium and Oidium parthenii |
Parthenium hysterophorus (Congress weed) |
|
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Uredo eichhorniae Alternaria eichhorniae, Cercospora piaropi, Rhizoctonia solani, |
Water hyacinth( Eichhornia crassipes )
|
Merits
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Eco friendly approach
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No residual effect
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Involves less cost when compared to other methods
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Provides long lasting effect
Demerits
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Multiplication involves more cost
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Control is very slow
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Limited success of control
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Very few host specific bio agents are available at present
Author
A.Suganthi, A.P.Sivamurugan and V.Manivannan
Tamil Nadu Agricultural University, Coimbatore