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 appropriate adoption of appropriate weed management strategies.
The information pertaining to the nature and habitat of weeds, ontogeny, viability, dormancy, propagation and dispersal of weeds, response to weeds to the environment and edaphic factors should be known for designing any weed control strategy.
Biological control involves the use 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 quality of bio agent, weed infestation, environmental factors etc. In biological control, it is not possible to eradicate weeds but weed populations can be reduced. This method is not useful for controlling 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 populations of natural enemies are released to manage target weeds when existing natural enemies are ineffective for the control. Although augmentation approach is popular in many countries, it is felt that this approach is not self sustaining, continuous input is required and the technique is labor 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 successfully disperse and locate its host plant
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Ability to kill or prevent its reproduction
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High reproductive ability
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 appropriate environmental conditions, some plant pathogens can cause considerable damage to susceptible weed hosts with the purpose of attaining weed populations under an economic threshold level (Barreto and Evans, 1997). These are native pathogens, cultured artificially and sprayed like post-emergence herbicides targeted at each season, especially 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 a 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) |
|
— |
Fusarium phallidorosium and Oidium parthenii |
Parthenium hysterophorus (Congress weed) |
|
— |
Uredo eichhorniae Alternaria eichhorniae, Cercospora piaropi, Rhizoctonia solani, |
Water hyacinth (Eichhornia crassipes) |
Merits
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, APSivamurugan and V.Manivannan
Tamil Nadu Agricultural University, Coimbatore