Biological Control of Weeds
Aeschynomene virginica, Canabis sativa, Chbndrilla juncea (skeleton weed), Convolvulus arvensis, Cyperous rotundus, Eichhornia crassipes (water hyacinth), Hydrilla verticillata, Lantana camera, Nymphaea odorata, Panicum dicotomiflorum, Pistia statiotes, Rubus sp., Rumexa crispus, Solarium dulcambra, Xanthium strumarium. Pathogens and insect pests are applied in control of most of the weeds in both agricultural and aquatic systems. Pathogens of some of the aquatic weeds are given in Table 13.5.
Mycoherbicides
Biological potential of plant pathogens has been realized in recent years. They can be trained to behave as biocontrol agents. Some of them are host specific and even beyond the species to the variety or biotype level (Freeman, 1982). Plant pathologists are making efforts to characterize plant pathogens and their introduction in those areas where weed is a serious problem. If the pathogens could minimize the disease they can be recommended as mycoherbicides.
Table 13.4. Pathogens of aquatic weeds.
Plant species | Pathogens |
Algae (Cyanobacteria) | Cyanophages (viruses) and bacteria |
Alligator weed (Atternathea philoxeroides) | Stunt virus |
Hydrilla (Hydrilla verticillata) | Aspergillus sp., Penicillium sp., Phytophthora parasitica, Trichoderma sp. |
Salvinia (Salvinia sp.) | Alternaria sp. |
Water hyacinth (Echhornia crassipes) | Acremonium natum, Alternaria eichhorniae, Bipolaris stenospila, Cercospora piaropi, C. rodmanii, Fusarium roseum |
Water lettuce (Pistina stratiotes) | Virus |
Templeton et. al. (1979) sprayed the spores of C. gloeosporioides (1.5 x 106/ml) by aeroplane, in fields at the rate of 96 liter water per hectare and estimated about 95-100 per cent killing of weeds in 17 rice fields. Orr et al., (1975) used a host specific pathotype of Phytophthora citrophthora as a biocontrol agent for Morrenia odorata (milk weed vine) in Citrus spp. Templeton et al. (1979) reviewed the biocontrol of weeds where fungi have been used as myco-herbicides.
Freeman (1977) enumerated five characteristics of plant pathogens that make them desirable candidates as biocontrol agents for aquatic weeds : (i) they are numerous and diverse, (ii) they are often host specific, and their use of control noxious species would not endanger desirable plant species, (c) they are easily disseminated and self maintaining; therefore, there is no need of reapplication, (iii) they have ability to exert a limiting influence on plant populations without eliminating the species, and (iv) they are non-pathogenic to animals.
The technique of using insects for disease control originated in China. Chinese used predator ants to control certain insect pests of citrus. This practice continued through the ages, and even in modern time, it is still in use. Citrus growers purchased colonies of predatory ants (e.g. Oecophylla smaragdina) to introduce in citrus leaf feeding insects (Clausen, 1956; Me Cook, 1982).
Moreover, insects play a valuable role in the eradication of valuable weeds but complete eradication has not been achieved by this method. Insects attack the host and multiply rapidly until much plant growth is destroyed. Due to food shortage the number of insects decreases, consequently weeds reappear. Again weed is invaded by insects causing a resultant decreases in weed number. Therefore, the ratio of weed number and insect population fluctuates (Arora and Sharma, 1989). A few cases are described as below :
(ii) Hypericum perforatum (goat weed), a serious weed found in California and pacific North-West is controlled by Chyroline hyperici (goat weed bettle).
(iii) Eichhornia crassipes (water hyacinth), is controlled by an insect, Neochetina olehhorniae. This insect was introduced from Latin America to India. The female lays eggs on petiole of water hyacinth. Larvae feed upon petiole and the adults feed on leaves resulting in destruction of the whole plant.
(iv) Lantana, a pasture pest and poisonous plant is found throughout the world. In Hawaii, caterpillars of Plusia verticillata was introduced for its control. Larvae of seedfly, Agromyza lantanae, eat many berries and cause others to dry so that birds could not carry them. The lace bug, Teleonemia scrupulosa was most effective insect to control Lantana but this insect could not yield much success (Arora and Sharma, 1989).