Biological Control of Weeds

Weeds are the unwanted plant which grow in agricultural fields, ponds, lakes, etc. and have bad effects on the flora and fauna present/growing in their vicinity. However, in agricultural fields, nutrients supplied to specific crop are absorbed by weeds and results in poor supply of nutrients to crop plants. Similarly, in paddy fields, they do so. Larvae of the harmful insects e.g. malarial mosquito survive around the aquatic weeds growing in ponds or lakes. Although synthetic herbicides have been formulated which have phytotoxic and mutagenic effects on many agricultural plants and in turn enter in animal and human system. Therefore, use of synthetic herbicides is being discouraged in many countries. An alternative method of control of weeds has been developed which is the use of "microbial herbicides" or "bioherbicides".

Some of the noxious weeds are :
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.

In India, biocontrol of water hyacinth has been much emphasized (Anonymous, 1987). In the U.S.A. plant pathologists and weed scientists made the joint efforts foi- biocontrol of weeds by using fungal pathogens. Conway (1976) used Cercospora rodmanix as a biocontrol agent of water hyacinth in the lakes in Florida (U.S.A.). Control of A. viroinica growing in rice field by Collectotrichum gloeosporioides has also been reported (Anonymos, 1978). University of Arkansas and the Upjohn Company have made a cooperative effort to prepare the microbial herbicide by using C. gloeosporioides (Templeton et al, 1980).

Table 13.4. Pathogens of aquatic weeds.

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.

A B International Institute of Biological Control, U.K. has been developing a mycoherbicide for the control of Rottboellia cochinchinensis which is a serious weed. An isolate of Colletotrichum from Thailand was selected against R. cochinchinensis. Field trails have demonstrated that the mycoherbicide has potential for controlling this weed, particularly in combination with low does of chemical herbicides (Natural Resource Institute, Newsletter 5, 1992).

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.

Insects as Biocontrol Agents

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 :

(i) Senecio jacobaea, a serious weed in wetter areas has been controlled by introducing Tyrea jacobaea, cinnaber moth, in California (U.S.A.) and by Pegohylemyia seneciella (seedfly) in Australia.
(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).