In this article we will discuss about the top three methods used for controlling aquatic weeds. The methods are: 1. Physical Control and Utilization of Aquatic Weeds 2. Aquatic Weed Control through Herbicides 3. Biological Control of Aquatic Weeds.
Method # 1. Physical Control and Utilization of Aquatic Weeds:
Physical cutting and removal of aquatic weeds, followed by then- rapid, commercial utilization are, perhaps, the best complementary approaches to aquatic weed management, at least in India and like developing countries. The economic utilization of aquatic weeds is, in fact, an indirect way of motivation of the owners and users of water bodies to adopt their physical removal.
Manual labour is often used for harvesting, digging, chaining dredging, and netting of aquatic weeds. Sometimes, physical barriers are also floated in water to contain the floating weeds within a portion of the water body for its subsequent removal from there.
To reduce the dependence on human labour and harvest the weeds rapidly, several kinds of aquatic weed harvesters have been developed, varying in design and capacity to suit local conditions. The onshore machines, including cranes are also used to lift masses of weeds from the weedy water bodies at convenient points.
The physical control of weeds has some distinct advantages over their herbicidal management as follows:
1. It yields immediate results, without any waiting periods.
2. It removes the nutrient load from water each time the weeds are taken out. This in turn helps to discourage the growth of future weeds.
3. It does not allow the weeds to rot inside water (a process called putrefaction), avoiding thus its fouling, enhancement in BOD (= biological oxidation demand), and increase in nutrient loads.
4. It is a non-selective procedure; so it has less chances of inducing ecological shifts in weed species.
A major limitation of the physical control of aquatic weeds is that the harvested weeds regrow fast, making it essentially a continuous, manual or machine intensive process. More so, when an attempt is made to dig out the roots and rhizomes of aquatic weeds from the bottom of waterbodies, it requires their drying for considerable period.
Even then the weeds regrow after the water is let-in again. Nevertheless, when we develop in near future some real, large scale economic usage of aquatic vegetation, the negative considerations of the physical control procedures will automatically become less pertinent.
The utilization of aquatic weeds is now the main concern of the weed scientists. The Government agencies should encourage further intensive research on the subject to make physical control of aquatic weeds a practical proposition. Aquatic weeds harvested from each ha of land weigh in tonnes. What is needed is to be able to use this huge biomass for some economic purpose.
At the present, in India, certain specific aquatic weeds are already put to some economic uses, but this does not commensurate with the size of- the problem. Typha, Phragimites, and Scirpus spp. are widely harvested and sold for making thatches, baskets, ropes, screens, mats, etc. Typha shoots mixed with mortar make strong structural material.
The, pith of Aeschynomene aspera is used for insulating solar hats, stuffing life-jackets, and making bottle corks. Carex spp. provide weaving material for making carpets. Attempts are often made to feed waterhyacinth to pigs and some other animals, but this is not adequate. Much research is needed to achieve the goal of using aquatic weeds as animal feed.
An important bottleneck in this process is their very high moisture content which makes their transport to the site of their use or processing very costly. Other difficulties in the use of aquatic weeds as animal feed encountered are their high mineral levels and possible presence of toxic elements in them. Another good possibility of large scale use of weeds is as organic manure.
Thus far only waterhyacinth has been exploited for the purpose. Again, high water content of aquatic weeds is a hurdle both in their transport as well as their aerobic decomposition inside the manure pits. Therefore, it is recommended to let the harvested waterhyacinth dry in the sun to a semi-dry stage on the shore itself before transporting it to the pits.
Also, in the pits alternate layers of dung be provided as inoculum to hasten the biomass decomposition. Bacterial concentrates to expedite aerobic decomposition of aquatic weeds in the pits shall be welcome when these are developed by the scientists in near future.
Some other promising usages of aquatic weeds are as paper pulp, biogas, edible protein, vegetative mulch, and soil amendments. Presently these are only of academic interests.
Method # 2. Aquatic Weed Control through Herbicides:
A number of aquatic herbicides and algicides are marketed and used in the western countries to keep their water bodies weed free. In India, herbicidal control in aquatic ecosystems is very much limited chiefly because of the fact that our waterbodies are subjected to multipurpose uses while the herbicides pose specific safety limitations.
No single herbicide can be safe to the varied uses of a waterbody. Still, at least in certain situations, one could choose amongst the following herbicides and algicides to control certain aquatic weeds and algae better than before.
i. Acrolein and Aromatic Solvents:
These are total weed killers, used for destroying the submersed weeds in irrigation canals. The treated water is highly unsafe to fish and for its potable uses. After killing the submersed weeds these compounds, however, leave the treated water by volatilization, before the water had chance to reach the farm lands. The effective concentrations of the two herbicides are 15-80 ppm.
ii. Anhydrous Ammonia:
Better known to us a fertilizer, anhydrous ammonia can be used in confined waterbodies to obtain non-selective kill of submersed weeds and water-bloom (phytoplanktons). The chemical is highly toxic to fish. Therefore, it is recommended to treat only 10-12% of the pond and lake area at a time with it to let the fish move to the untreated portions. Being volatile, anhydrous ammonia leaves the water soon after damaging the weeds whence the fish can return to the treated weedfree area, conveniently.
iii. Dalapon:
Combined with Amitrole, dalapon forms a very effective herbicide against cattails. Also, with 2,4-D it forms a good, wide-spectrum herbicide for the treatment of bank-weeds.
iv. 2, 4-D:
It is widely recommended and used to sink waterhyacinth mats in non-irrigation waters. In no way the treated water should find way to cropped areas. Besides waterhyacinth, 2,4-D is effective in controlling certain other notorious, broadleaf aquatic weeds.
v. Paraquat and Diquat:
In irrigation water bodies, instead of 2,4-D one must choose paraquat and diquat to control water hyancinth since the treated water is then safe for irrigation of the crops. Both paraquat and diquat are, however, unsafe to fish. Also, the treated water is not good for drinking and its other potable uses.
vi. Diuron:
Diuron finds its place in aquatic eco-systems in sterilizing drainage ditches, irrigation channels, and ditch banks, with normal precautions. The effective rate is 10 kg ha-1.
vii. Endothall (Na and K Salts):
The Na and K salts of endothall are used for the destruction of existing stands of submersed weeds. These are safe to fish and other aquatic fauna. Considerable handling precautions are, however, needed with endothall to prevent injury to the work crew.
viii. Dichlobenil:
It is primarily used to treat the exposed bottom soils of canals to kill the rhizomes and roots of aquatic weeds. Its effective rates are 5-10 kg ha-1. The canal can be refilled with water about five weeks later.
ix. Algicides:
‘Copper Sulphate Pentahydrate’ (CSP), terbutryn, simazine, dichlone, and diuron form good, aquatic algicides. Of these, CSP (01-10 ppm), terbutryn (0.05-0.1 ppm), and diuron (0.2-0.4 ppm) are suited for treating fish ponds. CSP can also be used for treating the drinking waterbodies and water storage tanks.
Besides algae, it will also effectively damage hydrilla. Simazine is good only for treating house drains, park fountains, and like places. Dichlone (0.02-0.15) can be used safely in recreational waters and paddy fields to control algae.
Method # 3. Biological Control of Aquatic Weeds:
Here only specific examples of aquatic weed control shall be cited.
i. Herbivorous Fish:
Of the several herbivorous fish, grasscarp, also called, white amur or Chinese carp (Ctenopharyngodon idella), is a voracious feeder of several submersed and emerged aquatic vegetation. After several years of intensive research now it is established as a non-specific, bio-agent of several aquatic weeds in canals and large lakes in the tropics and subtropics.
The daily aquatic vegetation consumption of the grass carp is equal to its own body weight, which is about 1 kg when it is one year old. Its body weight reaches up to 32 kg at its full length of 1m or so. Unfortunately water hyacinth (E. crassipes) is not consumed by it. Grasscarp is native to cold rivers of China and Siberia where it breeds freely but shows a poor appetite.
In warm waters of the tropics and subtropics on the other hand, it develops a very good appetite for aquatic vegetation but turns to be, fortunately, a very shy breeder. Therefore, for weed control purposes it is bred artificially and released in water when the fingerlings are about 100 g each. About 1500 fingerlings must be released per ha area of water surface to obtain effective weed control.
Where older fingerlings are used, the stocking rates can be proportionately reduced. When the weedy vegetation in a water body is reduced by the carp to a low level, one could either transfer these to a new weedy waterbody or sell these as meat.
ii. Insect Bioagents:
At least three aquatic weeds have been successfully brought tinder the banner of bio-control, using specific insect bioagents (Table 16.1).
