Biological control of weed is a long-term solution which is most effective as part of an integrated weed management approach. Biological control is rarely a ‘silver bullet’, as not all weeds have biological control agents that would be considered safe for introduction in India.
There are three main methods used for control of weeds- biological, mechanical, and chemical. Mechanical control includes mowing, hoeing, cultivation, and hand pulling. Chemical control is the use of herbicides. You can also use a combination of these three methods in an integrated weed management approach which some authorities would consider as the fourth method.
Biological Control of weed is necessary because herbicide application has been the most widely adopted management tool against many rangeland and pasture weeds. Unfortunately, the marginal economic rewards and inaccessibility of many of the infested rangelands has limited the value of chemical application.
Simultaneously, the elimination of desirable broad leaved species and the potential contamination of the water table by herbicidal application are deleterious side effects. To be successful, an introduced biological control agent need not kill its weedy host outright. Biological control using insects is slower than other weed control methods. Biological Control of weeds provides specific insects that attack only certain weeds. Not all weed problems can be addressed by the insects.
The biological control of weeds involves the use of living organisms, such as insects, herbivorous fish, other animals, disease organisms, and competitive plants to limit their infestations. Currently, there are two approaches to this method of weed control, viz., (i) the classical biological control philosophy approach, and (ii) the bio-herbicide philosophy approach. Any biological control approach shall yield best results when it is integrated with a comprehensive weed management programme.
The classical biological weed control aims at restoring the balance between the target alien weed and its natural enemies in the ecosystem by purposeful introduction of suitable, exotic bioagents. The immediate response to successful introduction of a native bioagent is a decrease in the weed population, followed after some period by a decline in the bioagent population due to its food shortage.
Then often the weed population tends to recover. The process continues in a cyclic fashion till the bioagent-weed populations get stabilized at a low level. At that time, man enters the cycle and he puts the cleared area to some economic use.
The biological weed control should not be expected to eliminate the target weed from an area; in fact, success of biological control of weeds depends upon continued presence of the weed existing in small numbers and shifting with time. The method being a slow operating one, is presently used mainly in non-crop situations.
On crop lands, even if successful, the bioagent will not get opportunity to work on the host weed because of frequent use of insecticides and fungicides in modern agriculture. Otherwise, Cyperus rotundus can possibly be controlled in crop fields with a moth borer, Bactra verutana. Another promising example of selective biocontrol in crop fields is complete denudation of Ludwigia parviflora ‘waterpurslane’ by Haltica cyanea Web. ‘steelblue beetle’, in rice fields.
An important aspect of biological weed control is that at a time, it is applicable to the control of only one major weed species that has spread widely. Also, one should be prepared to accept cyclic resurgence of the weed under attack by the classical biocontrol approach. Further, while with perennial weeds the main objective of biocontrol is the destruction of the existing vegetation, in the case of annual weeds prevention of their seed production is generally more important.
A Successful Bioagent:
1. Host Specificity:
Biocontrol agents recommended for weed control should be host- specific, i.e. they should not attack other plant species, particularly the economic ones. The bioagents released so far (chiefly insects) had successfully passed the ‘starvation tests’ before they left the laboratories for field use. They were found to prefer to starve to death rather than feed upon plant species other than the target weed.
Starvation test is a good safeguard against an introduced bioagent becoming a pest on some economic crop plant. For example, Teleonemia scrupulosa, though found a very good insect bioagent for the control of lantana, yet in India it is found likely to damage teak (Tectona grandis) and sesame (Sesamum indicum).
Therefore, it was not released in the country. Similarly, zygogramma bicolorata Pallister, although an effective, leaf eating bioagent of Parthenium hysterophorus, ‘carrotgrass’, it is found to attack sunflower (Helianthus annus) in India, because of which it was not released. However, there is also a view that unless the teak trees and sesame crop were located next to some large tract of lantana under a biocontrol programme, and T. scrupulosa bred there prolifically, the damage to the two economic plants would only be transient.
Similarly, feeding a few plants of sunflower by Z. bicolorata beetle did not justify dropping of the project of biocontrol of carrotgrass with this insect. In fact, in recent years the importance of subjecting a promising bioagent to very rigorous starvation tests has been questioned by certain biological weed control workers. According to these scientists, in the first instance even when a bioagent engaged itself in feeding upon a plant of economic value, it does not mean it will also breed upon it.
Secondly, one must weigh the damage and losses caused by the target weed and difficulty in controlling it by other means, against some possible damage that may be inflicted by the bioagent to some plant of economic value. In substance, a latitude is recommended in the conventional starvation tests of the otherwise promising bioagents, although not too liberally.
2. Bioagent Hardiness:
It is important that a classical bioagent withstands elements of its new environment in which it is transplanted. This includes freedom from its own parasites and predators. Further, the bioagent should be hardy enough to survive short and long periods of food shortage when the target weed population is brought to a low level.
This has a significant implication in the case of carp which cannot survive even a short starvation period. To continue their survival, the grasscarp must be shifted to a new weedy water body soon after the weed biomass in the previous one is lowered sufficiently.
3. Feeding Habit:
Great stress has been placed on the selection of insects as bioagents that attack either the flowers or seeds of the weed or bore into their stems. Insect bioagents with such feeding habits have been found more host specific and efficient in controlling weeds than the leaf or root feeders. But lately certain leaf eating insects have been found equally valuable in the control of weeds, particularly of the perennials.
Likewise, insects that attack roots are likely to be very effective on perennial weeds but insufficient attention has been paid to them. In any case, the destructive capacity of a bioagent on the weed should be very high so that its small population could destroy large weed infestations.
4. Ease of Multiplication:
High rate and ease of natural reproduction of bioagent is important in insects, pathogens, snails, and competitive plants. But it is very much unwanted in the case of carp employed for aquatic weed control because their excessive population may compete with cultivated fish. Fortunately, all present researches indicate that a bioagent carp like the Chinese grasscarp, is extremely slow, natural breeder in the tropics and sub-tropics, although under artificially controlled conditions it can be multiplied rapidly.
Types of Classical Bioagents:
1. Insects:
Insects have been more frequently used than other animals for the biological control of weeds. The first successful example of weed control with deliberately introduced insects was reported from Hawaii in 1902 when a project for the control of Lantana camara L. was initiated with a moth, Crocidosema lantana Buck. Since then insects belonging to widely different families and genera have been employed to control some other noxious weed species.
Alien weeds have been the main subject of biocontrol with insects imported from the evolutionary centres of the weed concerned. But there is at least one example when a native weed has been brought under control with an alien insect species. The weed is manuka, Leptospermum scoparium Forester, and it was controlled on its native land, New Zealand, with mealybug Eriococcus orariensis Hoy, introduced from Australia. Insects selected as biocontrol agents of weeds are largely host specific i.e., one insect species is employed to destroy only one weed species.
2. Carp Fish:
Certain fresh water carp fish consume large quantities of aquatic weeds and they are very suitable for relieving water bodies of their massive growth of noxious, aquatic vegetation. Whiteamur (Chinesegrasscarp) Ctenopharyngodon idella Val is the most promising species of carp for aquatic weed control. Whiteamur eats more than its own body weight in weeds daily and it grows at a rate of 5 kg/year, attaining 50 kg or more body weight at its full size.
The high potentiality of white amur for aquatic weed control has been tested in many countries, including India, and it has successfully cleared many weed-infested waterbodies. Unlike insect bioagents, the herbivorous fish are not much food-specific. They feed upon a wide range of plants and, therefore, they control a variety of aquatic weeds, simultaneously.
This food habit of grass carp may be undesirable in waterbodies having some plants of economic value, like waterchestnut (Trapa natans) and lotus (Nelumbo sp.) which, of course, are also classed as weeds in most situations. A good review on this bioagent is given by Gupta (2001).
3. Plant Pathogens:
Many fungi are observed to attack specific weed species, but examples of successful, practical weed control with fungi are few. Of late, it has been discovered that Acacia glauca could be controlled successfully by injecting a suspension of spores of Cephalosporium zonatum Sawada.
The ranchers of Hawaii are already using this practice on commercial scale. Control of skeleton weed (Chondrilla juncea L.) by introducing a rust causing fungus, Puccinia chondrillina Bubak & Syd in Australia is another example of successful biological control of weeds employing fungi.
4. Competitive Plants:
Certain plants may be highly competitive in nature; capable of suppressing some specific weed/s without causing any meaningful hindrance to our interests. Yeo (1972) found that Eleocharis acicularis ‘slender spikerush’ possessed these two characters. It is a short, very fast growing, aquatic plant that could cover the canal bottom rather rapidly, disallowing the obstructive tall weeds from establishing there. Likewise Panicum purpurascens Raddi is found competitive to Typha spp. in marshlands. More recently marigold (Tagetes spp.) plants have shown potential of displacing parthenium.
Classical Biocontrol of Weeds:
1. Lantana (Lantana camara L.):
Lantana was the first weed controlled successfully with certain insect bioagents in Hawaii. The insect greatly curtailed the spread of lantana and thinned its existing infestations. Hypena jussalis Guenoe is an excellent feeder of lantana leaves. It defoliated all sizes of lantana plants rapidly. Since then many other insects have been screened for the destruction of lantana in Hawaii, and the weed has been brought under control on her different islands.
Biological control of lantana has also been adopted successfully in Australia and other places. In Australia, three highly successful insect biocontrol agents are- hispine beetles (Octotoma scabripennis Guerin and Uroplata girardi Pic.) and tingid or lantana bug (Teleonemia scrupulosa Stal). These insects are causing severe damage to the susceptible taxa of Lantana, which are receding fast there. In Andhra Pradesh (India), lantana has been successfully controlled with some of these insects, besides the lantana seed fly (Ophiomya lantanaea).
2. Pricklypear (Opuntia Spp.):
In Australia, biocontrol of O. inermis with a moth, Cactoblastis cactorum (Berg) transformed 24 million ha land from wilderness to a scene of prosperous endeavour. In Tamil Nadu and Maharashtra (India), 40,000 ha of land infested with Opuntia dillenii was recovered from the weed by releasing Dactyloplius tomentosus Auct. (D. ceylonicus Green) ‘cochineal scale insect’ as its bioagent.
Other species of Opuntia were not attacked by this insect. The primary damage to Opuntia with insects can be combined with secondary attacks by bacterial and fungal parasites for further eroding the weed. These secondary bioagents are Cleosporium anatum E & E, Phyllosticta concava Seav. and Montegnella oppuntiorum Speg.
3. Carrotgrass (Parthenium Hysterophorus L.):
It is an exotic alien weed which is considered one of the seven most dreaded weeds of the world. It first appeared in India in 1956 at Pune, it has spread alarmingly in almost every part of the country. It is primarily a weed of all possible neglected non-crop areas, including grazing grounds, where it has covered vast areas with devastating effects on health and environment.
Carrotgrass has now also spread to some agricultural lands, fallow fields, and orchards. Parthenium is poisonous and allergic in its effects on humans. The biological control of Parthenium using a Mexican beetle, Zygogramma biclorata, is found very promising.
Large patches of land heavily infested with this menace have been cleared off by the beetle in several parts of the country. Its optimum level of population release is 50-100 beetles/plant of carrotgrass. Management of Parthenium through biocontrol may be considered as the most preferred, cost-effective, and self-sustaining method.
4. Alligatorweed (Alternanthera Philoxeroides):
Alligatorweed is a very prolific, aquatic plant of the tropics and sub- tropics. It has been very effectively controlled with fleabeetle (Agasicles hygrophyla Selman & Vogt) and several bayous have been cleared of this weed there. The fleabeetle larvae feed on the leaves of alligatorweed and finally bore into its bare stems to pupate. Vogtia molloi Pastrana, have proved very effective in controlling the alligatorweed.
5. Waterhyacinth (Eichhornia Crassipes):
Waterhyacinth, a worldwide aquatic weed, infests transplanted paddy fields in many countries, including India. Several attempts have been made on the biological control of this weed; both by pathogens and insects. Its larvae feed upon young leaves and apical buds of waterhyacinth, rather severely. Besides the hyacinthmoth, two beetles, viz., Neochetina eichhorniae Warner, and N. bruchii Atustache, are also damaging to waterhyacinth, but these are not as fast in building their populations as the hyacinthmoth.
6. Salvinia (Salvinia Molesta):
In Kerala, fresh water courses and paddy fields have been cleared of this noxious fern, using curculionid beetle (Cyrtobagous salviniae Sands) as a very effective bioagent. The beetle is native of South America. It is released in waterways by collecting some beetle-infested Salvinia plants from either the beetle rearing ponds or some old infestations, and scattering these on the target, salvinia infestations.
It takes 4-6 months for the salvinia mats to turn yellow, and another six months to sink, completely. The young larvae of the beetle damage the terminal buds, rhizomes, and petioles of salvinia. The biological control of salvinia has made lives of over 5 million people in Kerala more comfortable than before.
Bioherbicide:
Bioherbicides, in variance with the Bioagents, are herbicidal chemicals released by some living organisms. At the present, these organisms comprise largely certain microbes (fungi/ bacteria) because of which the bioherbicides are also termed as Mycoherbicides. The possibility of controlling weeds with exudates of certain higher plants, called allelochemicals.
The mycoherbicides were first promoted by certain corporate in the USA as ‘made to order’ products to control specific noxious weeds. Sometimes, the mycoherbicides are also offered as fermentation products of the relevant microbes. In either case, the farmer is advised to spray the formulation, after making recommended dilutions, on the target weed just like a herbicide, soon after procuring it, to obtain best results.
It may be brought home here that mycoherbicides act like synthetic, contact herbicides, requiring repeat applications to tackle the new flush of the weed, if any. Some proprietary bioherbicide products used in the western world, along with their active microbes and the target weeds. All the released bioherbicides are supposed to be highly host-specific.
The bioherbicide philosophy differs from the classical biocontrol philosophy in certain ways as follows:
1. Bioherbicide remains active only on the current weed populations, without any chance of cyclic perpetuation of the weed (or of the bioagent); each new flush of the weed thus requiring retreatment with it.
2. Bioherbicide can be developed for selective control of weeds in a crop just like any other selective herbicide, which is not the case with the classical philosophy bioagents.
3. The development of bioherbicides is of great interest to industrialists since it involves every season requirement of the product for field use. In variance with it, the classical biological control approach has no incentive to the private, profit-oriented organizations; it must depend solely upon public sector support.
Parasitic weeds, allergenic weeds, crop mimics, and narcotic plants are excellent targets for activating the bioherbicidal approach. Besides, it has always been the goal of scientists to develop products also for broad-spectrum weed control in agriculture, competitive to chemical herbicides. But thus far it has been elusive, save a few individual weeds that have been successfully curbed with bioherbicides in agriculture and horticulture.
One way to increase the virulence of bioherbicides is to increase the spore load in the spray. Another, and probably better, way is to genetically modify the microorganism by genetic engineering to a very high potency level. No doubt the transgenic bioherbicides will have to face serious scrutiny, and may be required to be equipped with some fail-safe mechanism in tandem with the enhancement genes.
Biological Versus Other Methods of Weed Control:
The initial cost of biological weed control appears to be high, but in comparison to the cost of developing a new herbicide, it is quite reasonable. Unfortunately, at the present the biological control of weeds is given a last resort consideration for dealing with some difficult weed problems on grasslands and in non-crop situations where other weed control methods have either failed or were found infeasible.
The last resort approach to biological control methods has been largely due to fear of man about the introduced bioagent becoming a pest on some economic plant which was not included in the starvation tests. Also, an equally important reason for it is the conflicting interests of man in the target plant. A plant may be a serious weed in one area, but it may be of high value as an animal feed, soil binder, or as a fence plant in the neighbourhood.
Prickly pear (Opuntia spp.), for example, is a serious weed of rangelands in many areas and it can be controlled, as though by magic, by Cactoblastis cactorum. But in the neighbouring areas, where this plant is used either as fence or as a forage plant, the people may object to the release of the insect for its control. Whereas physical and chemical methods of weed control can be limited to the terrain to be cleared of a given weed, the insect and other bioagents can move the non-target lands and destroy the host plant there.
For this reason, in Hawaii, Cactoblastis is not allowed to be released for the control of Opuntia. Likewise in Australia, lantana is not allowed to be controlled by biological methods in her rangelands where it is a noxious weed, just because in banana plantations it forms a useful cover plant.
Despite such limitations of biological control of weeds, with improvements in technology and the relative costs of weed damage and value of the benefits from the control of weeds, it is now considered a folly to treat biological control of weeds as the last resort method. In fact, because of its eco-friendly nature, the biological control of weeds is bound to get new impetus, particularly when there is growing concern about the pollution hazards associated with, at least, certain pesticides.