2. Underlying principles and procedures for biological weed control

#### 2.1. Underlying principles

environmental damage [5]. Also according to [6] weeds disrupt the ecology and the functioning of rangeland plant communities and decrease the quality of services and commodities obtainable from this diverse and important natural resource. In the developing countries, weeding accounts for up to 60% of the total pre-harvest labor input and this is usually by use of simple hand tools [7]. Weeds are generally defined as plants growing where they are not wanted. Popular methods of weed control such as mechanical and chemical are known to be: expensive, energy and labor intensive, require repeated applications, and are unsuitable for managing wide spread plant invasions in ecologically fragile conservation areas or low-value habitats, such as range lands and many aquatic systems. Also mechanical methods cause soil disturbance that may eventually lead to erosion; chemical herbicides cause environmental pollution that pose dangers to human health and wildlife, and certain weed species have developed resistance to some chemical herbicides [1, 8]. Biological approach to weed control dates back from 1795 when Dactylopius ceylonicus was introduced to control drooping prickly pear (Opuntia vulgaris Miller) over a large area of land; and since then biological control of weeds have been mainly through the classical strategy of introducing natural enemies from

Biological control agents usually target their specific natural enemy weeds. Recently due to certain favorable environmental [15], health, economic and sustainability reasons; foreign and native organisms that attack weeds are being evaluated for use as biological control agents that may be used to complement conventional methods especially where some weeds have developed resistance to chemical control. Wheeler et al. [16] reported that their international team discovered and tested numerous new species of potential biological control agents that could attack different plant tissues such as defoliators, sap-suckers, stem borers, and leaf- and stemgall formers. Many successful biological weed control programs in many parts of the world have demonstrated the potency of this approach and support the concept that natural enemies can contribute to the reduction of plant growth and reproduction [17, 18]. Wapshere et al. [19] classified biological approach to weed control as follows: the classical or inoculative method which is based on the introduction of host-specific exotic natural enemies adapted to exotic weeds; the inundative or augmentative method which is based on the mass production and release of native natural enemies usually against native weeds; the conservative method which is based on reducing numbers of native parasites, predators and diseases of native phytophages that feed on native plants; and the broad-spectrum method which is based on the artificial manipulation of the natural enemy population so that the level of attack on the weed is restricted to achieve the desired level of control. According to McFadyen [5] classical method is the predominant method in weed biocontrol. He further explained that classical method involves the introduction and release of agents in form of exotic insects, mites or pathogens to give permanent control, while inundative involves the releases of predators, use of bioherbicides and other integrated pest management which usually are not as widely used as the classical method. Also there are three different techniques for applied biocontrol: (i) conservation—protection or maintenance of existing populations of biocontrol agents; (ii) augmentation—regular action to increase populations of biocontrol agents, either by periodic releases or by environmental manipulation; and (iii) classical biocontrol—the importation and release of exotic biocontrol agents, with the expectation that the agents will become established

areas of co-evolution [9–14].

6 Biological Approaches for Controlling Weeds

and further releases will not be necessary.

The underlying principle behind biological approach to weed control is based on some research works that reported that exotic plants become invasive because they have escaped from the insect herbivores and other natural enemies that limit their multiplication and distribution in their native regions [23–25]; however some other factors may contribute to the tendency for particular plant species to become invasive [26–28]. Therefore biological control involves using specific natural enemies that can diminish the development and reproduction of their prey organism and put some limitations to them [29]. McFadyen [5] stated that the predominant approach to classical biological weed control involves the importation, colonization, and establishment of exotic natural enemies (predators, parasites, and pathogens) to diminish and maintain exotic pest populations to densities that are economically insignificant [30, 31].

#### 2.2. General procedures

Some authors have outlined general procedures to be followed when embarking on classical biological weed control programs as follows: (i) evaluate the ecology, economic impact of the weed and potential conflicts of interest; (ii) survey the organisms that are already attacking the weed in the new habitat in order to distinguish accidentally introduced agents and so eliminate such from future evaluation; (iii) carry out literature search and other forms of survey to identify natural enemies attacking the weed in its native region; (iv) screen the possible biological control agents in the foreign country to determine host range and


when compared to other methods; however certain factors have slowed down the rate of adoption. These factors include: long time of establishment-usually 20 years or more to ensure success, inadequate or no records of the extent of pre-biological control weed infestations that should serve as a guide for a new biocontrol program, discouraging story of poorly implemented weed bio-control programs. A lot of success stories however have been documented [35]. Lack of information about previous successfully implemented biological control of weeds often lead to untested theories becoming established dogma and this negatively influence the decisions to or not apply it [36]. For instance Mcfadyen [35] stated that it was believed that biological control of trees is difficult, but many examples of trees controlled by insects have been reported [37, 38]. Also classical biological control has been viewed as unsuitable for weeds of annual crops or other frequently disturbed environments [39, 40],

Overview of Biological Methods of Weed Control http://dx.doi.org/10.5772/intechopen.76219 9

Some researchers have reported that there are evidences showing that some agents introduced for exotic weed control have attacked non target, native plants [43, 44]; and this situation has raised concerns among biological control workers and weed scientists as well as the governments [5, 43, 45, 46]. Opposition to biological approach to control of weeds has also contributed to slowing down the rate of adoption and practice; this is because some researchers and weed control scientists believe that it is difficult to estimate the cost or the feasibility of biocontrol [47]. Based on a study carried out in South Africa, it was reported that some of the weed biocontrol projects have provided practical solutions to problems e.g. the development of Stumpout for the treatment of wattle stumps and the use of C. gloeosporioides for the control of H. sericea. However other projects have been less successful and have resulted in the rejection of potential agents for various reasons and these include C. albofundus on A. mearnsii, X. campestris on M. aquaticum and G. nitens on R. cuneifolius [48]. Vurro and Evans [49] identified legislative hurdles, technological and commercial constraints as limitations to the adoption of biological weed control in Europe. Olckers [50] stated that limited budgets in many countries have also helped to slow than the rate

4. Examples of successful biological control of weeds with introduced

One thousand one hundred and forty-four individuals (mostly entomologists and plant pathologists) have ever attended the International Symposia on Biological Control of Weeds (ISBCWs); and out of these, 450–550 weed biological control experts have been actively involved in research and development efforts over the last 50 years mainly from USA, Canada, Australia, South Africa and New Zealand [51]. McFadyen [5] reported that biological approach to weed control has a long history and a good success rate of 94. A comprehensive list of agents and their target weeds have been documented by Winston et al. [52]. Culliney [1] presented potential benefits estimated for some proposed or initiated biological control programs targeting invasive weeds. Frequently cited examples of successful approach to biological weed control are the prickly pear cacti (Opuntia; spp.) in Australia, eradicated by an imported moth (Cactoblastis cactorum) and rangeland in California, Oregon, Washington, and British Columbia controlled by St. John's wort Hypericum perforatum (millepertuis perforé) [53].

however there are many examples of successful control of crop weeds [41, 42].

of adoption and practice of biological approach to weed control.

insects and pathogens

Table 1. Summary of steps normally followed when introducing a biological control agent in a classical/inoculative biological control weed program.

specificity, and to remove nonspecific agents from further consideration; (v) carry out further tests of promising candidates in quarantine after introduction to ensure host specificity and eliminate predators, parasites, and pathogens that may have been introduced with them; (vi) embark on mass rearing of host-specific agents; (vii) release the host-specific agents; (viii) carry out post-release evaluation to determine establishment and effectiveness of agents; and (ix) redistribute agents to other areas where control is required [5, 32–34]. Wapshere et al. [19] presented a summary of steps normally followed when introducing a biological control agent in a classical biological control weed program as in Table 1.
