Biological Control - Clean Technology

Biological Control

The use of insecticides and herbicides, particularly in the context of agricultural usage, has been responsible for a number of instances of pollution and many of the chemicals implicated are highly persistent in the environment. Though there has been a generalised swing away from high dosage chemicals and a widespread reduction in the use of recalcitrant pesticides, worldwide there remains a huge market for this class of agrochemicals. As a result, this is one of the areas where biotechnological applications may have significant environmental impact, by providing appreciably less damaging methods of pest management. The whole concept of biological control took a severe blow after the widely reported, dis-astrous outcome of Australia’s attempts to use the Cane Toad (Bufo marinus) to control the cane beetle. However, in principle, the idea remains sound and con-siderable research effort has gone into designing biological systems to counter the threat of pests and pathogens. Some of these, in respect of soil-borne plant pathogens and biopesticides, are discussed elsewhere in this work and, accord-ingly, do not warrant lengthy reiteration here.

 The essence of the specifically environmental contribution of this type of bio-intervention lies in its ability to obviate the need for the use of polluting chemicals and, consequently, leads to a significant reduction in the resultant instances of contamination of groundwater or land. However, one of the major limitations on the effective use of biocontrols is that these measures tend to act more slowly than direct chemical attacks and this has often restricted their use on commercial crops. In fairness, it must be clearly stated that biotechnology per se is not a central, or even necessary, requirement for all of biological control, as many methods rely on whole organism predators, which, obviously, has far more bearing on an under-standing of the ecological interactions within the local environment. However, the potential applications of biotechnology to aspects of pest/pathogen/organism dynamics, as examined in other sections of this book, has a supportive role to play in the overall management regime and, thus, there exists an environmental dimension to its general use in this context.

 Biological control methods can provide an effective way to mitigate pesticide use and thus the risk represented to the environment and to public health. In addition, unlike most insecticides, biocontrols are often highly target-specific reducing the danger to other nonpest species. Against this, biological measures typically demand much more intensive management and careful planning than the simple application of chemical agents. Success is much more dependent on a thorough understanding of the life-cycles of the organisms involved and can often be much more of a long-term project. In addition, though high specificity is, generally, a major advantage of biocontrol measures, under some circumstances, if exactly the right measure is not put in place, it may also permit certain pests to continue their harmful activities unabated. Considering the huge preponderance of insect species in the world, a large number of which pose a threat to crops or other commodities and thus represent an economic concern, it is small wonder that the global insecticide market has been estimated at over $8 billion (US) per year. Accordingly, much of the biological control currently in practice relates to this group of animals.

Whole-organism approaches

There are three main ways in which whole-organism biological pest control may be brought about. Classical biological control, as with the previously mentioned Cane Toad, requires the importation of natural predators and is principally of use when the pest in question is newly arrived in an area, often from another region or country, having left these normal biological checks behind. Another form of control involves conservation measures aimed at bolstering the predatory species, which may be a valuable approach when natural enemies already exist within the pest’s range. However, the third method, augmentation, is more relevant to the concepts of biotechnology and refers to means designed to bring about the increase in effectiveness of natural enemies to a given pest. This may consist simply of artificially rearing them in large numbers for timed release or may extend to more intensive and sophisticated measures like the modification, either by selective breeding or genetic manipulation, of the predator such that it is better able to locate or attack the pest.

 One attempt at augmentation which has been tried commercially is the pro-duction of parasitic nematodes. Juvenile stages of the nematodes, which are then only around 500 Âµm long and 20 Âµm wide, can enter soil insects and many carry pathogenic bacteria in their guts. Once ingested, these bacteria pass out of the nematode and multiply inside the insect, typically causing death within a few days. Five species of nematode were made available on the US agricultural mar-ket, namely Steinernema carpocapsae, S. riobravis, S. feltiae, Heterorhabditisbacteriophora and H. megidis, each being effective against different groups of insects. Despite much research and development effort, the results were largely unpredictable, with success against many of the target species, like wireworms and root maggots, proving elusive. One avenue of potential application for this technology may, however, lie in the control of cockroaches, which have been found to be the most vulnerable species to augmented nematode attack (Georgis 1996). However, there still remain some technical problems to overcome in terms of ensuring a level of parasite delivery before widespread uptake is likely. Aug-mentation is, obviously, a highly interventionist approach and relies on a regime of continual management to ensure its effectiveness.

 There is also a role for the engineered application of biologically derived chem-icals in this sector. One example of this is the growing interest in Azadirachtaindica, the neem, a plant which is found naturally in over 50 countries aroundthe world including India, where its medicinal and agricultural value has been known for centuries. The compound azadirachtin has been identified and isolated from the plant and it has been shown to have broad spectrum insecticidal proper-ties, acting to disturb larval moults and preventing metamorphosis to the imago. Additionally, it also seems to repel many leaf-eating species, and trials involving the direct foliar application of azadirachtin has shown it to be an effective way of protecting crop plants (Georgis 1996). This duality of action makes it a par-ticularly appealing prospect for wide-scale applications, if suitable methods for its production can be made commercially viable.

Semiochemical agents

However, perhaps one of the best examples of the use of such biological technolo-gies in pest control is the development of isolated or synthesised semiochemical agents.

 Semiochemicals are natural messenger substances which influence growth, development or behaviour in numerous plant and animal species and include the group known as pheromones, a number of which are responsible for sexual attraction in many insects. This has been successfully applied to control various forms of insect pests, either directly to divert them from crops and trap them, or indirectly to trap their natural enemies in large numbers for introduction into the fields for defence.

 For example, crops worldwide suffer severe damage as a result of a number of pentatomid insects, amongst which are several of the common brown stink bugs of North America (Euschistus spp.). They arrive late in the growing season and often cause major harm before detection. A major part of biocontrol involves obtaining a thorough understanding of their migration patterns and to help achieve it in this case, a pheromone, methyl 2E,4Z-decadienoate, has been produced commercially to aid trapping. The early success of this is being developed to extend its scope in three main directions. Firstly, to capture and eliminate the pests themselves, secondly, to harvest predatory stink bugs for bioaugmentative control programmes and thirdly, to identify more pheromones to widen the number of phytophagous stink bug species which can be countered in this way.

 As something of an aside, one interesting and somewhat unusual use has been proposed for this technology. The Siberian moth Dendrolimus superans is a vigorous defoliating pest of northern Asian coniferous forests and, though it does not presently occur in North America, its arrival is much feared. In an attempt to provide a first line of defence against this potential threat to native woodlands, it has been suggested that a blend of Z5,E7-dodecadienol and Z5,E7-dodecadienol, which has been shown to act as a powerful sex attractant for male Siberian moths, be deployed at US ports of entry.

 However, as illustrated by the case of another pentatomid, Nezara viridula, the southern green stink bug, the use of this approach to biological control is not universally applicable. These insects are major agricultural pests affecting a variety of field crops, vegetables, fruits and nuts. While it has been known for sometime that sexually mature males produce an attractant pheromone, the active ingredients of which have been identified, early attempts to use this knowledge to exclude them from crops have been of only limited effectiveness. As a result, an alternative method of Nezara control has been suggested involving the genetic engineering of its gut symbionts to produce a reduced tolerance of environmental stress. Preliminary work at the Agricultural Research Center, Beltsville, USA has isolated and cultured in vitro a gram-negative bacterium from the mid-gut of the pest insect, which appears to be a specific symbiont and has been putatively iden-tified as a species of Yokenella. This kind of application of transgenic technology may increasingly be the future of biological control for species which do not respond favourably to pheromone trapping.

 Not all approaches to biocontrol truly qualify as environmental biotechnolo-gies, at least not within the frame of reference used in this book. However, where the use of biological systems results in reduced insecticide use and thus a cor-responding lowering of the attendant pollution potential, the net environmental gains of the application of biotechnology are clear.