Biological control – The struggle for sustainable options

Potentially biopesticides offer a sustainable solution to agricultural systems. Multinational corporations made initial investments, but in recent years have been lured by the biotechnological fixes such as genetic modification. Here Jeff Waage explains why this is so—and in the following article offers a possible way forward for biopesticides.

Towards the end of the last century, scientists began to experiment with the production and release of natural enemies for insect control. In 1878, Metchnikoff experimented with producing and releasing the green muscardine fungus Metarhizium he had seen killing beetles in cereals(1). Interest grew in the 20th century, for instance with studies on the production and release of predators and parasitoids, such as the moth egg parasite, Trichogramma species. Population ecologists began to interact with biological control specialists to explore the conditions under which natural enemies might be expected to control pest populations by themselves or require augmentation.
    Not surprisingly, the use of natural enemies as biological control agents was set back by the growing use of synthetic organic chemical pesticides in the 1950s. A few decades later, however, it re-emerged. This rebirth of interest was driven by a new need to find replacements for chemical pesticides in integrated pest management (IPM) systems.
    Priority was therefore given to biological agents that could be produced and used like chemicals. Bacillus thuringensis (Bt) is a good example. While it is a living organism, which can reproduce in insects, Bt’s development as a product has been focused on its toxic protein crystal, which acts much like a pesticide. Bt is not naturally adapted to crop habitats, and it does not appear to persist there, and therefore is not likely to cause disease outbreaks (epizootics) like some other insect-killing (entomopathogenic) bacteria, fungi and viruses. Biological control technologies, built in this manner on a ‘chemical paradigm’, while promising, do not even approach the potential seen for biological control by scientists and ecologists earlier this century.
    The effectiveness of a biopesticide depends on two factors: its capacity to kill pests and its capacity to reproduce on pests and therefore continue to compound its killing action; in ecological terms, these are called functional and numerical responses(2). To date, biopesticide development has tended to concentrate only on the direct killing action of insect pathogens. Increasingly, however, the value in biological control of making use of a pathogen’s power of reproduction is being realised(3). This involves moving from regular application of the biopesticide to strategic application early in the crop season, to establish populations that recycle over the whole season and check the growth of invading pest populations. This strategy greatly extends the utility of biopesticides and can be much cheaper for farmers.

Biopesticides in IPM
The development of biopesticides and mass released predators and parasites has meant a lot to IPM. Not only have these agents done the job of controlling pest populations, but unlike many insecticides, they have allowed local natural enemy populations to recover and to have an impact as well. In fact, in many instances where biological agents have been mass-produced and released, it is not clear which of these two factors has had the greater benefit. It may be that, in many crop systems, released biological agents act more as ‘environmental remediation products’ than as ‘pest control products’. Some IPM specialists point out that, independent of their true value in the field, the mere existence of biological agents for crop protection helps governments to implement pesticide-reduction and IPM policies, because they are not seen to be leaving farmers without tools for pest control.
    The role that biological agents play in IPM is well illustrated by the use of Bt against the diamondback moth (DBM), Plutella xylostella, in Asian highlands. Chemical control of this pest of cabbages and related crops has led to pesticide treadmills in many areas. Because the pest is alien to tropical highlands, IPM systems today are based on the introduction of several parasitic wasps from its area of origin, particularly Diadegma semiclausum. Bt is another important part of the system, particularly in the transition from chemical dependence to IPM, because, unlike most chemicals, it does not kill the adult wasp.
    However, while these biological elements of IPM have been widely distributed in Asia, IPM has been slow to start because farmers lack an understanding the biology of these agents and the effect of pesticides on the wasps. A Farmer Field School approach has proven valuable in countries like Indonesia, Philippines and Vietnam to help farmers understand why biological control will be effective only when chemical control is reduced and Diadegma wasps are conserved, and how Bt helps this to happen(4).
    The risk that some IPM specialists perceive is that agents like Bt, if not introduced in conjunction with farmer training or skilled extension, will simply be used like chemical pesticides. Regular spraying of Bt may be unnecessary, uneconomical and may lead quickly to resistance.

Expanding access and use
The growth of interest in IPM is creating a demand for biological control technologies worldwide. 
    Some years ago, an expectant world looked to the agrochemical industry to develop these technologies for mass distribution. Multinational corporations made substantial exploratory investments in bacteria, fungi, viruses, nematodes, parasites and predators, but in the past few years, these have largely been abandoned. Reasons include poor competition with chemical products; small, difficult markets; and the lure of biotechnology, such as genetic modification, as an alternative investment.

How to realise the promise
To begin this process, we will need to make use of what biological agents are available, like Bt. But we will also need to look towards future biological control technologies which do not model products on chemical pesticides but exploit that feature of biological agents that makes them inherently superior to chemicals, their capacity to reproduce and spread themselves. Some systems already do this, for instance, parasites and predatory mites released in Europe to build up in glasshouses over a season; viruses in Brazil which can control several pest generations; and a new fungal product in Africa which may cycle on its target locust and grasshopper populations(5). Not only new products, but new approaches to use, registration, marketing, end-user training and distribution will be required to create a dependable supply of high quality biological control technologies as components of sustainable IPM. The promise of biological control discovered earlier this century is still there to be realized.

References
1. P. DeBach, Biological control by natural enemies, Cambridge University Press, 1979, 323pp.
2. M.B. Thomas and J.K. Waage, Integrating biological control and host plant resistance breeding–a scientific and literature review, Technical Centre for Agricultural and Rural Cooperation of the European Union (CTA), Wageningen, Netherlands, 1996, 99pp.
3. J.K. Waage, Biological control in the year 2000. In: Pest Management and the Environment in 2000 (eds. Abdul Aziz, A.S. Kadir and H.S. Barlow) CAB International, Wallingford, 1992, p.329-340.
4. S. Williamson, Understanding natural enemies; a review of training and information in the practical use of biological control, Biocontrol News and Information, 1998, 19(4) N117–N126.
5. Green Muscle registered, Africa newsletter January 1999, CABI Africa Regional Centre, Nairobi, 1999, p4.

Jeff Waage is Director of Biological Pest Management, CABI Bioscience UK Centre, Sil-wood Park, Buckhurst Road, Ascot, Berks. SL5 7TA, UK, tel. +44 (0)1344 872999, fax +44 (0)1491 829123, bioscience-ascot@cabi.org, www.cabi.org

[This article first appeared in Pesticides News No.45, September 2000, page 9]