December 1998, Volume 19 No. 4
Plant-Microbe Interactions and Biological Control
According to the editors, the aim of this book* is to "...discuss promising strategies and approaches to the development of effective biological controls for plant diseases, based on plant-microbe interactions". From this and the publishers blurb, it would appear that the 19 chapters in the book deal exclusively with biological control of plant disease. However, somewhat lost within the body of the book are two comprehensive and well researched chapters addressing biocontrol of weeds using microorganisms: one, an up-to-date review covering both the classical and bioherbicide approaches; the other, an in-depth introduction to the formulations currently in use or under investigation for bioherbicide production.
The greater part of the book is, indeed, devoted to the actual and potential use of microorganisms (viruses, bacteria, fungi) for plant disease control; consisting of an ill-assorted (eclectic) mixture of highly specialized chapters (e.g. `Control of cucumber mosaic virus using viral satellites'; `Genetic analysis of selected antifungal metabolites produced by Pseudomonas aureofaciens'), and more general review-type chapters (`Biological control of Fusarium wilt'; `Biological control with Trichoderma species'; `Biological control strategies for Sclerotinia diseases'), which should appeal to a much wider audience.
The book also claims to be both a practical reference for a range of scientists, involved directly or indirectly in agriculture, and a standard text for graduate students. It succeeds only partly in this objective since many of the chapters are far too narrow and specialized for the latter readers, and probably also for most agronomists and soil/crop scientists. However, it could serve as a valuable reference source and thus would be an important addition to both university and agriculturally-inclined libraries. Certainly, the price puts it beyond the reach of many of the readers at which it is aimed.
Conservation Biological Control
In one of the classic texts on biological control, DeBach (1964: `Biological Control of Insect Pests & Weeds') defined conservation biological control as environmental modification to protect and enhance natural enemies, and this is the central focus of this new book*. In chapter 1, we are reminded that conservation biological control is probably the oldest form of biological control of insects and the example is cited of the Chinese, in 900 AD, placing nests of the predaceous ant Oecophylla smaragdina in mandarin orange trees to reduce the abundance of foliage insect pests.
A major review of the scattered literature on this subject was way overdue, the last most accessible general work being that of Rabb, Stinner & van den Bosch in 1976 (Conservation and augmentation of natural enemies, pp. 233-254 in `Theory and practice of biological control' edited by Huffaker & Messenger). Nonetheless, Barbosa has successfully co-ordinated an international group of authors to provide a comprehensive coverage of a wide selection of topics from the ecological basis of the subject, and its practical application, through to the constraints on uptake and the problems of compatibility with the economics of cropping systems. There is, however, a lot of interesting science in this book and this reviewer thinks that the content of many of the well referenced chapters will be of interest to ecologists as well as biological control specialists. A whirlwind tour of some of the major sections of the book will provide a flavour of what to expect.
Chapter 1 sets the scene with the historical background to the subject. However, it moves rapidly on to highlight one of the major dilemmas with this field of applied science: how to get policy makers and farmers to adopt some of the principles which have emerged from research. As it stands, conservation biological control has a lot to offer the agricultural sectors of countries which are now challenged to reduce pesticide usage and adopt integrated pest management. However, all too frequently, the basic principles of conservation run counter to production practices. Also, conservation biological control is facing competition from other biologically based management tactics (including genetically engineered crops) which are, or maybe, less demanding for farmers to implement.
The parallel of conservation biological control with conservation biology is the subject of the next chapter, with a discussion of important concepts such as island biogeographic theory, species richness, meta populations and keystone species. Nonetheless, whilst these subjects have much to tell us about the scientific basis of conservation, the author concludes: "political forces, legal proceedings, policy decisions and economic pressures often dictate the level of success or failure in a management plan for preservation or recovery of species and their habitats". Thus, again, we are reminded that it is policy that matters in the end.
Chapters 3-6 focus on the ecological basis of conservation biological control. Chapter 3 draws attention to the fact that a limited number of pest species are dominant in agroecosystems and then goes on to discuss the implications for the structure and composition of natural enemy communities. An important practical message is here for those sceptics who think that conservation biological control is too complicated to implement: "a relatively narrow suite of pests reduces the number of natural enemies that need to be targeted for conservation and may facilitate the use of a small number of effective conservation tactics that are also cost-effective". The many ways in which plants (plant patch structure, diversity and single plants) can influence the searching behaviour and population dynamics of parasitoids and invertebrate predators are reviewed in remaining chapters of this section. Whilst many of the case studies cited are, often by necessity, reductionist in approach, the authors strongly emphasize that plant factors will not act independently and that single plant traits cannot be easily `engineered' into a tactic for conservation biological control.
The theme of chapters 7-9 is the control or manipulation of the size and distribution of crop patches and landscape features. Chapter 7 emphasizes the need for a better understanding of the ecology of natural enemies outside of agroecosystems for their effective conservation, and chapter 8 shows how artificially sown weed strips can provide essential resources for parasitoids and invertebrate predators. After revisiting some ecological theory (particularly the diversity-stability and enemies hypotheses for pest outbreaks), chapter 9 successfully shows how habitat manipulation research may be undertaken and translated into practical guidance to farmers. This reviewer found this section particularly stimulating as the authors address the major problem of the gap between research and implementation in conservation biological control.
Chapter 10 examines the important subject of the influence of genetically engineered crops, particularly pest resistant Bt-expressing cultivars, on the interaction of existing biological control agents with pests. In general terms, the authors conclude that conservation biological control will benefit from the use of genetically modified cultivars because less insecticide will be used on the crops. This subject is, however, not clear cut and at best controversial [e.g. see BNI 19(2), 38N-41N]. Chapter 11 discusses the compatibility of pesticides with natural enemies. The use of pesticides is likely to continue for the foreseeable future. Thus, with increasing worldwide concerns about the environmental impact of these chemicals there is now pressure to develop protocols that realistically predict their effects on non-target beneficial organisms.
The remaining chapters of this book (12-20) are devoted to reviews of either particular types of pests (mobile insect pests, chapter 12; weeds, chapter 20) or taxonomic groups of natural enemies. The latter reviews bring the literature together on a wide range of organisms in particular farming systems - entomopathogenic nematodes, entomopathogenic fungi, ants, coccinellids, spider mites and microbial antagonists.