Anantanarayanan Raman, Carl W. Schaefer and Toni M. Withers, eds.
Science Publishers
Enfield, NH
2005, 817 pp.
$148.00
ISBN: 1-57808-262-5

Of all plant-herbivore interactions, perhaps the most complex and poorly understood are those involving arthropods that induce galls. A surprising large number of arthropod species induce galls on plant species spanning the phylogenetic range from cycads to asters, and researchers have studied these gall systems to address diverse issues ranging from plant physiology to evolutionary ecology and systematics. Given the diversity of gall-inducing arthropods, their host plants, and research addressing these species, it may be difficult to keep current with research being done with gall systems. Fortunately for gall enthusiasts, every decade or so a new edited volume is published offering summaries of the most recent findings. The most recent addition is Biology, Ecology, and Evolution of Gall-inducing Arthropods. Vols. 1 and 2.

This two-volume set is largely devoted to insects, with mites representing the only noninsect arthropods known to induce galls. Nevertheless, the volumes systematically address all known taxa of gall-inducing arthropods, with 19 chapters dedicated to gall-inducing Acari, thrips, aphids, whiteflies, psyllids, scale insects, tingids, Coleoptera, cecidomyiids (2 chapters), tephritids, chloropids, fergusoninids, Lepidoptera, sawflies, braconids, chalcidoids, fig-pollinating wasps, and cynipids. An additional chapter addresses dipteran leaf miners and the leaf mining habit as an ancestral trait that may have preceded gall formation in some dipteran lineages. Two chapters are dedicated to patterns of diversity in gall-inducing arthropods: one focuses on the use of gall inducers as weed biological control agents and another deals with dual aseptic culture of gallers and their host plants.

The book begins nicely with the editors providing a quick, but informative sketch of the history of cecidology. They continue with an overview of the field, covering the definition of a gall, characteristics of gall inducers and their host plants, evolution of the gall-inducing habit, and some of the hypotheses proposed to explain how galls are induced. One of the most intriguing mysteries of gall-inducing arthropods is how they force plants to do their bidding and grow the galls that provide them food and shelter. Some hints have emerged and most point to phytohormones manipulation by arthropods, with indoleacetic acid (IAA) playing a central role. The editors briefly summarize some of the data that implicate IAA in gall formation and present two competing hypotheses on the action of IAA.

The chapters addressing the 18 gall-inducing taxa are arranged phylogenetically beginning with mites and finishing with cynipids. Because of unique characteristics of each taxon, chapters are arranged differently, but most begin with an overview of the group, offering comments on biology, diversity of gall inducers within the group, taxonomy, life cycles, geographic distribution, and phylogenetic relationships of gall inducers in the group, as well as their relationships to closely related taxa that do not induce galls. Many chapters also have sections on host plant associations, gall induction and structure, and evolution of gall inducing in the group. Some chapters contain little more than basic information, which is forgivable given how little is known about particular taxa. For example, chapters on gall-inducing whiteflies (by Byrne), tingid bugs (Schaefer), and braconid wasps (Wharton and Hanson) are all short and discuss the limited information on gall-inducing members of these otherwise well-known taxa. The inclusion of chapters on each of these groups clearly shows that the editors were committed to assembling a comprehensive collection, particularly when previous editors/authors did not acknowledge these taxa as including gall formers.

In contrast to short offerings for a few taxa, chapters on the well-studied groups of gall inducers are brimming with information. The chapter on scale insects is an excellent review of this taxon whose gall-inducing members have radiated largely in Australia and may not be familiar to many from other parts of the world. Coleoptera and Lepidoptera are not frequently gall inducers, and this allows authors to address virtually all known gall-inducing species and offer details on their host-plant associations and biology. The chapter on fig-pollinating agaonids is a concise summary of the intricate biology of fig wasps and the community of insects reliant on their galls. The inclusion of this chapter is particularly refreshing because agaonids are often overlooked as gall inducers. The authors of the chapters on the biology and ecology of cecidomyiids and cynipids had the challenge of summarizing the two largest and most diverse gall-inducing groups; however, they capably address diversity, host plant associations, life cycles, and population dynamics among other topics. In addition, each chapter addresses some of the unique characteristics of these groups with large sections dedicated to gall midges that have symbiotic relationships with fungi (i.e., ambrosia galls) and the community ecology of oak gall wasps, which can be exceedingly complex with upward of 20 parasitoids and inquilines relying on one cynipid host species.

The few criticisms I have of the volumes are minor compared with the valuable information they offer; however, it would have been useful to see more images of galls. One of the most interesting aspects of galls is the diversity of gall structure, and it can be constructive to consider the adaptive significance of particular forms or shapes. Readers could have benefited had each chapter contained enough photographs or sketches to thoroughly represent the taxa. Also, the book seems to overlook some recent findings that provided significant insight into gall formation (Doss et al. 2000, Sopow et al. 2003), but perhaps these and similar references fell outside the scope of the chapters on their respective taxa. Furthermore, given the mystery of gall formation, it would have been very useful to have an entire chapter dedicated to how galls are formed that could have summarized recent findings, explored potential mechanisms, and offered insight from a range of taxa.

Nevertheless, this two-volume set is a great reference and portal into the world of galls. It seems to be geared toward gall researchers and specialists on the various taxa addressed, but students of plant-herbivore interactions or evolutionary ecology would have much to gain by reading some of the chapters. Having read the books cover to cover, I gained considerable useful information, but the real fun for me was learning, or being reminded of, cool details of gall biology that continue to stoke my interest in gall inducers, such as galls that secrete honeydew to gain protection from ants or decoy larval chambers formed in some cynipid galls that dupe parasitoids into wasting eggs on empty chambers that do not contain hosts. Similarly revealing was the entire chapter on fergusoninids, a poorly known group of flies, which have allied with mutualistic nematodes to form galls on myrtaceous plants in Australia and southeastern Asia. The flies have species-specific associations with nematodes that seem to induce galls for fly larvae in return for dispersal services provided by adult flies. In the words of the authors, the relationship between fergusoninids and their nematodes may be "… one of the most unusual examples of cospeciation and coevolution" yet discovered. As others read these volumes, I suspect they too will be fascinated by similar details of the biology of gall-inducing arthropods and feel compelled to enter the wonderful world of galls.

References

Doss R. P, Oliver J. E, Proebsting W. M, Potter S. W, Kuy S, Clement S. L, Williamson R. T, Carney J. R, DeVilbiss E. D. Bruchins. 2000.Insect-derived plant regulators that stimulate neoplasm formation. Proc. Nat. Acad. Sci. U.S.A. 2000, 97:6218-6223.

Sopow S. L, Shorthouse J. D, Strong W, Quiring D. T. 2003. Evidence for long-distance, chemical gall induction by an insect. Ecol. Lett. 2003, 6:102-105.

J. F. Tooker
Department of Entomology
Pennsylvania State University
University Park, Pennsylvania

Environmental Entomology
Vol. 36, No. 2, April 2007