Insect Migration: Tracking Resources Through Space and Time. – book reviews
David L. Gibo
One measure of the robustness of a field of research is its appeal across a wide range of disciplines. By this measure, the field of insect migration is in very good health. There is information in Insect migration likely to interest experts in aerodynamics, agriculture, animal behavior, biogeography, climatology, ecology, entomology, forestry, genetics, meteorology, physiology, and many other disciplines. The book is a collection of papers presented at a symposium on insect migration held during the 1992 XIX International Congress of Entomology (ICE) in Beijing. The symposium focused on three areas in which significant progress had been made during the 1980s – the relation of insect migration to weather and climate, physiological adaptations of insects to migration, and forecasting the appearance of migrant pest species. Insect migration is logically organized, well written, and contains generally clear illustrations, informative tables, and a good index. However, most of the material deals with pest species. Well known non-pest migrants, such as the monarch butterfly, Danaus plexippus, and the large milkweed bug, Oncopeltus fasciatus, are discussed in depth in only a few chapters. I noted just a single brief reference to migratory dragonflies. Researchers in insect migration are presented with numerous taxa that have independently evolved an array of specialized physiological, morphological, and behavioral adaptations in response to a wide variety of environmental challenges. Therefore, it is hardly surprising that limited resources are focused on the subset of economically important migrant species. This causes a problem because most migrant pest species attack crops that have had their genetics, morphology, life cycles. habitats, and distributions (many are introduced) greatly modified due to human activity. It is not apparent that findings from studies on selected pest species can be easily extrapolated to the much greater numbers of migratory non-pest species. That being said, Insect migration is an important book that relates the current state of our knowledge in a rapidly developing field of research.
The first section of the book covers insect migration in relation to weather and climate. The first two chapters are excellent overviews of this topic for Europe, Africa, and North America. The authors also introduce what have proved to be the three most intractable questions presented by most migrant species of insects: (1) Do the migrants have a significant degree of control over their direction of displacement, (2) is there an eventual return of the migrants or the descendants to the source area, and (3) is the observed migration adaptive? Our uncertainty of the answers to these three basic questions for most migrant species is a theme that runs through the rest of the book. Possible biases caused by focusing on pest species also crop up early. In Chapter 1, D. E. Pedgley, D. R. Reynolds, and G. M. Tatchell propose that the simplest hypothesis accounting for long-distance wind-borne migration is that flight behavior is not affected by wind direction as such, although variation in air temperature with wind direction may influence take off and flight duration. However, earlier in the chapter the authors state that they have generally excluded two groups from consideration: (1) large day-flying insects that appear to have directional migrations, exerting some control of their direction of displacement by flying largely within their flight boundary layer, or at least keeping within it when winds aloft prevent progress towards a preferred heading, and (2) northern temperate species that orientate approximately northwards in spring and summer and southwards in autumn. Species of butterflies and dragonflies are among the best studied examples of these two groups, inclusion of which would probably have provided a series of counter examples for their hypothesis. Nevertheless, a test of the null hypothesis that flight behavior is not affected by wind direction (i.e., the migrants exhibit no obvious control over their direction of displacement) is exceedingly important and should be one of the first things attempted in a field research program.
Chapters in the second section of the book deal with adaptations to migration and contain most of the references to non-pest species. Since non-pest migrants are usually native species that have evolved to exploit specific native plants, it is not surprising that several chapters in this section emphasize the differences among migratory species rather than perceived similarities. Gatehouse and X.-X. Zhang show in Chapter 10 that few generalities apply in insect migration with respect to flight behavior and the relationship between migration and reproduction. J. N. McNeil, M. Cusson, J. Delisle, I. Orchard, and S. S. Tobe conclude in Chapter 13 that there is evidently no universal scenario for physiological coordination of migratory flight behavior in Lepidoptera. They also question whether flights on flight mills actually reflect migratory propensity. Finally, in Chapter 14, R. Dudley decries the absence of data on the flight characteristics of migrating insects in free flight and questions the relevance of generalized models of flapping flight.
The third section of the book explores the current state of our ability to forecast insect migration. Because the techniques described often depend upon developments in database systems, computer modeling, development and deployment of remote sensing technology, and sharing information among different governments, the timeliness of the material is most likely to have been compromised by the three-year delay in publishing the symposium papers. In any case, all five chapters in the third section emphasize the limits of the current state of forecasts in providing helpful information to interested parties. Since forecasting pest outbreaks amounts to applied ecology, it is not surprising to learn that the most useful forecasts, those that predict outbreaks in the long-term and with high resolution (e.g., in this particular region, three months from now), are most likely to be inaccurate. Furthermore, in section four, Chapter 20, J. I. Magor states in an overview of the system for forecasting outbreaks of the desert locus (Schistocerca gregaria) “No satisfactory method of testing pest forecasts has been developed to date.” This is hardly encouraging since S. gregaria is among the most studied migrants in the world.
In the final chapter of the book, V. A. Drake, A. G. Gatehouse, and R. A. Farrow list 12 important insights about insect migration that have emerged over the last few decades and attempt to bring everything together in a holistic conceptual model. Given that one of the unifying themes of the book is our ignorance of many important aspects of insect migration, that a number of contributors have argued that few generalities seem to apply, particularly when non-pest species are considered, attempting to fashion a single, encompassing model of the phenomenon may be a bit premature. But I could be wrong. In any case, I highly recommend Insect migration because of its wealth of information, numerous case studies, theoretical models, and the intellectual stimulation resulting from many, and occasionally contradictory, views expressed by different contributors.
DAVID L. GIBO ERINDALE COLLEGE, UNIVERSITY OF TORONTO Zoology Department 3359 Mississauga Road Mississauga, Ontario, Canada L5L 1C6
COPYRIGHT 1996 Ecological Society of America
COPYRIGHT 2004 Gale Group
