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BOOK REVIEW
Drake, V. A., and A. G. Gatehouse (eds.). 1995. Insect
Migration: Tracking Resources Through Space and Time. Cambridge
University Press, Cambridge. xvii + 478 p. ISBN 0-521-44000-9.
Hardback. $74.95.
Migration is a fundamental component of many insect
life cycles, and multidisciplinary studies have begun to clarify
some major deficiencies in our understanding of it. Two recent
books succeed in summarizing these studies and fitting them into
patterns that are roughly complete or at least have the missing
pieces identified. One is by Hugh Dingle and deals with all of
animal migration (Migration: the Biology of Life on the Move,
Oxford Univ. Press, New York, 1996, 474 p.). The other, the subject
of this review, has 41 authors and deals only with insects.
Insect Migration stems from a symposium at the XIX
International Congress of Entomology in Beijing (1992). The coherence
of its 21 chapters and the currentness of its references demonstrate
the editors’ success in persuading the symposium participants
to modify and update their contributions. The book has a slight
geographical bias toward eastern Asia and Australia meant to counterbalance
the biases toward Africa and North America of previous reviews.
(Little is reported about insect migration in South America, but
little is known.) The species discussed usually migrate above
their flight boundary layer, where wind direction is the principal
determinant of direction of movement. Weak fliers have little
choice but to use the wind for transport; strong fliers, such
as Monarch butterflies and desert locusts, can progress in any
direction near the ground or they can take advantage of favorable
winds at higher altitudes and travel fast at low cost.
Many migratory species are most damaging in geographic
areas they cannot occupy continuously because temperature or moisture
is only seasonally or erratically favorable. One theme of the
book is the evolutionary dilemma posed by insects that move into
temporarily favorable habitats from which neither they nor their
descendants are known to return (the so-called “pied piper”
effect, proposed by Rabb & Stinner in 1979). Such one-way
migration can be viewed as an inadvertent consequence of the insects
using uncertain winds as a transport system, or it can be viewed
as a consequence of our knowing too little about the insects’
movements. Indeed, in a few cases further study has revealed return
movements in what were thought to be pied piper species. In other
cases, a nomadic life style may be a means of continually exploiting
habitats of erratic suitability.
Insect Migration has three main groups of chapters.
The first and largest group describes insect migration in relation
to weather and climate, with one or more chapters on Africa and
Europe, North America, eastern Asia, and Australia. In West Africa
seasonal reversal of windborne migration has been demonstrated
for bugs, moths, and flies as well as several species of grasshoppers.
Much migration in Africa is more complex and involves seasonally
changing wind patterns in area of shifting habitat suitability.
The complexity of windborne migration is well illustrated by studies
of desert locust (Schistocerca gregaria) and African armyworm
(Spodoptera exempta). Their movements cannot be simply summarized,
and there is no easily grasped return of migrant genes from their
source areas. Some migrations are fatal, with insects carried
into deserts, poleward in autumn, and out to sea. Yet staying
put does not insure survival either.
Eastern North America is much more favorable for
regular to-and-fro seasonal migration than is Africa and Europe,
there being no Sahara or Mediterranean. Spring poleward migrations
are on northward, fast-moving, low-level jets at altitudes of
200 to 1000 m and take 2 to 3 nights. Autumn equatorward migrations
are nearer the surface (100-300m) on slower southward airflows
and are limited by how rapidly temperatures fall behind the front.
Passage of successive fronts may be required for migrants to reach
overwintering areas. Circumstantial evidence supports a north-in-spring,
south-in-fall migration for many pest species. Such migrations
have been confirmed experimentally for potato leafhopper (Empoasca
fabae) and black cutworm (Agrotis ipsilon).
Two planthoppers in eastern Asia migrate northward
and northeastward in spring and summer attacking rice in areas
where they cannot overwinter. Some migration to the southwestward
in autumn has been detected, but whether these are significant
contributors to the gene pool of individuals that move northeastward
the following year is uncertain. Three chapters deal with migrations
of the oriental armyworm (Mythimna separata) in northeastern China,
Korea, and Japan. In China there seems to be enough return migration
to make the northward migrations adaptive. Whether the outbreaks
in Korea and northern Japan are an adaptation is not addressed.
Three chapters deal with important Australian migrants:
common armyworm (Mythimna convecta), native budworm (Helicoverpa
punctigera) and Australian plague locust (Chortoicetes terminifera).
In each case adaptive return migration is possible but unproved.
Especially in the case of the native budworm, which is adapted
to the ephemeral environments of the arid inland, migration may
be the only way to maintain a permanent, though nomadic, population.
A second group of chapters concerns adaptations for
migration. A. G. Gatehouse and X. -X. Zhang review the literature
on potential for migration, including when migration occurs relative
to reproduction, reproductive status of migrants sampled during
flight, duration of pre-reproductive period, and free or tethered
flight performance. They conclude that not much is known but that
females of some species vary extensively in what it takes to initiate
facultative migration and in migratory potential itself. K. Wilson
outlines types of temporal and spatial variation in habitat and
their expected effect on the evolution and maintenance of migratory
potential. He reports that many studies suggest adaptive variation
in migratory potential, but that few provide a convincing association
with habitat heterogeneity and these mostly with temporal heterogeneity.
J. Colvin describes laboratory studies of the genetics and ontogeny
of two correlates of migratory potential in the cotton bollworm
(Helicoverpa armigera): duration of tethered flight and of the
pre-reproductive period. J. N. McNeil et al. discuss the physiological
integration of migration in Lepidoptera-specifically, the roles
of juvenile hormone titer and the accumulation and mobilization
of lipids. R. Dudley, in a chapter dealing with the aerodynamics
and energetics of migratory flight, notes that data on free flying
migrants are few and describes how a motorboat can be used to
collect such data on butterflies and day-flying moths as they
migrate across large lakes.
A third group of chapters treats forecasts of migrant
pests. Two chapters in this group concern general problems and
techniques of forecasting, specifically operational aspects and
the use of computer-based geographic information systems. Other
chapters deal with specific forecasting systems-for brown planthopper
in China, rice planthoppers in Japan, locusts and grasshoppers
in West Africa and Madagascar, and desert locust throughout its
invasive area.
The book concludes with a “holistic conceptual
model” of insect migration by V. A. Drake, A. G. Gatehouse,
and R. A. Farrow. They identify these four components of a migration
system and list for each the processes that are involved: migration
arena (the space, geographical and vertical, within which a population’s
migration takes place), population trajectory (space/time population
demography that results from migration), migration syndrome (the
physiological, morphological, and behavioral traits that implement
migration and determine the fitness of the migrants), and genetic
complex (the genes that underlie the migration syndrome and their
interactions and modes of inheritance). In this manner they succeed
in describing, classifying, and illustrating the complexities
of insect migration. Their conceptual model becomes an organizing
framework for migration studies that accommodates what is known
and reveals what is not.
In words borrowed from the title, this book tracks
studies of insect migration through space (worldwide) and time
(the past 25 years). It summarizes and integrates the results
of these studies and cites the important primary literature. Insect
Migration belongs on the bookshelf of those interested in insect
ecology and life histories as well as those who must propose control
strategies for migratory insect pests.
Thomas J. Walker
Department of Entomology and Nematology
University of Florida
Gainesville, Florida 32611-0620