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Exploiting Invertebrate Intimacy

Posted on: Wednesday, 4 June 2008, 03:00 CDT

By Lowenberger, Carl

EXPLOITING INVERTEBRATE INTIMACY Big Fleas Have little Fleas: How Discoveries of
Invertebrate Diseases Are Advancing Modern Science. Elizabeth W. Davidson.
University of Arizona Press, Tucson, 2006. 208 pp., illus. $17.95 (ISBN
9780816525447 paper). The field of invertebrate immunity and pathology goes back
to the beginning of the 1900s. In Big Fkas Have Little Fleas: How Discoveries of
Invertebrate Diseases A Are Advancing Modern Science, Elizabeth W. Davidson
approaches the discovery of invertebrate pathogens and their effects on pest
invertebrate hosts from the perspective of "usefulness," giving an account of
how we have used and exploited these pathogenic microorganisms for our own
purposes.

Davidson, a professor at Arizona State University, begins the book with a
description of silkworm diseases, moving into the development and understanding
of "germ theory" to explain infectious disease, a major concept in its own
right. She then discusses the discovery of Bacillus thuringiensis (Bt) as a
lethal agent for lepidopteran larvae (caterpillars), and the effort to develop
this bacterium as a biological control agent. Bacillus thuringiensis is no
longer merely an odd microbe; its genes have been copied and cloned into other
organisms to create transgenic plants resistant to feeding by lepidopteran
pests, and its use in agriculture is massive.

Although Bt toxicity is largely limited to the Lepidoptera, Davidson describes a
related bacterium, Bacillus thuringiensis israeliensis (Bti), that is toxic to
dipteran larvae. Bti, and to a lesser extent Bacillus sphaericus, became the
major microbial insecticides used to reduce populations of mosquito and black
fly larvae, an intervention that has undoubtedly saved the lives of millions of
people who would have contracted malaria and onchocerdasis and alleviated the
suffering of many more. These products continue to be the foundation of
antimosquito and antiblack fly programs throughout the world.

Bacteria were not the only microorganisms being discovered for use in pest
control during the 20th century. Davidson explores some of the initial studies
on the identification and use of viruses to control caterpillars that cause
significant damage to crops and forests. Beetles (Coleoptera) also cause
substantial problems in certain crops. To combat beetle pests of palms,
scientists identified, characterized, and commercialized viruses lethal to
beetles to reduce tree damage to acceptable levels. One virus, first isolated
from the rhinocerous beetle of the genus Oryctes, was transported around the
world for the control of various related pest species. It is sobering to note
that relatively few regulations existed at the time to control or regulate the
movement of these novel biological control agents, and it might be considered
fortunate that beneficial species were apparently not significantly affected.

Insects and other invertebrates have, however, evolved mechanisms to protect
themselves against invading pathogens. Davidson helpfully describes the
preliminary studies of Metchnikoff on phagocytosis in starfish, immunology in
general, and the concept of vaccines, as well as the understanding of innate
immunity in both vertebrates and invertebrates. This includes the expression of
antibiotic proteins first identified in in sects in the 1980s, which has spawned
a whole field of studies into innate immunity, the recognition of nonself, and
the evolution of immune responses in invertebrates and vertebrates alike. The
initial studies of cecropins from Lepidoptera and lysozymes from Diptera have
led to similar studies in all classes of organisms.

Humans have played a part in the evolutionary stories by moving many pests
around the world, often necessitating measures to control those same organisms
in their new environments. These pests include insects, their control agents,
and organisms that cause illness in cultivated and wild shellfish. Moreover,
humans are responsible for transporting human cholera to previously cholerafree
regions of the world.

In some cases, pathogens of invertebrates have teamed up to become more
efficient killers. Such is the case of a family of nematodes that infects
insects. When the nematodes penetrate an insect, they release compounds to
inactivate the immune response of the insect, in the process releasing some
specific bacteria they have transported with them. The bacteria proliferate once
the innate immune response of the insect is knocked down, and the nematode and
bacteria reproduce to huge numbers at the insect's expense. When the resources
are exhausted, the remaining nematodes feed on some of the bacteria, then
millions leave the corpse to look for new potential hosts. This association of
nematode and bacteria has been exploited and used in biological control programs
to reduce agricultural damage caused by lepidopteran and coleopteran pests to
acceptable levels. The biotechnology industry is exploiting the compounds
produced by nematodes and bacteria to develop new drugs, all of them possible
because of a very successful symbiotic association between two lowly organisms.

Some of the chapters describe the use of bacterial pathogens to reduce pest
insect populations that cause losses in human agriculture. It should be noted,
however, that beneficial insects such as honeybees have bacterial and viral dis
eases of their own, which may result in hive death and reductions in
agricultural production. Despite hives' innate immune defenses, the loss of
colonies or apiaries to bacteria, viruses, or mites is often severe. We still
have not found measures to protect hives from these pathogens, and we cannot
expect insects we consider beneficial to be exempt from having them.

Fungal diseases of invertebrates also abound, some of which have been developed
and exploited to control pests such as the gypsy moth or locusts. Scientists'
keen observations of sick insects led to the discovery of these compounds, which
are now sprayed from airplanes to control outbreaks of these pest insects. The
associations described above are symbiotic, and we have exploited these close
linkages to achieve biological control of a problem species. Often, though,
humans created the problem in the first place through extensive monocultures or
by moving organisms to regions where natural controls are in short supply.

Davidson has done a good job of describing the historical context in which these
pathogens have been developed as controls. She also portrays the individuals
from a range of countries who have worked together in their own symbiotic
relationships to solve the problems. It should be noted that many activities of
scientists, such as sending live biological material from laboratory to
laboratory and continent to continent, are not as simple today as they may have
been only a few decades ago.

Big Fleas Have Unk Fleas is not an indepth text for researchers on biochemical
methodologies, large-scale production processes, or the biochemistry of
pestcontrol agent interactions. There are many more-specialized texts for their
purposes. What Davidson has produced is an overview and a recent history of the
development of the "little fleas," or the microbial pathogens, that we have
exploited to control the big fleas-the pests that cause us no end of grief,
either directly (mosquitoes that transmit parasites) or indirectly (pests that
reduce our food supply). As such, this book is valuable for the novice entering
the field-it puts some initial discoveries and concepts in perspective. This
text also gives the research scientist or agronomist who uses these products
regularly a historical context. It describes the chain of events that has
allowed us to identify, produce, and exploit specific aspects of these lowly
microbes to develop effective microbial agents to control pest populations.
There will always be big fleas. This text suggests there will also always be
more little fleas that can be exploited for our purposes.

CARL LOWENBERGER

Carl Lowenberger (e-mail:

clowenbe@...) works in the biology

department at Simon Fraser University

in Burnaby, British Columbia.

doi:10.1641/B580314

Include this information when citing this material.

Copyright American Institute of Biological Sciences Mar 2008

(c) 2008 Bioscience. Provided by ProQuest Information and Learning. All rights
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