The field of invertebrate immunity and pathology goes back to the beginning of the 1900s. In Big Fleas 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 onchocerciasis and alleviated the suffering of many more. These products continue to be the foundation of antimosquito and anti–black fly programs throughout the world.
Bacteria were not the only microorganisms being discovered for use in pest control during the 20th century. David-son 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 insects 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 cholera-free 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 diseases 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 Little Fleas is not an in-depth text for researchers on biochemical methodologies, large-scale production processes, or the biochemistry of pest-control 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.