Annual Review of Entomology - Volume 45, 2000

Certain fly larvae can infest corpses or the wounds of live hosts. Those which are least invasive on live hosts have been used therapeutically, to remove dead tissue from wounds, and promote healing. This medicinal use of maggots is increasing around the world, due to its efficacy, safety and simplicity. Given our low cultural esteem for maggots, the increasing use and popularity of maggot therapy is evidence of its utility. Maggot therapy has successfully treated many types of chronic wounds, but much clinical and basic research is needed still. In this review, the biology of myiasis and the history of maggot therapy are presented, the current status of our understanding and clinical use of medicinal maggots is discussed, and opportunities for future research and applications are proposed.

Studies of the production of stream insects are now numerous, and general factors controlling the secondary production of stream communities are becoming evident. In this review we focus on how life-history attributes influence the production dynamics of stream insects and other macroinvertebrates. Annual production of macroinvertebrate communities in streams world-wide ranges from approximately 10⁰ to 10³ g dry mass m⁻². High levels are reported for communities dominated by filter feeders in temperate streams. Filter feeding enables the accrual and support of high biomass, which drives the very highest production. Frequently disturbed communities in warm-temperate streams are also highly productive. Biomass accrual by macroinvertebrates is limited in these streams, and production is driven by rapid growth rates rather than high biomass. The lowest production, reported for macroinvertebrate communities of cool-temperate and arctic streams, is due to the constraints of low seasonal temperatures and nutrient or food limitation. Geographical bias, paucity of community-wide studies, and limited knowledge of the effects of biotic interactions limit current understanding of mechanisms controlling stream productivity.

Most insect cell membranes seem to contain uniporters that facilitate the diffusion of amino acids into and out of the cells. In addition to these passive diffusion systems, all but one of the insect tissues studied to date seem to contain at least one amino acid–cation symport system that allows their cells to accumulate certain amino acids from the extracellular medium. cDNAs encoding three such symporters have very recently been cloned and sequenced. The deduced amino acid sequence of each insect symporter was determined to be homologous to that of symporters mediating the transport of the same or related substrates in mammalian tissues.

Social wasps (Hymenoptera: Vespidae) forage for water, pulp, carbohydrates, and animal protein. When hunting, social wasps are opportunistic generalists and use a variety of mechanisms to locate and choose prey. Individual foragers are influenced by past foraging experience and by the presence of other foragers on resources. A forager’s ability to learn odors and landmarks, which direct its return to foraging sites, and to associate cues such as odor or leaf damage with resource availability provide the behavioral foundation for facultative specialization by individual foragers. Social wasps, by virtue of their behavior and numbers, have a large impact on other organisms by consuming them directly. Indirect effects such as disruption of prey and resource depletion may also be important. Community-level impacts are particularly apparent when wasps feed upon clumped prey vulnerable to depredation by returning foragers, or when species with large, long-lived colonies are introduced into island communities. A clearer understanding of these relationships may provide insight into impacts of generalist predators on the evolution of their prey.

The blood-brain barrier (BBB) ensures brain function in vertebrates and insects by maintaining ionic integrity of the neuronal bathing fluid. Without this barrier, paralysis and death ensue. The structural analogs of the BBB are occlusive (pleated-sheet) septate and tight junctions between perineurial cells, glia and perineurial cells, and possibly between glia. Immature Diptera have such septate junctions (without tight junctions) while both junctional types are found in the imago. Genetic and molecular biology of these junctions are discussed, namely tight (occludin) and pleated-sheet septate (neurexin IV). A temporal succession of blood barriers form in immature Diptera. The first barrier forms in the peripheral nervous system where pleated-sheet septate junctions bond cells of the nascent (embryonic) chordotonal organs in early neurogenesis. At the end of embryonic life, the central nervous system is fully vested with a blood-brain barrier. A blood-eye barrier arises in early pupal life. Future prospects in blood-barrier research are discussed.

Many agroecosystems are unfavorable environments for natural enemies due to high levels of disturbance. Habitat management, a form of conservation biological control, is an ecologically based approach aimed at favoring natural enemies and enhancing biological control in agricultural systems. The goal of habitat management is to create a suitable ecological infrastructure within the agricultural landscape to provide resources such as food for adult natural enemies, alternative prey or hosts, and shelter from adverse conditions. These resources must be integrated into the landscape in a way that is spatially and temporally favorable to natural enemies and practical for producers to implement. The rapidly expanding literature on habitat management is reviewed with attention to practices for favoring predators and parasitoids, implementation of habitat management, and the contributions of modeling and ecological theory to this developing area of conservation biological control. The potential to integrate the goals of habitat management for natural enemies and nature conservation is discussed.

The antennal lobe of insects has emerged as an excellent model for olfactory processing in the CNS. In the present review we compile data from areas where substantial progress has been made during recent years: structure-function relationships within the glomerular array, integration and blend specificity, time coding and the effects of neuroactive substances and hormones on antennal lobe processing.

Recent advances on the biochemistry of flight-related lipid mobilization, transport, and metabolism are reviewed. The synthesis and release of adipokinetic hormones and their function in activation of fat body triacylglycerol lipase to produce diacylglycerol is discussed. The dynamics of reversible lipoprotein conversions and the structural properties and role of the exchangeable apolipoprotein, apolipophorin III, in this process is presented. The nature and structure of hemolymph lipid transfer particle and the potential role of a recently discovered lipoprotein receptor of the low-density lipoprotein receptor family, in lipophorin metabolism and lipid transport is reviewed.

A number of landmark events in applied entomology are listed together with some insect-related studies that have had a major impact on biology in general. In large part, however, advances in our understanding of insects have depended on technological advances, especially in the second half of the century. The exponential increase in the ease and extent of communication has been critical. Sometimes, as in the field of insect/plant relations, the ideas of a few individuals have been critical with technological advances having a facilitating role. Elsewhere, as in the study of olfaction, major changes in understanding have been directly dependent on new technology. Very brief accounts of the impacts on insect-related science of developments in the fields of radio, radioactivity, immunology, imaging techniques, and chemical analysis are given. Despite the importance of technology, the lovers of their insects continue to have a key role.

Entomopathogenic nematodes are excellent biological control agents. Utilization of these nematodes is developing rapidly with almost a doubling of newly described species in the past five years. Advances in molecular biology and phylogenetic reconstruction have revolutionized understanding of population structure, identification, genetic improvement, systematics, and the symbiosis between entomopathogenic nematodes and their bacteria. Population structure provides the most fundamental information for reliable identification of species and unique genetic variants. Such information could be further assessed for nematode potential as biological control agents. Phylogenetic reconstruction is an important approach for understanding multitrophic interactions among entomopathogenic nematodes, symbiotic bacteria, and their insect hosts. Phylogenetic reconstruction is also important for the development of a natural and stable type of systematics, which can provide guidelines for selecting appropriate entomopathogenic nematode species for particular biological control programs.

Culicoides biting midges are among the most abundant of haematophagous insects, and occur throughout most of the inhabited world. Across this broad range they transmit a great number of assorted pathogens of human, and domestic and wild animals, but it is as vectors of arboviruses, and particularly arboviruses of domestic livestock, that they achieve their prime importance. To date, more than 50 such viruses have been isolated from Culicoides spp. and some of these cause diseases of such international significance that they have been allocated Office International des Épizooties (OIE) List A status. Culicoides are world players in the epidemiology of many important arboviral diseases. In this context this paper deals with those aspects of midge biology facilitating disease transmission, describes the factors controlling insect-virus interactions at the individual insect and population level, and illustrates the far-reaching effects that certain components of climate have upon the midges and, hence, transmission potential.

Most models of optimal progeny size assume that there is a trade-off between progeny size and number, and that progeny fitness increases with increasing investment per young. We find that both assumptions are supported by empirical studies but that the trade-off is less apparent when organisms are iteroparous, use adult-acquired resources for reproduction, or provide parental care. We then review patterns of variation in progeny size among species, among populations within species, among individuals within populations, and among progeny produced by a single female. We argue that much of the variation in progeny size among species, and among populations within species, is likely due to variation in natural selection. However, few studies have manipulated progeny environments and demonstrated that the relationship between progeny size and fitness actually differs among environments, and fewer still have demonstrated why selection favors different sized progeny in different environments. We argue that much of the variation in progeny size among females within populations, and among progeny produced by a single female, is probably nonadaptive. However, some species of arthropods exhibit plasticity in progeny size in response to several environmental factors, and much of this plasticity is likely adaptive. We conclude that advances in theory have substantially outpaced empirical data. We hope that this review will stimulate researchers to examine the specific factors that result in variation in selection on progeny size within and among populations, and how this variation in selection influences the evolution of the patterns we observe.

Insecticide resistance is an increasing problem in many insect vectors of disease. Our knowledge of the basic mechanisms underlying resistance to commonly used insecticides is well established. Molecular techniques have recently allowed us to start and dissect most of these mechanisms at the DNA level. The next major challenge will be to use this molecular understanding of resistance to develop novel strategies with which we can truly manage resistance. State-of-the-art information on resistance in insect vectors of disease is reviewed in this context.

This review examines how molecular markers can be used to increase our understanding of the mechanisms of plant resistance to insects and develop insect resistant crops. We provide a brief description of the types of molecular markers currently being employed, and describe how they can be applied to identify and track genes of interest in a marker-assisted breeding program. A summary of the work reported in this field of study, with examples in which molecular markers have been applied to increase understanding of the mechanistic and biochemical bases of resistance in potato and maize plant/pest systems, is provided. We also describe how molecular markers can be applied to develop more durable insect-resistant crops. Finally, we identify key areas in molecular genetics that we believe will provide exciting and productive research opportunities for those working to develop insectresistant crops.

Oviposition behavior in herbivorous and frugivorous insects and parasitoids is dynamic at the level of the individual, responding to variation in host quality and availability. Patterns of variation in egg load in response to host presence and quality suggest that ovarian development also responds to variation in the host environment. Ovarian dynamics are mediated by feedback from oviposition, by host feeding, and by sensory input from the host. The last of these mechanisms, host sensory cuing, is known to occur in three major orders and provides strong evidence that ovarian dynamics are adaptive by design. Conditions favoring host effects on ovarian development include trade-offs between egg production and either survival or dispersal, uncertainty in the host environment, and a correlation in host conditions between the time that oogenesis is initiated and the time that eggs are laid. Some host defenses block ovarian development, suggesting that ovarian dynamics in hostspecific insects should be viewed from a coevolutionary perspective.

This review follows progress in the analysis of cyclodiene insecticide resistance from the initial isolation of the mutant, through cloning of the resistance gene, to an examination of the distribution of resistance alleles in natural populations. Emphasis is given to the use of a resistant Drosophila mutant as an entry point to cloning the associated γ-aminobutyric acid (GABA) receptor subunit gene, Resistance to dieldrin. Resistance is associated with replacements of a single amino acid (alanine302) in the chloride ion channel pore of the protein. Replacements of alanine302 not only directly affect the drug binding site but also allosterically destabilize the drug preferred conformation of the receptor. Resistance is thus conferred by a unique dual mechanism associated with alanine302, which is the only residue replaced in a wide range of different resistant insects. The underlying mutations appear either to have arisen once, or multiply, depending on the population biology of the pest insect. Although resistance frequencies decline in the absence of selection, resistance alleles can persist at relatively high frequency and may cause problems for compounds to which cross-resistance is observed, such as the novel fipronils.

Annual cropping systems consist of a shifting mosaic of habitats that vary through time in their availability and suitability to insect pests. Agroecosystem instability results from changes that occur within a season with crop planting, development, and harvest. Further instability results from continuous alterations in biotic and abiotic insect life system components and from agricultural inputs. Changes to agroecosystems occur across seasons with changing agricultural practices, changing cropping patterns, and technological innovations. Much of this instability is a result of events unconnected with pest management.

The abilities of polyphagous pest species to move among and utilize different habitat patches in response to changes in suitability enable the pests to exploit unstable cropping systems. These pest characteristics determine the location and timing of damaging populations. Habitat suitability is influenced by plant species and cultivar, crop phenology, and agricultural inputs. Pest movement is affected by a suite of intrinsic factors, such as population age structure and mobility, and extrinsic factors, including weather systems and habitat distribution.

The life systems of three selected polyphagous pests are presented to demonstrate how an understanding of such systems in agricultural ecosystems improves our ability to predict and hence manage these populations.

In addition to the dorsal vessel (“heart”), insects have accessory pulsatile organs (“auxiliary hearts”) that supply body appendages with hemolymph. They are indispensable in the open circulatory system for hemolymph exchange in antennae, long mouthparts, legs, wings, and abdominal appendages. This review deals with the great diversity in the functional morphology and the evolution of these accessory pulsatile organs. In primitive insects, hemolymph is supplied to antennae and cerci by arteries connected to the dorsal vessel. In higher insects, however, these arteries were decoupled and associated with autonomous pumps that entered their body plan as evolutionary innovations. To ensure hemolymph supply to legs, wings, and some other appendages, completely new accessory pulsatile organs evolved. The muscular components of these pulsatile organs and their elastic antagonists were recruited from various organ systems and assembled to new functional units. In general, it seems that the evolution of accessory pulsatile organs has been determined by developmental and spatial constraints imposed by other organ systems rather than by changes in circulatory demands.

The hive of the honey bee is a suitable habitat for diverse mites (Acari), including nonparasitic, omnivorous, and pollen-feeding species, and parasites. The biology and damage of the three main pest species Acarapis woodi, Varroa jacobsoni, and Tropilaelaps clareae is reviewed, along with detection and control methods. The hypothesis that Acarapis woodi is a recently evolved species is rejected. Mite-associated bee pathologies (mostly viral) also cause increasing losses to apiaries. Future studies on bee mites are beset by three main problems: (a) The recent discovery of several new honey bee species and new bee-parasitizing mite species (along with the probability that several species are masquerading under the name Varroa jacobsoni) may bring about new bee-mite associations and increase damage to beekeeping; (b) methods for studying bee pathologies caused by viruses are still largely lacking; (c) few bee- and consumer-friendly methods for controlling bee mites in large apiaries are available.

Abundant natural enemies in tropical Asian irrigated rice usually prevent significant insect pest problems. Integrated pest management (IPM) extension education of depth and quality is required to discourage unnecessary insecticide use that upsets this natural balance, and to empower farmers as expert managers of a healthy paddy ecosystem. Farmers' skill and collaboration will be particularly important for sustainable exploitation of the potential of new, higher-yielding and pestresistant rices. IPM “technology transfer” through training and visit (T&V) extension systems failed, although mass media campaigns encouraging farmer participatory research can reduce insecticide use. The “farmer first” approach of participatory nonformal education in farmer field schools, followed by community IPM activities emphasizing farmer-training-farmer and research by farmers, has had greater success in achieving IPM implementation. Extension challenges are a key topic for rice IPM research, and new pest management technology must promote, rather than endanger, ecological balance in rice paddies.

Polyene hydrocarbons and epoxides are used as pheromone components and sex attractants by four macrolepidopteran families: the Geometridae, Noctuidae, Arctiidae, and Lymantriidae. They constitute a second major class of lepidopteran pheromones, different from the C10-C18 acetates, alcohols, and aldehydes commonly found in other species. They are biosynthesized from diet-derived linoleic or linolenic acids and are characterized by C17-C23 straight chains, 1-3 cis double bonds separated by methylene groups, and 0, 1, or 2 epoxide functions. Pheromone blends are created from components with different chain lengths, numbers of double bonds, and functional groups, or from mixtures of epoxide regioisomers or enantiomers, with several examples of synergism between enantiomers. Behavioral antagonists also limit interspecific attraction, with numerous examples of antagonism by enantiomers. This review summarizes the taxonomic distribution, mechanisms used to generate unique pheromone blends, and the identification, synthesis, and biosynthesis of these compounds.

The possibility of disrupting the chemical communication of insect pests has initiated the development of new semiochemicals, parapheromones, which are anthropogenic compounds structurally related to natural pheromone components. Modification at the chain and/or at the polar group, isosteric replacements, halogenation or introduction of labeled atoms have been the most common modifications of the pheromone structure. Parapheromones have shown a large variety of effects, and accordingly have been called agonists, pheromone mimics, synergists and hyperagonists, or else pheromone antagonists, antipheromones and inhibitors. Pheromone analogues have been used in quantitative structure-activity relationship studies of insect olfaction, and from a practical point of view they can replace pheromones when these are costly to prepare or unstable under field conditions.

African agriculture is largely traditional—characterized by a large number of smallholdings of no more than one ha per household. Crop production takes place under extremely variable agro-ecological conditions, with annual rainfall ranging from 250 to 750 mm in the Sahel in the northwest and in the semi-arid east and south, to 1500 to 4000 mm in the forest zones in the central west. Farmers often select well-adapted, stable crop varieties, and cropping systems are such that two or more crops are grown in the same field at the same time. These diverse traditional systems enhance natural enemy abundance and generally keep pest numbers at low levels. Pest management practice in traditional agriculture is a built-in process in the overall crop production system rather than a separate well-defined activity. Increased population pressure and the resulting demand for increased crop production in Africa have necessitated agricultural expansion with the concomitant decline in the overall biodiversity. Increases in plant material movement in turn facilitated the accidental introduction of foreign pests. At present about two dozen arthropod pests, both introduced and native, are recognized as one of the major constraints to agricultural production and productivity in Africa. Although yield losses of 0% to 100% have been observed on-station, the economic significance of the majority of pests under farmers’ production conditions is not adequately understood. Economic and social constraints have kept pesticide use in Africa the lowest among all the world regions. The bulk of pesticides are applied mostly against pests of commercial crops such as cotton, vegetables, coffee, and cocoa, and to some extent for combating outbreaks of migratory pests such as the locusts. The majority of African farmers still rely on indigenous pest management approaches to manage pest problems, although many government extension programs encourage the use of pesticides. The current pest management research activities carried out by national or international agricultural research programs in Africa focus on classical biological control and host plant resistance breeding. With the exception of classical biological control of the cassava mealybug, research results have not been widely adopted. This could be due to African farmers facing heterogeneous conditions, not needing fixed prescriptions or one ideal variety but a number of options and genotypes to choose from. Indigenous pest management knowledge is site-specific and should be the basis for developing integrated pest management (IPM) techniques. Farmers often lack the biological and ecological information necessary to develop better pest management through experimentation. Formal research should be instrumental in providing the input necessary to facilitate participatory technology development such as that done by Farmer Field Schools, an approach now emerging in different parts of Africa.

We discuss a framework for studying the evolution of morphology in insects, based on the concepts of “phenotypic plasticity” and “reaction norms.” We illustrate this approach with the evolution of some of the most extreme morphologies in insects: exaggerated, sexually selected male ornaments and weapons, and elaborate social insect soldier castes. Most of these traits scale with body size, and these scaling relationships are often nonlinear. We argue that scaling relationships are best viewed as reaction norms, and that the evolution of exaggerated morphological traits results from genetic changes in the slope and/or shape of these scaling relationships. After reviewing literature on sexually selected and caste-specific structures, we suggest two possible routes to the evolution of exaggerated trait dimensions: (a) the evolution of steeper scaling relationship slopes and (b) the evolution of sigmoid or discontinuous scaling relationship shapes. We discuss evolutionary implications of these two routes to exaggeration and suggest why so many of the most exaggerated insect structures scale nonlinearly with body size. Finally, we review literature on insect development to provide a comprehensive picture of how scaling relationships arise and to suggest how they may be modified through evolution.

Information about the phylogenetic relationships of Plecoptera is summarized. The few characters supporting monophyly of the order are outlined. Several characters of possible significance for the search for the closest relatives of the stoneflies are discussed, but the sister-group of the order remains unknown. Numerous characters supporting the presently recognized phylogenetic system of Plecoptera are presented, alternative classifications are discussed, and suggestions for future studies are made. Notes on zoogeography are appended. The order as such is old (Permian fossils), but phylogenetic relationships and global distribution patterns suggest that evolution of the extant suborders started with the breakup of Pangaea. There is evidence of extensive recent speciation in all parts of the world.

The Internet is affecting entomology teaching and research. Internet tools help students communicate and easily find and access information. Entomology instructors who adopt these tools may discover they are surprisingly time consuming to implement. Requiring students to use the Internet teaches them to glean from the glut of available information and to communicate electronically, both vital skills in today’s workplace. The Internet helps meet the growing need for distance education by providing a medium that allows students to conveniently access course materials and to communicate with the instructor and other students. Researchers benefit from using the Internet for one-to-one and one-to-many communication and from access to large cooperative databases, for example, in molecular biology and systematics. Perhaps the greatest impact on research will be the migration to the Web of journals and other specialized research literature. This may permit free access and will change the content and format of journal articles.

Circadian clocks are endogenous timing mechanisms that control molecular, cellular, physiological, and behavioral rhythms in all organisms from unicellulars to humans. Circadian rhythms influence many aspects of insect biology, finetuning life functions to the light and temperature cycles associated with the solar day. Genetic studies in the fruit fly Drosophila melanogaster have led to the cloning and characterization of several genes involved in the mechanism of the circadian clock. Periodic transcription and translation of these clock genes form the basis of a molecular feedback loop that has a “circa” 24-hour period. Rhythmic expression of clock genes in specific brain neurons appears to control behavioral rhythms in adult flies. However, clock genes are also expressed in other tissues, both within and outside of the nervous system. These observations prompted chronobiologists to investigate whether nonneural tissues possess intrinsic circadian clocks, what role they may be playing, and what the relationships are between clocks in the nervous system and those in peripheral tissues. Answers to those questions are providing important insights into the overall organization of the circadian system in insects.

Acceptance of the Internet as a common method for transferring entomological information has forced a re-evaluation of extension entomology’s role. The discipline of entomology was an early adopter of Internet-based information delivery and organization on both Gopher and the World Wide Web. New opportunities exist for immediate presentation and collection of information, but organization and categorization of information for timely retrieval remains a challenge. Metadata strategies, computer literacy, and integration of the Internet with current workflow paradigms promise to change the way extension entomology is done in the near future.