Annual Review of Entomology - Volume 51, 2006

Insulin-like peptides (ILPs) exist in insects and are encoded by multigene families that are expressed in the brain and other tissues. Upon secretion, these peptides likely serve as hormones, neurotransmitters, and growth factors, but to date, few direct functions have been demonstrated. In Drosophila melanogaster, molecular genetic studies have revealed elements of a conserved insulin signaling pathway, and as in other animal models, it appears to play a key role in metabolism, growth, reproduction, and aging. This review offers (a) an integrated summary of the efforts to characterize the distribution of ILPs in insects and to define this pathway and its functions in Drosophila and (b) a few considerations for future studies of ILP endocrinology in insects.

Prostaglandins and other eicosanoids are oxygenated metabolites of certain polyunsaturated fatty acids. These compounds are well known for their important actions in mammalian physiology and disease. Recent work has revealed the presence and biological actions of eicosanoids in insects and many other invertebrate animals. In insects, eicosanoids mediate cellular immunity to microbial and metazoan challenge. Notably, some infectious organisms secrete factors responsible for impairing host insect immune reactions by inhibiting biosynthesis of eicosanoids. Eicosanoids also act in insect reproductive biology, in ion transport physiology, and in fever response to infection as well as in protein exocytosis in tick salivary glands. Aside from ongoing actions in homeostasis, certain eicosanoid actions occur at crucial points in insect life histories, such as during infectious challenge and important events in reproduction.

Botanical insecticides have long been touted as attractive alternatives to synthetic chemical insecticides for pest management because botanicals reputedly pose little threat to the environment or to human health. The body of scientific literature documenting bioactivity of plant derivatives to arthropod pests continues to expand, yet only a handful of botanicals are currently used in agriculture in the industrialized world, and there are few prospects for commercial development of new botanical products. Pyrethrum and neem are well established commercially, pesticides based on plant essential oils have recently entered the marketplace, and the use of rotenone appears to be waning. A number of plant substances have been considered for use as insect antifeedants or repellents, but apart from some natural mosquito repellents, little commercial success has ensued for plant substances that modify arthropod behavior. Several factors appear to limit the success of botanicals, most notably regulatory barriers and the availability of competing products (newer synthetics, fermentation products, microbials) that are cost-effective and relatively safe compared with their predecessors. In the context of agricultural pest management, botanical insecticides are best suited for use in organic food production in industrialized countries but can play a much greater role in the production and postharvest protection of food in developing countries.

Thrips are among the stealthiest of insect invaders due to their small size and cryptic habits. Many invasive thrips are notorious for causing extensive crop damage, vectoring viral diseases, and permanently destabilizing IPM systems owing to irruptive outbreaks that require remediation with insecticides, leading to the development of insecticide resistance. Several challenges surface when attempting to manage incursive thrips species. Foremost among these is early recognition, followed by rapid and accurate identification of emergent pest species, elucidation of the region of origin, development of a management program, and the closing of conduits for global movement of thrips. In this review, we examine factors facilitating invasion by thrips, damage caused by these insects, pre- and post-invasion management tactics, and challenges looming on the horizon posed by invasive Thysanoptera, which continually challenge the development of sustainable management practices.

Plant diseases caused by, or associated with, phytoplasmas occur in hundreds of commercial and native plants, causing minor to extensive damage. Insect vectors, primarily leafhoppers, planthoppers, and psyllids, have been identified for relatively few phytoplasma diseases, limiting the capacity of managers to make informed decisions to protect crops and endangered indigenous plants. In the past two decades our knowledge of insect vector–phytoplasma interactions has increased dramatically, allowing researchers to make more accurate predictions about the nature and epidemiology of phytoplasma diseases. These better-characterized systems also may provide clues to the identity of insect vectors of other phytoplasma-associated diseases. We review the literature addressing the ecology of insect vectors, phytoplasma-insect ecological and molecular interactions, vector movement and dispersal, and possible management strategies with an emphasis on research from the past 20 years.

Insect odor and taste receptors are highly sensitive detectors of food, mates, and oviposition sites. Following the identification of the first insect odor and taste receptors in Drosophila melanogaster, these receptors were identified in a number of other insects, including the malaria vector mosquito Anopheles gambiae; the silk moth, Bombyx mori; and the tobacco budworm, Heliothis virescens. The chemical specificities of many of the D. melanogaster receptors, as well as a few of the A. gambiae and B. mori receptors, have now been determined either by analysis of deletion mutants or by ectopic expression in in vivo or heterologous expression systems. Here we discuss recent advances in our understanding of the molecular and cellular basis of odor and taste coding in insects.

Boreal peat bogs contain distinctive insects in addition to widely distributed generalists, including species restricted to bogs (tyrphobionts) and species characteristic of bogs but not confined to them (tyrphophiles). Bogs raised above the water table form characteristic habitat islands in southern boreal and temperate forest zones. Many bogs have persisted for hundreds and even thousands of years, preserving relict ecosystems related to subarctic biomes. The historical development and nature of individual bogs are reflected by differences among their insects, which are of great biogeographical and ecological interest. The environmental sensitivity of bogs also makes insects valuable as bioindicators. Moreover, few readily accessible bogs remain in a natural state. Given the scientific interest of bog insects and the fact that each relict bog habitat island is unique, further studies of the diversity of bog faunas are merited, and the conservation of these habitats should be strongly supported by entomologists.

Tremendous strides have been made regarding our understanding of how host plant chemistry influences the interactions between herbivores and their natural enemies. While most work has focused on plant chemistry effects on host location and acceptance by natural enemies, an increasing number of studies examine negative effects. The tritrophic role of plant chemistry is central to several aspects of trophic phenomena including top-down versus bottom-up control of herbivores, enemy-free space and host choice, and theories of plant defense. Furthermore, tritrophic effects of plant chemistry are important in assessing the degree of compatibility between biological control and plant resistance approaches to pest control. Additional research is needed to understand the physiological effects of plant chemistry on parasitoids. Explicit tests are required to determine whether natural enemies can act as selective forces on plant defense. Finally, further studies of natural systems are crucial to understanding the evolution of multitrophic relationships.

Phytophagous insects and their natural enemies make up one of the largest and most diverse groups of organisms on earth. Ecological processes, in particular negative indirect effects mediated by shared natural enemies (apparent competition), may be important in structuring phytophagous insect communities. The potential for indirect interactions can be assessed by analyzing the trophic structure of insect communities, and we claim that quantitative food webs are particularly well suited for this task. We review the experimental evidence for both short-term and long-term apparent competition in phytophagous insect communities and discuss the possible interactions between apparent competition and intraguild predation or shared mutualists. There is increasing evidence for the importance of trait-mediated as well as density-mediated indirect effects. We conclude that there is a need for large-scale experiments manipulating communities in their entirety and a greater integration of community and chemical ecology.

The past decade has produced an explosion of new information on the development, neuroanatomy, and possible functions of the mushroom bodies. This review provides a concise, contemporary overview of the structure of the mushroom bodies. Two topics are highlighted: the volume plasticity of mushroom body neuropils evident in the brains of some adult insects and a possible essential role for the γ lobe in olfactory memory.

Parasitoid wasps have evolved a wide spectrum of developmental interactions with hosts. In this review we synthesize and interpret results from the phylogenetic, ecological, physiological, and molecular literature to identify factors that have influenced the evolution of parasitoid developmental strategies. We first discuss the origins and radiation of the parasitoid lifestyle in the Hymenoptera. We then summarize how parasitoid developmental strategies are affected by ecological interactions and assess the inventory of physiological and molecular traits parasitoids use to successfully exploit hosts. Last, we discuss how certain parasitoid virulence genes have evolved and how these changes potentially affect parasitoid-host interactions. The combination of phylogenetic data with comparative and functional genomics offers new avenues for understanding the evolution of biological diversity in this group of insects.

Throughout its long evolutionary history, the Dopa decarboxylase gene (Ddc) has acquired a variety of functions in insects. The enzyme (DDC) catalyzes the production of the neural transmitters dopamine and serotonin. Not surprisingly, evidence of the enzyme's involvement in the behavior of insects is beginning to accumulate. In addition, DDC plays a role in wound healing, parasite defense, pigmentation, and cuticle hardening. A high degree of sequence conservation has allowed comparisons of the Ddc-coding regions from various insects, facilitating a number of recent studies on insect systematics. This review outlines the diverse functions of Ddc and illustrates how studies of this model system address many questions on insect neurobiology, developmental biology, and systematics.

Interest in trap cropping, a traditional tool of pest management, has increased considerably in recent years. In this review we propose a broader definition of trap cropping that encompasses the inherent characteristics of the trap crop plants themselves as well as the strategies associated with their deployment. Inherent characteristics of a trap crop may include not only natural differential attractiveness for oviposition and feeding, but also other attributes that enable the trap crop plants to serve as a sink for insects or the pathogens they vector. Successful deployment of trap crops within a landscape depends on the inherent characteristics of the trap crop and the higher value crop, the spatial and temporal characteristics of each, the behavior and movement patterns of insect pests, and the agronomic and economic requirements of the production system. Thus, trap cropping is more knowledge-intensive than many other forms of pest management. We review recent references on trap cropping, classify them according to their modalities and level of implementation, and provide a synthesis of the factors that influence the success of trap cropping. Last, we provide a list of recommendations and guidelines that should prove helpful in moving trap cropping forward to its full potential.

As phloem feeders and major vectors of plant viruses, aphids are important pests of agricultural and horticultural crops worldwide. The processes of aphid settling and reproduction on plants therefore have a direct economic impact, and a better understanding of these events may lead to improved management strategies. Aphids are also important model organisms in the analysis of population differentiation and speciation in animals, and new ideas on plant utilization influence our understanding of the mechanisms generating biological diversity. Recent research suggests that the dominant cues controlling plant preference and initiation of reproduction are detected early during the stylet penetration process, well before the nutrient supply (phloem) is contacted. Aphids regularly puncture cells along the stylet pathway and ingest cytosolic samples, and the cues stimulating settling and parturition likely are metabolites present in peripheral (nonvascular) plant cells. We discuss these findings and their implications for aphid evolution and management.

Invertebrate pathogens and their hosts are taxonomically diverse. Despite this, there is one unifying concept relevant to all such parasitic associations: Both pathogen and host adapt to maximize their own reproductive output and ultimate fitness. The strategies adopted by pathogens and hosts to achieve this goal are almost as diverse as the organisms themselves, but studies examining such relationships have traditionally concentrated only on aspects of host physiology. Here we review examples of host-altered behavior and consider these within a broad ecological and evolutionary context. Research on pathogen-induced and host-mediated behavioral changes demonstrates the range of altered behaviors exhibited by invertebrates including behaviorally induced fever, elevation seeking, reduced or increased activity, reduced response to semiochemicals, and changes in reproductive behavior. These interactions are sometimes quite bizarre, intricate, and of great scientific interest.

With world trade in agricultural commodities increasing, the introduction of exotic insects into new areas where they become pests will increase. The development and application of quarantine treatments or other mitigation approaches to prevent pest introduction in traded commodities raise many research and regulatory issues. The probit 9 standard for quarantine treatment efficacy has given way to risk-based alternatives. Varietal testing may have merit for some treatments or commodities but not for others. Development of generic treatments to control broad groups of insects or insects in all commodities can expedite new trade in agricultural products. Area-wide pest management programs lower pest levels before harvest and improve the quarantine security provided by any postharvest treatments. Systems approaches capitalize on cumulative pest mortality from multiple control components to achieve quarantine security in an exported commodity. Certain quarantine treatment technologies such as irradiation are not universally accepted, which is slowing their adoption. Standardized phytosanitary measures and research protocols are needed to improve the flow of information when countries propose to trade in a regulated commodity.

Tallgrass prairie (TGP) arthropods are diverse and abundant, yet they remain poorly documented and there is still much to be learned regarding their ecological roles. Fire and grazing interact in complex ways in TGP, resulting in a shifting mosaic of resource quantity and quality for primary consumers. Accordingly, the impacts of arthropod herbivores and detritivores are expected to vary spatially and temporally. Herbivores generally do not control primary production. Rather, groups such as grasshoppers have subtle effects on plant communities, and their most significant impacts are often on forbs, which represent the bulk of plant diversity in TGP. Belowground herbivores and detritivores influence root dynamics and rhizosphere nutrient cycling, and above- and belowground groups interact through plant responses and detrital pathways. Large-bodied taxa, such as cicadas, can also redistribute significant quantities of materials during adult emergences. Predatory arthropods are the least studied in terms of ecological significance, but there is evidence that top-down processes are important in TGP.

The mating system of each species is a unique, dynamic suite of interactions between the sexes. In this review I describe these interactions in the families of flies that contain blood-feeding species. A transition from the aerial swarm, with rapid copulae and no direct female choice, to substrate-based systems with lengthy copulae and opportunities for female choice is evident at both a phylogenetic scale and within nematoceran families under specific ecological conditions. Female monogamy is associated with the former, polyandry with the latter. I suggest that the intensity of sexual selection operating on males in systems where the probability of mating is low has favored male ability to control female receptivity. Reproductive success of males is universally correlated to successful foraging for sugar or blood and (in some species and ecological conditions) to body size. Understanding the ecological basis of the mating systems of these flies will help formulate integrative, sustainable, and biologically lucid approaches for their control.

Cannibalism among generalist predators has implications for the dynamics of terrestrial food webs. Spiders are common, ubiquitous arthropod generalist predators in most natural and managed terrestrial ecosystems. Thus, the relationship of spider cannibalism to food limitation, competition, and population regulation has direct bearing on basic ecological theory and applications such as biological control. This review first briefly treats the different types of spider cannibalism and then focuses in more depth on evidence relating cannibalism to population dynamics and food web interactions to address the following questions: Is cannibalism in spiders a foraging strategy that helps to overcome the effects of a limited supply of calories and/or nutrients? Does cannibalism in spiders reduce competition for prey? Is cannibalism a significant density-dependent factor in spider population dynamics? Does cannibalism dampen spider-initiated trophic cascades?