S. Bradleigh Vinson

Viruses and Parasitism in Insects

Publisher Summary This chapter focuses on viruses and parasitism in insects. In recent years, virus-like particles have been observed in the reproductive tracts of a large number of endoparasitic hymenoptera (i.e., wasps whose larvae develop inside host larvae), belonging to the families of braconidae and ichneumonidae. Some of these agents have now been characterized to the extent that one does not hesitate in referring to them as viruses, in which context they are discussed in the chapter; indeed, the most prevalent type of particle observed in braconid wasps appears to represent a highly unusual form of baculovirus. A unique feature of all parasitoid viruses, thus far discovered, is that they replicate only in the ovary (of wasps), from which they are transmitted to host larvae or eggs during oviposition. In affected species, large concentrations of virions can always be observed in either cell nuclei of the calyx, an epithelium situated between the ovarioles and oviduct, or within the lumen of calyces and oviducts. It is now apparent that the presence of virus particles in the ovaries of parasitoid wasps is not an isolatcd phenomenon. Indeed, it may represent the rule rather than the exception among certain groups of endoparasitic hymenoptera. The invariable presence of particulate calyx fluids in some parasitoid species suggests that these particles represent an integral component of the parasitoid life cycle.

Division of labour and specification of castes in the red imported fire ant Solenopsis invicta buren

Abstract Division of labour in Solenopsis invicta follows a familiar pattern: younger, smaller ants tend toward brood care while older, larger ants tend toward foraging. However, long-term observations of marked individuals reveal that length of nursing and foraging ‘careers’ and the age of transition between these activities vary considerably between and within size groups, and are related to length of life. Experiments with entire colonies show that larger ants are more likely than smaller ants to forage for insect prey. There are two main worker castes, ‘nurses’ and ‘foragers’, whose members span a wide age-size range, and a large ‘reserve’ subcaste, heterogeneous in age, size, and behaviour: reserves may nurse, forage, store liquid food, or relay food from nurses to foragers. The proportion of ants engaged in foraging decreases with colony size because many ants in large colonies are not exposed to recruitment signals.

Oviposition behaviour of Bracon mellitor, a parasitoid of the boll weevil (Anthonomus grandis). II. Associative learning

ABSTRACT. The parasitoid, Bracon mellitor, will probe with its ovipositor plant structures containing its host. The identification of methyl parahydroxy‐benzoate (methyl parasept), an antimicrobial additive to the semi‐synthetic diet used for rearing the host, as a releaser of this behaviour prompted an investigation of the parasitoids response to this chemical. Its absence from field hosts suggested the response was learned. Female parasitoids reared in host‐infested cotton plant buds from the fields or in methyl parasept‐free hosts did not respond to methyl parasept alone until exposed to hosts contaminated with it. This is taken to imply associative learning by the females reared from the methyl parasept‐free hosts. A few responses to methyl parasept were elicited from inexperienced females reared from methyl parasept‐contaminated hosts, but their responsiveness increased after they had been exposed to methyl parasept‐contaminated hosts as oviposition stimuli. These results point to the caution that must be exercised in the isolation and identification of kairomones.

Reproductive division of labor between hybrid and nonhybrid offspring in a fire ant hybrid zone.

Abstract.— Interspecific hybridization can often impose a substantial fitness cost due to reduced hybrid viability or fecundity. In social insects, however, such costs disproportionately impact reproductive offspring, whereas hybrids who become sterile workers can be functional, and even beneficial, colony members. Genomic imprinting of the paternal genome in reproductive, but not worker female offspring has been proposed as a mechanism to avoid genomic incompatibilities in hybrid queens in a hybrid zone between two fire ant species, Solenopsis geminata and S. xyloni. A study of allozyme variation demonstrated differences between the worker caste displaying a hybrid phenotype, and the winged queen caste displaying only the mothers phenotype. In this study, we investigate whether these differences are caused by genomic imprinting or genetic differences between castes by comparing variability of proteins to that of microsatellite markers. Workers and winged queens differed genetically at both classes of marker, indicating that allozyme differences were caused by underlying genetic differences between castes rather than differences in gene expression due to imprinting. Workers were F1 S. geminata X S. xyloni hybrids, whereas nearly all winged queens were of pure S. xyloni ancestry. Thus, S. xyloni within the hybrid zone appears to have evolved social hybridogenesis, in which the loss of worker potential in pure‐species offspring necessitates hybridization for worker production, but prevents hybrids from being represented in the reproductive caste.

Depression of prothoracic gland activity of Heliothis virescens by venom and calyx fluids from the parasitoid, Cardiochiles nigriceps

Abstract The braconid wasp, Cardiochiles nigriceps, successfully parasitizes the host larva, Heliothis virescens, by inhibiting host pupation. The parasitoid suppresses the function of the prothoracic gland and its competence to respond to prothoracicotropic hormone (PTTH) after larval-pupal commitment caused by a small surge of ecdysteroid. Prothoracic gland from both unparasitized normal and parasitized larvae before the small surge responded to PTTH in vitro by releasing ecdysteroid, whereas after this surge only the prothoracic gland of the unparasitized larvae released the ecdysteroid in the presence of PTTH. Both calyx and venom fluid of female wasps, which are injected into the host with the eggs at the time of oviposition, were required to depress prothoracic gland function. Prothoracic glands pre-incubated in vitro for 6 h with both calyx and venom fluids resulted in inhibiting the release of ecdysteroid by pupally committed prothoracic gland. The results suggest that the calyx and venom fluids are directly able to depress the function of the prothoracic gland and the competence of prothoracic gland to respond to PTTH.

Multiporous plate sensilla in antennae of the chalcidoidea (Hymenoptera)

Abstract The ultrastructure of one sensillar type in one group of insects is described so that comparisons of this structure with similar structures in different groups of insects might be made. The ultrastructure of multiporous plate sensilla (MPS) in antennae was studied in 13 species from 7 families of Chalcidoidea. The cuticle of the MPS consists of an elongated, porous plate elevated above the antennal surface. Two cuticular invaginations form 3 channels, a median channel and 2 lateral channels that extend through the length of the sensillum. The MPS are innervated by numerous neurons, more than 50 in some species. The dendrites extend towards the median channel at the proximal end of the sensillum. When the dendrites reach the median channel, they branch and turn towards the distal end of the sensillum, traversing its longitudinal axis. Pore tubules, which occur in the pore chambers, come in contact with the membranes of the dendritic branches. There are 2 types of MPS: (1) with a relatively thin porous plate and many pores occurring in females and males; (2) with a thicker porous plate and fewer pores occurring in females only. The MPS in the Chalcidoidea are compared with the MPS in other Hymenoptera and the functional significance of the 2 types of MPS in the Chalcidoidea is discussed.

Effect of the parasitoid, Campoletis sonorensis, on the growth of its host, Heliothis virescens

Abstract Growth of Heliothis virescens was stopped following parasitism by the ichneumonid parasitoid, Campoletis sonorensis . The source of the active factor was found to be the lateral oviducts of the female parasitoid. The active factor appears to be a protein.

Correlating the initiation of virus replication with a specific pupal developmental phase of an ichneumonid parasitoid

SummaryThe morphogenesis of virus particles associated with the calyx region of the lateral oviduct of the ichneumonid parasitoid Campoletis sonorensis is initiated during a specific phase of pupal development. The extent of body pigmentation proved to be the most reliable visual marker for correlation of induction of virus replication with particular phases of pupal development. Parasitoids removed from their pupal cases at 15 and 16 days after egg deposition were categorized into one of four developmental groups based on the degree of pigmentation. Viruses were not evident in the calyx cells of pupae representing the intial developmental stage, however, in all subsequent stages the concentration of particles gradually increased as development progressed. Observations of freeze-fractured tissue support previous suppositions that the viruses bud through the nuclear envelope following synthesis de novo within the nucleus.

Behavioral Chemicals in the Augmentation of Natural Enemies

The behavioral manipulation of natural enemies of pest insects has been a long standing dream among the biological control practitioners. Recent work concerning the behavior of some natural enemies of pest insects suggests that this dream may become a practical reality. Wright (1964, 1965) recognized the potential for pest management by the behavioral modification of insects through the use of non-toxic agents that modify the insects’ behavior. As stated by Wright (1965) these chemicals may effect control of a pest by inhibiting a correct response or eliciting an incorrect one. He coined the term “metarchon” for the introduced stimulus which includes chemical repellents and attractants as well as physical factors such as light and sound which interfere with the insects ability to communicate. While much effort is underway to develop methods of manipulating the pest insect, a different approach is needed for the manipulation of natural enemies. Instead of attempting to elicit an incorrect response or inhibit a correct one, the goal of entomophagous insect manipulation is to redirect or stimulate the response. The behavioral modification of certain arthropod enemies of insects through the use of chemicals has opened up new opportunities for the manipulation of these arthropods for the benefit of man. However, such an approach requires a working understanding of the behavior of entomophagous arthropods and the role played by chemicals in their behavioral patterns.

Plant Resources and Colony Growth in an Invasive Ant: The Importance of Honeydew-Producing Hemiptera in Carbohydrate Transfer Across Trophic Levels

Abstract Studies have suggested that plant-based nutritional resources are important in promoting high densities of omnivorous and invasive ants, but there have been no direct tests of the effects of these resources on colony productivity. We conducted an experiment designed to determine the relative importance of plants and honeydew-producing insects feeding on plants to the growth of colonies of the invasive ant Solenopsis invicta (Buren). We found that colonies of S. invicta grew substantially when they only had access to unlimited insect prey; however, colonies that also had access to plants colonized by honeydew-producing Hemiptera grew significantly and substantially (≈50%) larger. Our experiment also showed that S. invicta was unable to acquire significant nutritional resources directly from the Hemiptera host plant but acquired them indirectly from honeydew. Honeydew alone is unlikely to be sufficient for colony growth, however, and both carbohydrates abundant in plants and proteins abundant in animals are likely to be necessary for optimal growth. Our experiment provides important insight into the effects of a common tritrophic interaction among an invasive mealybug, Antonina graminis (Maskell), an invasive host grass, Cynodon dactylon L. Pers., and S. invicta in the southeastern United States, suggesting that interactions among these species can be important in promoting extremely high population densities of S. invicta.