Apoptosis in mosquito salivary glands: Sindbis virus-associated and tissue homeostasis.

Apoptosis is observed during a spectrum of conditions including exogenous virus infection and endogenous cellular turnover. Adult female Aedes albopictus mosquitoes challenged with increasing titres of Sindbis virus (SINV) via intrathoracic inoculation demonstrated that the injection dosage did not result in significantly different levels of virus growth or mosquito survival at day 10 post-infection. Tissues probed for apoptosis using an in situ TUNEL assay revealed SINV-associated apoptotic cells scattered throughout the proximal and distal regions of the salivary gland (SG) lateral lobes but which were not detected in the median lobe or the midgut and hindgut. Apoptosis was also identified in SG duct cells in both infected and uninfected mosquitoes, suggesting routine tissue homeostasis. SINV-associated apoptosis sequestered to the SG lateral lobes indicates a differential epithelial cell response to an arbovirus and provides insight into mosquito defence mechanisms against pathogens and SG infection barriers, hurdles to transmission of arboviruses of public health concern.

Ants provide nutritional and defensive benefits to the carnivorous plant Sarracenia minor.

Ants can have important, but sometimes unexpected, effects on the plants they associate with. For carnivorous plants, associating with ants may provide defensive benefits in addition to nutritional ones. We examined the effects of increased ant visitation and exclusion of insect prey from pitchers of the hooded pitcher plant Sarracenia minor, which has been hypothesized to be an ant specialist. Visitation by ants was increased by placing PVC pipes in the ground immediately adjacent to 16 of 32 pitcher plants, which created nesting/refuge sites. Insects were excluded from all pitchers of 16 of the plants by occluding the pitchers with cotton. Treatments were applied in a 2 x 2 factorial design in order to isolate the hypothesized defensive benefits from nutritional ones. We recorded visitation by ants, the mean number of ants captured, foliar nitrogen content, plant growth and size, and levels of herbivory by the pitcher plant mining moth Exyra semicrocea. Changes in ant visitation and prey capture significantly affected nitrogen content, plant height, and the number of pitchers per plant. Increased ant visitation independent of prey capture reduced herbivory and pitcher mortality, and increased the number of pitchers per plant. Results from this study show that the hooded pitcher plant derives a double benefit from attracting potential prey that are also capable of providing defense against herbivory.

Effects of elevated CO2 on foliar quality and herbivore damage in a scrub oak ecosystem.

Atmospheric CO2 concentrations have increased exponentially over the last century and continuing increases are expected to have significant effects on ecosystems. We investigated the interactions among atmospheric CO2, foliar quality, and herbivory within a scrub oak community at the Kennedy Space Center, Florida. Sixteen plots of open-top chambers were followed; eight of which were exposed to ambient levels of CO2 (350 ppm), and eight of which were exposed to elevated levels of CO2 (700 ppm). We focused on three oak species, Quercus geminata, Quercus myrtifolia, Quercus chapmanii, and one nitrogen fixing legume, Galactia elliottii. There were declines in overall nitrogen and increases in C:N ratios under elevated CO2. Total carbon, phenolics (condensed tannins, hydrolyzable tannins, total phenolics) and fiber (cellulose, hemicellulose, lignin) did not change under elevated CO2 across plant species. Plant species differed in their relative foliar chemistries over time, however, the only consistent differences were higher nitrogen concentrations and lower C:N ratios in the nitrogen fixer when compared to the oak species. Under elevated CO2, damage by herbivores decreased for four of the six insect groups investigated. The overall declines in both foliar quality and herbivory under elevated CO2 treatments suggest that damage to plants may decline as atmospheric CO2 levels continue to rise.

Quality or quantity: the direct and indirect effects of host plants on herbivores and their natural enemies.

Resource quality (plant nitrogen) and resource quantity (plant density) have often been argued to be among the most important factors influencing herbivore densities. A difficulty inherent in the studies that manipulate resource quality, by changing nutrient levels, is that resource quantity can be influenced simultaneously, i.e. fertilized plants grow more. In this study we disentangled the potentially confounding effects of plant quality and quantity on herbivore trophic dynamics by separately manipulating nutrients and plant density, while simultaneously reducing pressure from natural enemies (parasitoids) in a fully factorial design. Plant quality of the sea oxeye daisy, Borrichia frutescens, a common coastal species in Florida, was manipulated by adding nitrogen fertilizer to increase and sugar to decrease available nitrogen. Plant density was manipulated by pulling by hand 25 or 50% of Borrichia stems on each plot. Because our main focal herbivore was a gall making fly, Asphondylia borrichiae, which attacks only the apical meristems of plants, manipulating plant nitrogen levels was a convenient and reliable way to change plant quality without impacting quantity because fertilizer and sugar altered plant nitrogen content but not plant density. Our other focal herbivore was a sap-sucker, Pissonotus quadripustulatus, which taps the main veins of leaves. Parasitism of both herbivores was reduced via yellow sticky traps that caught hymenopteran parasitoids. Plant quality significantly affected the per stem density of both herbivores, with fertilization increasing, and sugar decreasing the densities of the two species but stem density manipulations had no significant effects. Parasitoid removal significantly increased the densities of both herbivores. Top-down manipulations resulted in a trophic cascade, as the density of Borrichia stems decreased significantly on parasitoid removal plots. This is because reduced parasitism increases gall density and galls can kill plant stems. In this system, plant quality and natural enemies impact per stem herbivore population densities but plant density does not.

Induced defensive response of myrtle oak to foliar insect herbivory in ambient and elevated CO2.

The rising level of atmospheric CO2 has stimulated several recent studies attempting to predict the effects of increased CO2 on ecological communities. However, most of these studies have been conducted in the benign conditions of the laboratory and in the absence of herbivores. In the current study, we utilized large octagonal chambers, which enclosed portions of an intact scrub-oak community to investigate the interactive effects of CO2 and insect herbivory on myrtle oak, Quercus myrtifolia. Specifically, we assessed the effects of ambient and elevated CO2 (2x current concentrations) on percent foliar nitrogen, C:N ratio, total relative foliar tannin content, and the presence of leaf damage caused by leaf mining and leaf chewing insects that feed on myrtle oak. Total foliar N declined and C:N ratios increased significantly in oaks in elevated CO2 chambers. The percentages of leaves damaged by either leafminers or leaf chewers tended to be lower in elevated compared to ambient chambers, but they co-occurred on leaves less than expected, regardless of CO2 treatment. Leaves that had been either mined or chewed exhibited a similar wounding or defensive response; they had an average of 25 and 21% higher protein binding ability, which is correlated with tannin concentration, compared to nondamaged control leaves, respectively. While the protein-binding ability (expressed as total percent tannin) of leaves from elevated CO2 was slightly higher than from leaves grown in ambient chambers, this difference was not significant.

Elevated CO2 lowers relative and absolute herbivore density across all species of a scrub-oak forest.

The unabated increase in global atmospheric CO(2) is expected to induce physiological changes in plants, including reduced foliar nitrogen, which are likely to affect herbivore densities. This study employs a field-based CO(2 )enrichment experiment at Kennedy Space Center, Florida, to examine plant-herbivore (insect) interactions inside eight open-topped chambers with elevated CO(2) (710 ppm) and eight control chambers with ambient CO(2). In elevated CO(2) we found decreased herbivore densities per 100 leaves, especially of leaf miners, across all five plant species we examined: the oak trees Quercus myrtifolia, Q. geminata, and Q. chapmanii, the nitrogen-fixing vine Galactia elliottii and the shrub Vaccinium myrsinites. Both direct and indirect effects of lowered plant nitrogen may influence this decrease in herbivore densities. Direct effects of lowered nitrogen resulted in increased host-plant related death and an increase in compensatory feeding: per capita herbivore leaf consumption in elevated CO(2) was higher than in ambient CO(2). Indirectly, compensatory feeding may have prolonged herbivore development and increased exposure to natural enemies. For all leaf miners we examined, mortality from natural enemies increased in elevated CO(2). These increases in host-plant induced mortality and in attack rates by natural enemies decreased leaf miner survivorship, causing a reduction in leaf miner density per 100 leaves. Despite increased leaf production in elevated CO(2) from the carbon fertilization effect, absolute herbivore abundance per chamber was also reduced in elevated CO(2). Because insects cause premature leaf abscission, we also thought that leaf abscission would be decreased in elevated CO(2). However, for all plant species, leaf abscission was increased in elevated CO(2), suggesting a direct effect of CO(2) on leaf abscission that outweighs the indirect effects of reduced insect densities on leaf abscission.

The influence of species identity and herbivore feeding mode on top-down and bottom-up effects in a salt marsh system.

In this study we investigated the potential importance of species identity and herbivore feeding mode in determining the strengths of top-down and bottom-up effects on phytophagous insect densities. In 1998, we conducted two factorial field experiments in which we manipulated host plant quality and intensity of parasitoid attack on three salt marsh herbivores, the planthoppers Prokelisia marginata and Pissonotus quadripustulatus (Homoptera: Delphacidae), which feed only on Spartina alterniflora and Borrichia frutescens, respectively, and the gall fly Asphondylia borrichiae (Diptera: Cecidomyiidae), which feeds only on B. frutescens. We increased plant quality through addition of nitrogen fertilizer, and decreased parasitism by trapping hymenopteran parasitoids continuously throughout the study. Herbivore densities were censused biweekly. Increasing plant quality through fertilization increased the density of all three herbivores within 2 weeks of treatment application, and higher densities were maintained for the duration of the study. Reduction of top-down pressure had no effect on either planthopper species, possibly because of compensatory mortality affecting the two species. In contrast, reduction of parasitism significantly increased the density of A. borrichiae galls, perhaps because development within gall tissue reduces the sources of compensatory mortality affecting this species. The results of this study show that the bottom-up effects of plant quality were strong and consistent for all three species, but the strength of top-down effects differed between the two feeding guilds. Thus, even for herbivores feeding on the same host plant, conclusions drawn regarding the relative importance of top-down and bottom-up effects may vary depending upon the feeding mode of the herbivore.

Experimental tests of trophic dynamics: taking a closer look.

The relative importance of bottom-up versus top-down forces, and the effect of productivity on community dynamics continue to be of much interest to ecologists. Trophic dynamic theories are difficult to test, as they require explicit knowledge of the many organisms involved, as well as the nature of the interactions between them. The Oksanen-Fretwell (OF) theory, which suggests that the relative roles of top-down and bottom-up factors vary with primary productivity, is well known in the literature, but is difficult to test rigorously. Recently, two experimental studies have tried to test OF theory. In this paper we discuss methodological problems associated with these studies that may weaken the conclusions drawn by the authors.