Instar-specific sensitivity of specialist Manduca sexta larvae to induced defences in their host plant Nicotiana attenuata

1. The time delay associated with the activation of induced defences is thought to be a liability for this type of defence because it allows herbivores to remove biomass before the defence is fully induced. When defences are costly and plants grow with competitors, however, it may be more advantageous not to induce defences too fast and motivate the herbivore to move to the neighbour when it is most voracious. 2. Such a strategy can only work when the costs for the herbivore of moving to a neighbouring plant are smaller than the costs of staying on a fully induced plant. For lepidopteran herbivores, both the sensitivity to induced defences and the costs of moving may vary considerably between instars and this variation may constrain the plant's defensive opportunities. 3. This study was designed to examine whether the cost of moving, mimicked by a starvation period of 8 h, was larger than the cost of staying on a fully induced plant for each larval instar of the specialist Manduca sexta feeding on induced and control tissues of Nicotiana attenuata. 4. For first‐ and second‐instar larvae, the costs of moving were larger than the costs of staying on a fully induced plant. In contrast, feeding on induced plant material retarded development in third‐instar larvae more than did starvation, indicating that in this instar the costs of leaving are smaller than the costs of staying on an induced plant. More than 98% of the lifetime leaf mass consumed by a M. sexta larva is consumed during the fourth and fifth instars, and during these instars larval development was not affected by either induced defences or starvation. Thus the third instar, the stage just before larvae cause the majority of damage, represents a window of sensitivity to induced defences during which larvae can be motivated to change plants. 5. These results suggest that N. attenuata plants, which commonly compete with conspecifics in nature, have the opportunity to manipulate the behaviour of the specialist herbivore M. sexta to minimise the fitness effects of inducing defences when these defences are most costly, i.e. when plants grow under intraspecific competition.     

Antennae in the hawkmoth Manduca sexta (Lepidoptera, Sphingidae) mediate abdominal flexion in response to mechanical stimuli

Flying insects rely on the integration of feedback signals from multiple sensory modalities. Thus, in addition to the visual input, mechanosensory information from antennae is crucial for stable flight in the hawkmoth Manduca sexta. However, the nature of compensatory reflexes mediated by mechanoreceptors on the antennae is unknown. In this study we describe an abdominal flexion response mediated by the antennal mechanosensory input during mechanical body rotations. Such reflexive abdominal motions lead to shifts in the animal’s center of mass, and therefore changes in flight trajectory. Moths respond with abdominal flexion both to visual and mechanical rotations, but the mechanical response depends on the presence of the mass of the flagellum. In addition, the mechanically mediated flexion response is about 200° out of phase with the visual response and adds linearly to it. Phase-shifting feedback signals in such a manner can lead to a more stable behavioral output response when the animal is faced with turbulent perturbations to the flight path.     

Pericardial cell ultrastructure in the tobacco hornworm: Manduca sexta L. (Lepidoptera: Sphingidae)

The ultrastructure of the pericardial athrocytes of fifth instar tobacco hornworm Manduca sexta L. (Lepidoptera: Sphingidae) was examined by transmission electron microscopy. These cells are highly specialized for the maintenance of hemolymph homeostasis by synthesis and secretion of some proteins, and uptake and degradation of others. We observed a striking radial zonation with numerous labyrinthine channels extending into the cell providing a large surface area for enhanced efficiency of endocytosis. Granular material was imported into the endosomal compartment by fusion of endocytotic vesicles from the basal region. Anatomical evidence supports the hypothesis that proteins are transported from the endosome to the lysosome, by maturation of the early endosome to form the late endosome/lysosome, as opposed to vesicular transport. The presence of catalase in athrocyte lysosomes is reported for the first time. Endocytosis in the athrocytes of insects is compared with endocytosis in vertebrate systems.     

Sequestration of ascorbic acid by the larval labial gland and haemolymph of the tobacco hornworm,Manduca sexta L. (Lepidoptera: Sphingidae)

Tissues from Manduca sexta were examined for the presence of l-ascorbic acid and l-gulonolactone oxidase. l-Ascorbic acid was found in eggs, larval labial gland, haemolymph, gut, muscle, cuticle, adult nervous tissue and gonads at concentrations ranging from < 10 to > 150 mg per 100 g wet tissue. No ascorbate was detected in larval fat body and Malphigian tubule or adult salivary gland. Concentrations in labial gland and haemolymph increased 80- and 10-fold, respectively, during the fifth larval instar such that the labial gland surpassed all other tissues in ascorbate concentration. Since tissues from insects reared on an l-ascorbate-deficient diet contained no detectable vitamin C and l-gulonolactone oxidase was absent from tissue extracts, the hornworm apparently acquired l-ascorbate solely from the diet.     

Pheromone pre-exposure and mating modulate codling moth (Lepidoptera: Tortricidae) response to host plant volatiles

Abstract 1 Two codling moth Cydia pomonella kairomonal attractants, ethyl (E,Z)‐2,4‐decadienoate (pear ester) and (E)‐β‐farnesene, were tested in an insecticide‐sprayed apple orchard and an orchard treated for mating disruption with synthetic pheromone (E,E)‐8,10‐dodecadienol (codlemone). Male captures with pear ester were higher in the pheromone‐treated than in the insecticide‐treated orchard, whereas captures with (E)‐β‐farnesene were not different. Subsequent wind tunnel experiments confirmed that pre‐exposure to sex pheromone codlemone increased the behavioural response of codling moth males to pear ester. This supports the idea that male attraction to the plant volatile pear ester and sex pheromone codlemone is mediated through the same sensory channels. 2 Pear ester is a bisexual codling moth attractant and even captures of female moths were significantly increased in the pheromone‐treated orchard. In the laboratory wind tunnel, pheromone pre‐exposure had no effect on female response to pear ester, but significantly more mated than unmated codling moth females flew upwind towards a pear ester source. Differences in mating status in insecticide‐treated vs. pheromone‐treated orchards may thus account for the differences in female trap captures with pear ester. 3 These findings are important with respect to monitoring of codling moth with pear ester in mating disruption orchards. They also emphasize the importance of host plant volatiles in pheromone‐mediated mating disruption, which has been neglected to date.     

Formation of insoluble and colloidally dispersed tannic acid complexes in the midgut fluid of Manduca sexta (Lepidoptera: Sphingidae): An explanation for the failure of tannic acid to cross the peritrophic envelopes of lepidopteran larvae

Magnesium and calcium ions, in concentrations comparable to those reported in the midgut fluids of lepidopteran larvae, bring about the precipitation of most of the tannic acid present in simple solutions buffered at pH 8.0 and 10.0, but not at pH 6.5. In contrast, when tannic acid is added to Manduca sexta midgut fluid, less than 31% of the tannic acid added to the gut fluid is converted to a form that can be centrifuged into a pellet. The rest remains in the supernatant solution in the form of a colloidal suspension. Very little of the tannic acid, if any, remains in true solution. We suggest that the tannic acid-containing phase that is produced when tannic acid is added to midgut fluid is a complex multi-molecular aggregate of indefinite chemical composition, incorporating varying amounts of tannic acid, surface-active phospholipids, proteins, and polyvalent metal ions. On the basis of this study, we further suggest that the failure of tannins to diffuse across the peritrophic envelopes of lepidopteran larvae is a result of the capacity of the peritrophic envelope to act as a physical barrier to insoluble and colloidally dispersed particles, not the presence of substances in the matrix that strongly adsorb polyphenols or the presence of an extensive network of fixed anionic sites in the matrix that acts as an electrostatic barrier to the passage of polyphenolate anions. Arch. Insect Biochem. Physiol. 39:109–117, 1998.© 1998 Wiley-Liss, Inc.     

Pollinator-mediated interactions between a pathogenic fungus, Uromyces pisi (Pucciniaceae), and its host plant,Euphorbia cyparissias (Euphorbiaceae)

The plant Euphorbia cyparissias is commonly infected by rust fungi of the species complex Uromyces pisi. When infected, E. cyparissias is unable to flower, but instead is induced by the fungus to form pseudoflowers. Pseudoflowers are rosettes of yellow leaves upon which the fungus presents its gametes in a sweet-smelling fungal nectar. We hypothesized that the fungi, as they are heterothallic, are dependent on insect visitation to cross-fertilize their mating types. We confirmed that insects are required with an insect exclusion experiment. We further hypothesized that pseudoflowers of U. pisi interact with uninfected true host flowers through insects during their period of co-"flowering" in early spring. We conducted artificial array experiments in the field to test whether the two species share insects and whether they influenced each other's insect visitation. Insects moved between true flowers and pseudoflowers, but true flowers received more visits over all. Pseudoflowers and true flowers did not influence each other's visitation rates in mixtures. However, shorter visits were observed on pseudoflowers in mixtures than monocultures, suggesting that true flowers might be competitors for pseudoflowers. Further experiments are needed to determine whether the similarity of pseudoflowers to true flowers is adaptive.     

Olfactory response of Anagrus,nigriventrls (HYM.: Mymaridae): Effects of host plant chemical cues mediated by rearing and oviposition experience

The egg parasitoid Anagrus nigriventris Girault is an important natural enemy of the beet leafhopper, Circulifer tenellus (Baker) which has a broad host range that includes cultivated sugar beets and the weed, Russian thistle. When parasitoids were reared on leafhopper eggs deposited in sugar beets, females were attracted to sugar beet volatiles and preferred them over a blank control and over Russian thistle volatiles in Y- tube olfactometer tests. No preference was detected for either plant by female wasps that were reared on Russian thistle. Wasps reared on Russian thistle and allowed an oviposition experience on sugar beet showed a strong attraction to sugar beet volatiles; however, wasps reared on sugar beets and allowed oviposition experience on Russian thistle demonstrated no preference between the two plants. The implications of parasitoid response towards plant volatiles and their importance in biological control are discussed.     

美国白蛾丝素蛋白基因的鉴定、时空表达及取食不同寄主植物后的表达响应

美国白蛾Hyphantria cunea Drury被列为我国林业检疫性害虫,其l～4龄幼虫具有吐丝结网幕的习性.为探究丝素蛋白基因的表达特性,本研究利用PCR技术克隆了美国白蛾的HcP25(Genbank登录号:OL625670)、HcFib-H(Genbank登录号:OL625672)、HcFib-L(Genbank登录号:OL625671)3条丝素蛋白基因,并进行生物信息学分析;利用RT-qPCR技术检测美国白蛾3条丝素蛋白基因的表达特性.结果表明:3条丝素蛋白基因序列比对均与车前灯蛾Arctia plantaginis丝素蛋白一致性最高,系统进化分析显示丝素蛋白HcP25、HcFib-H、HcFib-L在鳞翅目不同科之间在出现较大的分化.RT-qPCR试验结果显示HcP25、HcFib-H、HcFib-L 3基因相对表达量与网幕产生高峰期(1龄、2龄)相一致.3种丝素蛋白均在丝腺中特异性高水平表达,在头部及脂肪体中有少量表达.美国白蛾幼虫取食不同寄主植物后,3种丝素蛋白基因呈现不同的表达规律;取食杨树Populus L.处理组中HcP25与HcFib-H基因相对表达量极显著高于取食其它寄主处理,而取食山樱花Cerasus serrulata var.lannesiana和日本晚樱Cerasus serrulata处理组中HcFib-L基因表达量最高.研究结果为进一步探究丝素蛋白介导的美国白蛾对不同寄主的适应机制及其扩散机制奠定基础,也为开发美国白蛾防治新方法提供了潜在的基因靶标.     

Molecular characterization and expression pattern of Spodoptera litura (Lepidoptera: Noctuidae) vitellogenin, and its response to lead stress

Vitellogenin (Vg) cDNA from Spodoptera litura Fabricius was cloned and sequenced. The open reading frame (ORF) of Vg cDNA was 5247 nucleotides in length (GenBank Accession no. EU095334), which encoded for a protein of 1748 amino acids. S. litura Vg comprised three conserved regions (Vitellogenin-N domain, DUF1943 and von Willebrand factor type D domain (VWD)), a 17 amino-acid signal peptide and a RXXR cleavage signal (RTIR). The highly conserved GL/ICG motif, the DGXR motif and cysteine residues were found in the C-terminus of the Vg. Vg mRNA was found specifically in the female fat body. Vg expression was first transcribed in 6th day female pupae and levels increased with insect development. The maximum level of Vg mRNA appeared in 24-h-old adults. When S. litura larvae were exposed to lead (Pb) (25-200 mg Pb/kg), there was a significant inhibition in Vg of female adults. The start of Vg expression was advanced ahead by Pb, from 6th day pupae to 3rd day or 4th day pupae. Low levels of Vg in male adults were also induced by low concentrations of Pb (12.5 and 25 mg Pb/kg). These data show that Pb stress elicits an important Vg response in S. litura.     

Hepatitis B surface antigen (HBsAg) infection in a hemodialysis unit. II. Factors affecting host immune response to HBsAg.

Serum from 86 hemodialysis patients, 105 healthy hospital staff "at risk" and 160 regular hospital staff was screened for hepatitis B surface antigen (HBsAg) and antibody (anti-HBs). The combined prevalence of HBsAg and anti-HBs was higher in the staff of the artificial kidney unit (57.7%) than in the hemodialysis patients (33.7%). The healthy subjects with HBsAg infection responded significantly more often by producing anti-HBs compared with the hemodialysis patients. Twelve of 29 (41.4%) hemodialysis patients with HBsAg infection produced anti-HBs, while 17 (58.6%) remained positive for HBsAg. This differential response could not be attributed to age, sex, time spent undergoing hemodialysis, delayed cutaneous reactivity or response to phytohemagglutinin (PHA) or pokeweed mitogen (PWM). However, a much larger proportion of patients with HBsAg than with anti-HBs had previously received blood transfusions (88.2% v. 33.3%). Our results indicate that development of the chronic HBsAg carrier state or production of anti-HBs in uremic patients may be influenced by the route of immunization or the dose of antigen, or both. Although uremic patients maintain normal in vitro response to PHA and PWM, they may have depressed immunity in vivo because of a decreased total number of T-lymphocytes.