Peritrophic membrane structure and secretion in European corn borer larvae (Ostrinia nubilalis)

Peritrophic membrane (PM) secretion and formation occur primarily in the anterior region of the mesenteron in the European corn borer (Ostrinia nubilalis) as determined by light and electron microscopy. Nascent PM first became visible as fibrous linear chitin-containing structures stained with gold-labeled wheat germ agglutinin between and at the tips of the microvilli. No formed PM was visible at the foregut-midgut junction, but a thin single PM appeared first in the lumen between the stomodeal valves and the midgut epithelium. Just posterior to the stomodeal valves, multiple PMs were observed that became progressively thicker and more numerous in the mid and posterior regions of the mesenteron. The PM consists of an orthogonal chitin meshwork with openings slightly larger than the diameters of the microvilli. As it delaminates from the microvilli, the meshwork becomes embedded in proteinaceous matrix that greatly reduces the pore size of the PM.     

Regulation of pheromone biosynthesis in the "Z strain" of the European corn borer, Ostrinia nubilalis

The regulation of pheromone biosynthesis by the neuropeptide PBAN in the Z strain of the European corn borer, Ostrinia nubilalis, was investigated using labeled intermediates. Injection of radiolabeled acetate showed PBAN did not influence the de novo synthesis of saturated fatty acids in the gland. When deuterium-labeled myristic acid was topically applied to the gland, females injected with PBAN produced more labeled pheromone than did control females, indicating that PBAN controls one of the later steps of pheromone biosynthesis. Although more myristic acid was Delta11-desaturated in the gland in the presence of PBAN, this was counterbalanced by less Delta11-desaturation of palmitic acid, indicating that desaturase activity did not change overall. This change in flux of myristic acid through to pheromone was shown to be caused by increased reduction of fatty acid pheromone precursors occurring in the presence of PBAN.     

Age- and density-related oviposition behavior of the European corn borer,Ostrinia nubilalis (Lepidoptera: Pyralidae)

Age-related oviposition patterns ofOstrinia nubilalis were studied at three population densities in the laboratory by releasing newly eclosed adults in wire-screened cages and analyzing their oviposition throughout the adult stage with digital analysis. Oviposition sequences of individual females depositing egg masses were documented on the third and seventh nights after eclosion with a video camcorder. During a sequence, a female produced an egg in an average time of 15 or 26 s on the third and seventh nights, respectively, and completed depositing an egg mass the size of 20–39 eggs in an average time of 316 and 525 s, respectively. Females were not easily disturbed during egg mass deposition and pulsated their abdomen before deposition of each egg. Females produced few egg masses the first night after eclosion. Oviposition increased on nights 2 and 3 but declined steadily thereafter as females matured. Females older than 6 nights produced fewer egg masses; the proportion of egg masses with fewer than 20 eggs increased gradually. By the end of the adults' lifetime, nearly 100% of the egg masses had fewer than 20 eggs. The data are fundamental to our research to define the role of phytochemicals in modifying oviposition behavior of the European corn borer.     

Linking larval nutrition to adult reproductive traits in the European corn borer Ostrinia nubilalis

Throughout an organism's lifetime, resources are strategically allocated to many different functions, including reproduction. Reproduction can be costly for both sexes; females produce nutrient‐rich eggs, whereas males of many species produce large and complex ejaculates. In capital breeding insects, nutrients are mainly acquired during the larval period, yet allocation decisions impact the reproductive fitness of adults. The present study examines the effect of larval dietary nitrogen on both male and female reproductive traits in the European corn borer moth Ostrinia nubilalis Hübner, whose adults do not feed and whose males transfer a large, nitrogen‐rich spermatophore. One day post‐eclosion, O. nubilalis adults reared on one of three different diets (3.0%, 1.6%, or 1.1% nitrogen) are mated and two experiments are undertaken: one to measure nitrogen and carbon content of male ejaculates, and the other to determine female fecundity and fertility. Although male larval diet does not alter the percentage nitrogen content of adult somatic tissue, males reared on the higher nitrogen diet (3.0%) produce spermatophores with increased nitrogen relative to somatic nitrogen. Furthermore, females raised on the 3.0% nitrogen diet receive spermatophores with lower carbon : nitrogen ratios and thus more nitrogen. Overall, females lay more eggs as their larval dietary nitrogen increases, although they lay fewer eggs when their mates are raised on the higher (3.0%) nitrogen diet. This suggests that O. nubilalis females may use male‐derived nitrogen not to supplement egg production, but rather for somatic maintenance. Overall, the present study furthers our understanding of how larval diet can affect adult fitness in Lepidoptera.     

Dissemination of Nosema pyrausta in feral populations of the European corn borer,Ostrinia nubilalis

Epizootiological studies of Nosema pyrausta in natural European corn borer populations show that while vertical transmission is the primary way in which N. pyrausta is transferred from one host generation to the next, it is horizontal transmission that is responsible for the annual build-up of infection in each nonoverlapping generation. During the first generation, larval corn borer migration to adjacent corn stalks is minimal and increases in the prevalence of N. pyrausta within the population result from horizontal transmission of infection among borers that inhabit the same stalk. During the second generation, corn borer larvae actively disperse to other corn plants and this results in an increased level of infection. Factors which facilitate pathogen dispersal in this generation include (1) higher host densities, (2) longer periods of larval development, (3) lower mortality among young larvae, and (4) possible mechanical transmission by the braconid parasitoid, Macrocentrus grandii.     

Regulation of larval diapause by juvenile hormone in the European corn borer,Ostrinia nubilalis

The effects of juvenile hormone (JH) on the larval diapause of the European corn borer, Ostrinia nubilalis, were investigated. The larval period was prolonged and the development of gonads was retarded if non-diapause larvae were treated with JH. Pupation of diapausing larvae was accelerated by allatectomy. It was concluded that the diapause of this borer is largely regulated by JH.     

Behavioural and electrophysiological responses of the European corn borer Ostrinia nubilalis to host‐plant volatiles and related chemicals

The European corn borer Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) is a worldwide pest of maize (Zea mays L.) and other crops. The semiochemicals released by maize plants and structurally‐related compounds can be used by adult female O. nubilalis for host‐plant location and oviposition. Headspace volatile compounds emitted by watered and water‐deprived maize plants are collected and identified by their retention indices and mass spectra. The most abundant compounds from watered plants are limonene, linalool, benzoic acid, indole, β‐caryophyllene and acetophenone, whereas, in water‐deprived plants, limonene, acetophenone, hexanoic acid, benzoic acid and indole are dominant. In addition, (E)‐4,8‐dimethyl‐1,3,7‐nonatriene, 6‐methyl‐5‐hepten‐2‐one, anisole and 1‐carvone are undetected in the water‐deprived plants. Some of the identified compounds show electrophysiological activity (electroantennogram) in the antennae of both sexes, with the responses elicited by tridecane, tetradecane, dodecane, nonanal, decanal and 2‐ethylhexanol on males being particularly noteworthy. In a dual‐choice olfactometer, adult females show a preference for 2‐hexanol, heptanal, methyl salicylate, hexyl acetate, nonanal, methyl dodecanoate, β‐pinene and (E)‐2‐hexenyl acetate over hexane controls. Tetradecane, linalool, methyl hexanoate, methyl nonanoate, (Z)‐3‐hexenyl benzoate, tridecane, 2‐cyclopentylcyclopentanone, 3‐methylbutyl acetate, β‐myrcene and (Z)‐3‐hexenyl butanoate result in fewer females in the test arm compared with the control arm. No single compound displays an activity similar to watered maize plants, supporting the hypothesis that blends of volatiles in specific ratios are more effective than single volatile chemicals. The results of the present study suggest that methyl salicylate, which elicits also one of the highest electrophysiological responses in female antennae, plays a role in host preference by O. nubilalis females.     

Concerted evolution of male and female display traits in the European corn borer, Ostrinia nubilalis

Background  

Sexual reproduction entails the encounter of the sexes and the multiplicity of rituals is parallel to the diversity of mating systems. Evolutionary mechanisms such as sexual selection and sexual conflict have led to the elaboration of traits to gain attention and favours from potential partners. A paradox exists about how coordinated systems can evolve and diverge when there would seem to be a stabilising selection acting. Moth display traits – pheromones – constitute an advantageous model with which to address questions about the evolution of mating systems in animals. Both males and females can possess pheromones that are involved either in close- or long-range communication. Female and male pheromones appear to have different origins and to be under different evolutionary constraints, thus they might be envisioned as independently evolving traits. We conducted laboratory experiments to explore the role of scents released during courtship by males of the European corn borer, Ostrinia nubilalis.     

Distribution of putative neurotransmitters in the brain of the European corn borer, Ostrinia nubilalis.

Radioautographic localization of three putative neurotransmitters in the brain of larvae of the European corn borer was studied by electron microscopy. Acetylcholine and dopamine were found to be accumulated in median neurosecretory cells. Both acetylcholine and dopamine were preferentially found in glial cells as compared to neuronal cells. This may be indicative of active participation of glial cells in a blood-brain barrier in the intact brain. Gamma-aminobutyric acid was not accumulated by any particular cell in the brain, but was apparently excluded to the periphery of the ganglion.It is suggested that dopamine, or more likely an immediate metabolite may be involved in an endogenous time measuring system and/or diapause induction-termination.     

Molecular differentiation at nuclear loci in French host races of the European corn borer (Ostrinia nubilalis)

French populations of the European corn borer consist of two sympatric and genetically differentiated host races. As such, they are well suited to study processes that could be involved in sympatric speciation, but the initial conditions of host-race divergence need to be elucidated. Gene genealogies can provide insight into the processes involved in speciation. We used DNA sequences of four nuclear genes to (1) document the genetic structure of the two French host races previously delineated with allozyme markers, (2) find genes directly or indirectly involved in reproductive isolation between host races, and (3) estimate the time since divergence of the two taxa and see whether this estimate is compatible with this divergence being the result of a host shift onto maize after its introduction into Europe approximately 500 years ago. Gene genealogies revealed extensive shared polymorphism, but confirmed the previously observed genetic differentiation between the two host races. Significant departures from the predictions of neutral molecular evolution models were detected at three loci but were apparently unrelated to reproductive isolation between host races. Estimates of time since divergence between French host races varied from approximately 75,000 to approximately 150,000 years, suggesting that the two taxa diverged recently but probably long before the introduction of maize into Europe.     

Sex pheromone biosynthesis in the Asian corn borer Ostrinia furnacalis (II): Biosynthesis of (E)- and (Z)-12-tetradecenyl acetate involves Δ14 desaturation

Sex pheromone biosynthesis in the Asian corn borer Ostrinia furnacalis was studied by topical application of deuterium labelled fatty acids to the pheromone gland. The incorporation of the labelled acids into pheromone components and precursors was determined by gas chromatography with flame ionization detection and mass spectrometry in the selected ion monitoring mode. The labelling experiments suggest that the pheromone components (E)- and (Z)-12-tetradecenyl acetates are biosynthesized from palmitic acid by δ14 desaturation, followed by chain shortening (β-oxidation), reduction, and acetylation. This is the first confirmation of a Δ14 desaturase in an eukaryotic system.     

Ecdysteroid synthesis by testes of 5th instars and pupae of the European corn borer,Ostrinia nubilalis (Hubner)

Summary

Testes from young fifth instar Ostrinia nubilalis produced very small amounts of ecdysteroids while those from larvae that had purged their gut produced considerably more immunodetectable ecdysteroid in vitro. Larval testes that had fused produced 2.2 times more ecdysteroid than those that remained separate. It was the sheath of the testes rather than the contents that was physiologically active. Synthesis was questionable in testes from day-1 pupae and was not observed in testes from pharate pupae, from day-2 pupae or from pharate adults. Thus, synthesis only occurred at specific times in the life cycle. Ecdysteroid profiles for testes from wandering larvae whose testes had fused showed a net increase in all normally observed ecdysteroids, with the greatest increase being in 20-hydroxyecdysone. For testes from day-1 pupae, the nature of the ecdysteroid profile changed after 24 h of incubation, with some ecdysteroids showing increases and other decreases. There appear to be considerable differences among species regarding the times of testis synthesis and the amounts and nature of the ecdysteroids synthesized.     

Sex-linked pheromone receptor genes of the European corn borer, Ostrinia nubilalis, are in tandem arrays

Background

Tuning of the olfactory system of male moths to conspecific female sex pheromones is crucial for correct species recognition; however, little is known about the genetic changes that drive speciation in this system. Moths of the genus Ostrinia are good models to elucidate this question, since significant differences in pheromone blends are observed within and among species. Odorant receptors (ORs) play a critical role in recognition of female sex pheromones; eight types of OR genes expressed in male antennae were previously reported in Ostrinia moths.

Methodology/Principal Findings

We screened an O. nubilalis bacterial artificial chromosome (BAC) library by PCR, and constructed three contigs from isolated clones containing the reported OR genes. Fluorescence in situ hybridization (FISH) analysis using these clones as probes demonstrated that the largest contig, which contained eight OR genes, was located on the Z chromosome; two others harboring two and one OR genes were found on two autosomes. Sequence determination of BAC clones revealed the Z-linked OR genes were closely related and tandemly arrayed; moreover, four of them shared 181-bp direct repeats spanning exon 7 and intron 7.

Conclusions/Significance

This is the first report of tandemly arrayed sex pheromone receptor genes in Lepidoptera. The localization of an OR gene cluster on the Z chromosome agrees with previous findings for a Z-linked locus responsible for O. nubilalis male behavioral response to sex pheromone. The 181-bp direct repeats might enhance gene duplications by unequal crossovers. An autosomal locus responsible for male response to sex pheromone in Heliothis virescens and H. subflexa was recently reported to contain at least four OR genes. Taken together, these findings support the hypothesis that generation of additional copies of OR genes can increase the potential for male moths to acquire altered specificity for pheromone components, and accordingly, facilitate differentiation of sex pheromones.     

The role of juvenile hormone in the larval diapause of the European corn borer,Ostrinia nubilalis

The possible role of juvenile hormone (JH) in the induction and termination of larval diapause in the European corn borer, Ostrinia nubilalis, was investigated using topical applications of both JH I and a JH mimic as well as by monitoring JH titers with the Galleria bioassay. Neither JH nor the JH mimic ZR515 was capable of influencing diapause termination when administered topically. The Galleria bioassay revealed little or no JH in the hemolymph of mid diapause (>30 days) insects, indicating no demonstrable role for JH in diapause maintenance. When ZR515 was administered to nondiapause, newly ecdysed fifth instar larvae the pupal molting cycle was delayed. By use of photoperiodic regimes we were able to show that the molting delay was not equivalent to diapause induction. The Galleria bioassay showed differences in JH titer profiles between diapause and nondiapause animals during the final larval stadium. The nondiapause insects showed titers that decline rapidly to trace amounts following the molt to fifth instar then rose prior to pupation. The diapause insects had generally higher titers and exhibited a more gradual decline after the molt. No evidence was obtained to support the hypothesis that JH plays a key role in the induction, maintenance, or termination of larval diapause.     

Intercrop effects on the distribution of populations of the European corn borer, Ostrinia nubilalis, in maize

Field investigations conducted from 1982 to 1984 to assess intercrop effects on the distribution of Ostrinia nubilalis (Hübner), Lepidoptera, Pyralidae, in maize revealed that the incidence of damage by O. nubilalis was influenced significantly by the proximity of maize to potato. Maize fields adjacent to potato fields had a lower incidence of early season damage by O. nubilalis and a higher incidence of late-season damage than maize isolated from potato.     

Sex pheromone blend discrimination in two races and hybrids of the European corn borer moth, Ostrinia nubilalis

Abstract. The response specificity of males of two races, and hybrids, of the European corn borer moth, to a large series of doses/ratios of E- and Z11-14:OAc, was determined in a sustained-flight tunnel. For both races an area of peak response was determined, which included the natural ratio eliciting peak levels of source contact, and other treatments eliciting response profiles over the behavioural sequence not significantly different from the peak. Consistent with studies on other moth species response specificity was controlled by two threshold effects, one affecting locking-on to odour plumes of lower doses and off-ratios containing lower proportions of the E or Z isomer than the natural blend, and the other resulting in arrestment of upwind flight to higher doses and off-ratios containing higher proportions of the E or Z isomer man the natural blend. A comparison of the size of the areas of peak response showed that males of the univoltine Z race (UZ) using a 3:97 E:Z mix displayed greater specificity and sensitivity than did males of the bivoltine E race (BE) responding to a 99:1 E:Z pheromone mix. At doses higher than those eliciting peak response (<100 μg) response specificity was lower for both races, but especially for the BE race, with increased levels of upwind flight and source contact occurring to off-ratios. Finally, consistent with a previous behaviour/genetic study on this species, F₁ hybrid males displayed peak levels of source contact not only to their natural isomer ratio (65:35 E:Z), but also to an expanded range of doses of ratios ranging from 2% to 98% E.