Integrating multiple criteria for the characterization of Psammotettix populations in European cereal fields.

The wheat dwarf disease is among the most damaging diseases in cereals. Its aetiological agent is the Wheat dwarf virus (WDV), which is exclusively transmitted from plant to plant by leafhoppers from the genus Psammotettix (Hemiptera, Cicadellidae). The parameters linked to the WDV/Psammotettix pathosystem are still poorly understood. We studied Psammotettix individuals collected in wheat and barley fields in France and, as a comparison, from grassland at agroecological interface in West Slovenia. Species identity of males and females has been determined using multiple criteria. In the first step, the characterization of the collected individuals included recordings of vibrational signals used in mating behaviour and morphometric analyses. In addition, a 442 nt sequence of the mitochondrial cytochrome oxydase I (COI) gene was obtained for some individuals and compared to COI sequences of the Psammotettix leafhoppers available in public databases. In the cereal fields in France, P sammotettix alienus was the most numerous species; however, it sometimes occurred together with Psammotettix confinis, while in the grasslands in Slovenia, the third syntopic species in Psammotettix community was Psammotettix helvolus. The temporal parameters of the P. alienus male calling song that were measured in this study were very similar to those measured in a previous study. The local biotic and/or abiotic parameters most likely influence the life history of Psammotettix leafhoppers, and the proportion of viruliferous individuals collected in cereal fields was 14.9%, while leafhoppers collected in Slovenia were virus-free. Taken together, results show that more detailed information on population structure of Psammotettix leafhoppers is crucial for providing an insight into the epidemiology of wheat dwarf disease.

Genetic differentiation of geographically separated populations of the southern green stink bug Nezara viridula (Hemiptera: Pentatomidae).

Genetic variation in the southern green stink bug Nezara viridula (Linnaeus) from 11 geographically separated sampling locations (Slovenia, France, Greece, Italy, Madeira, Japan, Guadeloupe, Galapagos, California, Brazil and Botswana) was studied by sequencing 16S and 28S rDNA, cytochrome b and cytochrome c oxidase subunit I gene fragments and random amplified polymorphic DNA (RAPD) analysis. Sequencing revealed 11 distinct haplotypes clustering into lineages A, B and C. Lineage C was characteristic for a single analysed specimen from Botswana. Lineage B was detected in Japan, and it probably arose in Asia. Haplotypes of European and American specimens belonged to lineage A; specimens from France, Slovenia, Madeira and Brazil shared highly similar haplotypes (>99%) from subgroup A1, while all the specimens from Greece, California, Galapagos and Guadeloupe shared a haplotype from subgroup A2. RAPD data were more variable but consistent with mtDNA sequences, revealing the same clustering. They separated the Botswanian specimen from Japanese specimens and from a group of more closely related specimens from Europe and America. Sequence and RAPD results both support the African origin of N. viridula, followed by dispersal to Asia (lineage B) and, more recently, by expansion to Europe and America (lineage A). RAPD analysis revealed two highly supported subgroups in Japan, congruent with mtDNA lineages A2 and B, suggesting multiple colonization of Japan. Invariant sequences at the 28S rDNA combined with other results do not support the hypothesis that cryptic (sibling) species exist within the populations investigated in this study.

Variability of vibratory signals and mate choice selectivity in the southern green stink bug.

The southern green stink bug, Nezara viridula, has a complex mate recognition system that implicates chemical and acoustic signals. To localise a sexual partner acoustically, a male and female alternate between the male courtship song (MCrS) and the female calling song type 1 (FCS1). Although previous research has revealed that both signals show geographical variability, until now no studies have explored the form of this variability. We analysed the temporal and spectral characteristics of MCrS and FCS1 pulse trains of males and females from a French and a Guadeloupe population. Pulse train duration of the MCrS varied within and between populations. Likewise, spectral and temporal parameters of FCS1 varied within and between the two populations. Although females did not show any behavioural responses to pre-recorded MCrS, males responded to pre-recorded FCS1 by emitting a higher number of MCrS per minute. Furthermore, males modulated the repetition rate of their MCrS pulse trains to match those of the FCS1. All males responded to FCS1 from French and Guadeloupe females despite the temporal differences in these songs; however, they responded with a shorter latency and a higher rate of MCrS/FCS1 pulse trains to the songs of females from their own population. In choice experiments with two alternatives, responses to FCS1 were inhibited when males received a simultaneous female call from another Pentatomidae sympatric species, Acrosternum hilare. We conclude that, although males prefer FCS1 from their own population, they recognise FCS1 from French and Guadeloupe N. viridula females as species-specific female calls. Variability of vibratory signals might play a role in mate choice.

Temporal and spectral properties of the songs of the southern green stink bug Nezara viridula (L.) from Slovenia.

Substrate born songs of the southern green stinkbug Nezara viridula (L.) from Slovenia were recorded and analysed. The male calling song is composed of narrow-band regularly repeated single pulses and of broad-band frequency modulated pulses grouped into pulse trains. The female calling song is characterised by broad-band pulsed and narrow-band non-pulsed pulse trains. A frequency modulated pre-pulse precedes the narrow-band pulse train. A frequency-modulated post-pulse usually follows the pulse train of the male courtship song. The male calling song triggers broad-band pulse trains of the female courtship song. The female also produces a repelling low-frequency vibration that inhibits male calling and courtship. The male rival song is characterised by prolonged pulses with a typical frequency modulation.

Green stink bug Nezara viridula detects differences in amplitude between courtship song vibrations at stem and petiolus.

Green stink bug Nezara viridula courtship songs are transmitted through plants as substrate vibrations. The amplitude of the vibrations is different at different distances from the source of vibration and at different locations on the plant. Amplitudes of the local vibration were measured on stem and petioli of the bean plant (Phaseolus vulgaris) with a Laser-Doppler vibrometer. Differences of the amplitudes of vibration between adjacent points around the nodes were large enough to release differential nerve activities of vibration receptor cells of different legs. There was no correlation between the signal amplitude and the distance from the singing bug, however; the differences in amplitudes of vibrations between the stem and the adjacent petioli of leaves potentially permitted direction finding in the green stink bug males.

Vibratory songs of hybrids from Brazilian and Slovenian populations of the green stink bug Nezara viridula.

Males and females of the green stink bug Nezara viridula produce species and sex specific vibrational signals. The songs of bugs from geographically isolated population differ in their temporal characteristics. Hybrids were produced between the bugs from Brazilian and Slovenian populations in order to examine the levels of genetic inheritance of their vibratory songs. Hybrid males and females produced songs which are distinctly different from parental songs and these differences can be attributable to genetic factor. The results show that in some parameters the hybrid songs are intermediate between the parental types. Several song parameters are apparently sex-linked. It remains to be established whether observed genetically determined differences in vibratory songs also indicate that cryptic species exist within the taxon N. viridula.

Vibrational directionality in the southern green stink bug, Nezara viridula (L.), is mediated by female song.

We tested the hypothesis that male southern green stink bugs, Nezara viridula (L.), use substrate-borne songs to locate females. We recorded the responses of bugs on plants to the vibrations caused by a prerecorded female song and by an artificial sound. The female song caused males to walk, to respond with the calling and courtship songs and to approach the source of the song with characteristic search behaviour at junctions between branches on the plants. At a junction, a searching male stopped, stretched his legs and antennae and compared the vibratory signals on the two branches, with different combinations of legs and antennae. The males then left the junction and approached the source of the vibration. Males located the loudspeaker significantly more frequently in the presence than in the absence of vibratory stimuli on cyperus, Cyperus alternifolius L., and beans, Phaseolus vulgaris L. Vibrational directionality was also elicited by artificial pure tones whose spectral and temporal parameters were similar to those of natural female song. Females showed no reaction to vibratory stimulation and no vibrational directionality. We discuss possible mechanisms underlying vibrational directionality in the light of expected signal changes during transmission through plants. Copyright 1999 The Association for the Study of Animal Behaviour.

Neurons projecting from the brain to the corpora allata in orthopteroid insects: anatomy and physiology.

Retrograde and orthograde labeling of neurons projecting to the corpus allatum was performed in locust, grasshopper, cricket, and cockroach species in order to identify brain neurons that may be involved in the regulation of juvenile hormone production. In the acridid grasshopper Gomphocerus rufus L., and the locusts Locusta migratoria (R.&F.) and Schistocerca gregaria Forskal, the corpora allata are innervated by two morphologically distinguishable types of brain neurons. One group of 9-13 neurons (depending on species) with somata in the pars lateralis extend axons via the nervus corporis cardiaci 2 and nervus corporis allati 1 to the ipsilateral corpus allatum, whereas two cells in each pars lateralis have bilateral projections and innervate both glands. No direct connection between the pars intercerebralis and corpus allatum has been found. In contrast, neurons with paired axons innervating both glands are not present in Periplaneta americana (L.) and Gryllus bimaculatus de Geer. Instead, two cells in each pars lateralis project only to the gland contralateral to their somata. Electrophysiological experiments on acridid grasshoppers have confirmed the existence of a direct conduction pathway between the two glands via the paired axons of four cells that have been identified by neuroanatomy. These cells are not spontaneously active under experimental conditions. Ongoing discharges in the left and right nerves are unrelated, suggesting that the corpora allata receive independent neuronal inputs from the brain.

Nervous control of juvenile hormone biosynthesis in Locusta migratoria.

In Locusta migratoria migratorioides R. and F., two types of brain neurons innervate the juvenile hormone (JH)-producing corpora allata (CA). Thirteen cells in each pars lateralis (PL) innervate the ipsilateral CA, while four cells (two in each PL) innervate both glands. We investigated possible influences of these two neuronal types on JH production by a newly developed method. A radiochemical assay was used to measure hourly JH production by a CA with intact nerve connections to the brain. Then, changes in hormone production due to selective nerve stimulation or transection were assessed. In control preparations JH production per h remained approximately constant for at least 9 h. Simultaneous electrical stimulation of all neurons innervating one CA (i.e., 13 ipsilateral plus 4 bilaterally innervating cells) always inhibited JH production, while their transection led to a rapid progressive increase in JH biosynthesis in CA from females with oocytes longer than 4.5 mm. Thus, there is strong neurally mediated inhibition of the CA at certain phases of the vitellogenic cycle. The dramatic effects of nerve transection show that in vitro rates of JH production are an unreliable indicator of in vivo levels. Selective stimulation of the four neurons innervating both CA suggests that they do modulate JH biosynthesis but the effect varies qualitatively depending on the phase of the vitellogenic cycle.