Five distinct electrical penetration graph waveforms characterising the feeding behaviour of the leafhopper Cicadulina mbila Naudé (Homoptera: Cicadellidae) on maize (Zea mays L.) were obtained using a DC based system. The waveforms were distinguished by spectral features and by statistical analysis of their median voltages, durations and time to first waveform recording. By changing the polarity of the system voltage and the level of the input resistor it was shown that the waveforms are mainly determined by the electromotive force (emf) component. Based on the correlation between waveforms and the fine structure of the stylet pathways observed by transmission electron microscopy, insect's activities have been associated with five waveforms: stylet pathway formation (waveform 1), active ingestion (waveform 2), putative stylet work (waveform 3), salivation (waveform 4) and passive ingestion (waveform 5). Like waveform E1 and E2 of aphids, waveforms 4 and 5 of C. mbila correspond to feeding activities in sieve tubes. However, unlike aphids which probe briefly in non‐vascular cells, waveform 2 corresponds to active ingestion in cells, where the cell content is partially ingested and hence the organelles' integrity severely affected. These observations suggest that this specific feeding feature, typical of leafhoppers, determines their ability to acquire geminivirus virions located in the plant cell nucleus.
Spatial and temporal distribution of Maize streak virus (MSV, family Geminiviridae, genus Mastrevirus) was monitored in the vector species Cicadulina mbila and the nonvector species C. chinaï using conventional and real-time quantitative polymerase chain reaction. Sustained feeding on MSV-infected plants showed that virus accumulation reaches a maximum in C. chinaï, but not in C. mbila. After a 3-day acquisition access feeding period (AAP), MSV was detected in the gut, the hemolymph, and the head of C. mbila, but only in the gut of C. chinaï. Similarly, Digitaria streak virus (genus Mastrevirus), which is not transmitted by either of the two species, was only detected in the gut. MSV was detected in the hemolymph of C. mbila 3 h after the beginning of the AAP. Although viral DNA progressively decreases in the vector and nonvector species after a 3-day AAP, MSV DNA remained stable in the salivary glands of C. mbila.
The whitefly Bemisia tabaci is a worldwide pest considered to be a complex of cryptic species. Its identification is primarily based on mitochondrial COI sequences. Microsatellite markers can also be used to identify this species and to study its population structure. Moreover, B. tabaci species harbour quite specific endosymbiont communities that can constitute additional markers for identification. Previous studies have demonstrated the occurrence of two B. tabaci species on the island of La Réunion: the exotic MEAM1 and the indigenous IO. A recent field survey revealed unusually high densities of whiteflies in Saint Pierre, and this could reflect the introduction of a new B. tabaci species. To test the putative introduction of a new invasive species, B. tabaci individuals were collected at Saint Pierre and at six additional sites. Among these samples, analyses were conducted on mitochondrial COI, microsatellites, and endosymbiont communities. Based on the mitochondrial COI marker, we detected in Saint Pierre two subgroups (Eastern and Western) characteristic of the invasive MED species. This is the first time these subgroups have been found in La Réunion. The analysis of microsatellite data from the seven sites around the island showed that (i) the geographical distribution of MED seems confined to Saint Pierre, (ii) no hybrids were detected between MED individuals and the resident species, and (iii) the MED population, while exhibiting two mitochondrial haplotypes, has a homogenous nuclear genetic background. The two MED subgroups differing in their mitochondrial haplotype also specifically differed in their endosymbiotic diversity.
IntroductionHuanglonbing (HLB) is the most serious disease of citrus in the world, associated with three non-cultivable phloem-restricted bacteria Candidatus Liberibacter asiaticus (CLas), Ca L. africanus (CLaf) and Ca L. americanus (CLam). CLas is transmitted by the Asian citrus psyllid Diaphorina citri, and has spread to several countries. The African psyllid Trioza erytreae, the vector of CLaf occurs in Africa and neighbouring islands. Only two major citrus-growing regions - Australia/New Zealand and the Mediterranean Basin - are still HLB-free in the world. However, T. erytreae has recently been introduced into continental Europe (Portugal and Spain) and has become a potential threat to citrus production. The transmission of CLas by T. erytreae had been postulated but never tested. To evaluate the risk of T. erytreae transmitting CLas, comparative transmissions of CLas by T. erytreae and D. citri were assessed.MethodsTransmission tests were performed on excised leaves and seedlings of Citrus volkameriana with different inoculation access periods (in series) for both insect species. Quantifications of bacterial titers were made in excised leaves, seedlings three and six months after inoculation and on individual insects.ResultsOur results showed that T. erytreae was able to efficiently acquire CLas. Furthermore, T. erytreae carried significantly higher bacterial titers than D. citri, and was able to efficiently transmit the bacteria to seedlings at a similar rate that D. citri highlighting the high risk of spread of the most aggressive variant of HLB (CLas) by T. erytreae in Europe.DiscussionThus, extreme precautions to prevent any entry of CLas into Europe should be adopted.
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