Submitted to: PLoS One
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/23/2013
Publication Date: 3/26/2013
Citation: Ammar, E.D., Hall, D.G., Shatters, R.G. 2013. Stylet morphometrics and citrus leaf vein structure in relation to feeding behavior of the Asian citrus psyllid Diaphorina citri, vector of citrus huanglongbing bacterium. PLoS One. 8(3):e59914. Doi:10.1371/journal.pone.0059914. Interpretive Summary: The feeding behavior and stylet length were studied in nymphs and adults of the Asian citrus psyllid (ACP), vector of Las bacterium associated with citrus huanglongbing (HLB) disease. Younger ACP nymphs feed only on young citrus leaves on smaller veins or on the sides of the midrib, whereas adults can feed anywhere on the veins of young or old citrus leaves. These nymphs can reach the phloem because the distance to the phloem is shorter from the sides of the midrib compared to that from the top, and is considerably shorter in younger than in older/mature leaves. Electron microscopy indicated that the width of the maxillary salivary canal in first instar nymphs may not be wide enough for Las bacteria to go through during salivation (and inoculation of Las bacterium) into host plants. This may explain previous studies indicating that older ACP nymphs and adults can transmit HLB bacterium whereas younger nymphs probably cannot.
Technical Abstract: The Asian citrus psyllid (ACP), Diaphorina citri (Hemiptera: Psyllidae), is the primary vector of the phloem-limited bacterium Candidatus Liberibacter asiaticus (LAS) associated with huanglongbing (HLB, citrus greening), considered the world’s most serious disease of citrus. Stylet morphometrics of ACP nymphs and adults were studied in relation to citrus vein structure feeding sites on Valencia orange leaves. ACP nymphs preferred to settle and feed on the lower (abaxial) side of young leaves either on secondary veins or on the sides of the midrib, whereas adults preferred to settle and feed on the upper (adaxial) or lower secondary veins of young or old leaves. Early instar nymphs can reach and probe the phloem probably because the distance to the phloem is considerably shorter in younger than in mature leaves, and is shorter from the sides of the midrib compared to that from the center. Additionally, the thick-walled ‘fibrous ring’ (sclerenchyma) around the phloem, which may act as a barrier to ACP stylet penetration into the phloem, is more prominent in older than in younger leaves and in the center than on the sides of the midrib. The majority (80–90%) of the salivary sheath termini produced by ACP nymphs and adults that reached a vascular bundle were associated with the phloem, whereas only 10–20% were associated with xylem vessels. Ultrastructural studies on ACP stylets and LAS-infected leaves suggested that the width of the maxillary food canal in first instar nymphs is wide enough for LAS bacteria to traverse during food ingestion (and LAS acquisition). However, the width of the maxillary salivary canal in these nymphs may not be wide enough to accommodate LAS bacteria during salivation (and LAS inoculation) into host plants. This may explain the inability of early instar nymphs to transmit LAS/HLB in earlier reports.