Background: Salivary sheaths, also known as stylet sheaths or stylet tracks, are essential features of the piercing-sucking feeding mechanism of plant-feeding hemipteran insects, many of which are vectors of economically important plant viral and bacterial pathogens. Although knowledge of their structure and function is incomplete, these salivary sheaths are frequently used by researchers to study hemipteran's feeding behavior, host preference, or host resistance, because these sheaths remain in the plant tissues after the insect withdraws its stylets following its feeding or probing on these tissues. However, in most cases, it is not known how long these salivary sheaths may last in plant tissues after their deposition by the feeding insects. An earlier report suggested that the salivary sheaths of the Asian citrus psyllid, Diaphorina citri (Hemiptera, Liviidae), vector of the devastating huanglongbing (citrus greening) disease bacterium, start to dissipate 1 week after their deposition in citrus leaves.
Methods and results: Here, using epifluorescence microscopy of cross sections in citron leaves, we found that D. citri salivary sheaths show signs of degradation in 3-4 weeks and become mostly degraded by 5-6 weeks, following their deposition by the psyllids into citrus tissues. Degradation of the salivary sheath starts at or near the "flange" area close to the leaf surface and continues gradually inward through the intercellular part of the sheath, within the mesophyll tissue, but apparently does not extend to the deeper or intracellular parts of the sheath in or near the phloem. Staining citron leaf sections with the fluorescent stain calcofluor white, which stains fungi, or propidium iodide (DNA/RNA stain) suggested that the degraded parts of the older salivary sheaths are not associated with fungi or bacterial accumulations.
Conclusion: We speculate that degradation of the salivary sheaths may be due to enzymatic activities in the host plant, especially in the extracellular matrix of the mesophyll tissue.
Keywords: Citrus greening; hemiptera; huanglongbing disease; psyllids; salivary sheaths.