Salivary deposits and plant damage associated with specific probing behaviors of the potato leafhopper, Empoasca fabae, on alfalfa stems
1990
Kabrick, Laura R. | Backus, E. A. (Elaine A.)
The feeding behaviors and salivary deposits of the potato leafhopper, Empoasca fabae (Harris), on stems of a susceptible cultivar of alfalfa (Medicago sativa, L.) were studied in relation to the cellular damage and initiation of hopperburn they cause. An AC electronic monitoring system (EMS) was used to quantify the number and duration of probes made on alfalfa stems. Light and electron microscopy were then used to examine the effects of these probes. Most probes were short in duration as determined by EMS and comprised primarily of multiple-cell laceration behavior (represented by the Ia waveform). Probes containing still-stylet ingestion behavior (represented by the Ic waveform) were longer in duration and occurred less frequently. There was a close correspondence between the number of probes containing the Ic waveform and number of salivary deposits. Histological examinations of the stems revealed six types of salivary deposits but only one of these resembled the solid, distinct (or ‘true') salivary sheaths characteristic of sheath-feeders. Less than 25% of E. fabae probes left any salivary deposit, and only 1% left true sheaths. Therefore, it seems unlikely that these deposits could physically block the phloem to cause hopperburn. Cellular disruptions, such as abnormally divided and enlarged cells, were found in the vascular cambium and phloem, as well as other tissues. Enlarged vascular cambial cells appeared to crush the phloem cells, and thus probably caused hopperburn. This microscopic damage was initiated by vigorous, active probing, along with the copious watery saliva that is secreted during the behavior represented by Ia. These observations strongly support Medler's (1941) hypothesis that hopperburn is caused by a substance in the insect's saliva that induces plant cells to enlarge and block the phloem.
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