Characterization and correlation of DC electrical penetration graph waveforms with feeding behavior of beet leafhopper, Circulifer tenellus

Feeding behavior of beet leafhopper, Circulifer tenellus (Baker) (Homoptera: Cicadellidae), was studied with a DC electrical penetration graph. Nine different electrical penetration graph waveforms associated with feeding were identified and characterized. Waveforms were correlated with specific feeding behaviors using a number of techniques, including high magnification video recording, honeydew analysis, stylectomy, and histological processing. Waveforms were grouped into three phases based on feeding behavior: pathway phase (waveforms A, B1, B2, and C), non-phloem ingestion phase (waveform G), and phloem phase (waveforms D1, D2, D3, and D4). No ingestion was found to occur during waveforms A, B1, B2, and C. Waveform G was associated primarily with ingestion of xylem sap and occasionally with ingestion of mesophyll sap. Stylet tips were located in phloem during waveforms D1, D2, and D3, and waveforms D2 and D3 were correlated with ingestion of phloem sap. Waveform D4 probably also plays a role in phloem ingestion, because D4 is very brief and always occurs embedded in either waveform D2 or D3. In contrast to most other homopteran insects, rate of honeydew production (and hence rate of ingestion) was much lower on phloem than on xylem. More rapid rates of ingestion are expected on phloem, because its high turgor pressure drives sap into the feeding insect whereas the negative pressure of xylem sap is expected to cause a slow rate of ingestion. The very slow ingestion rate of beet leafhopper feeding on phloem suggests that it is not able to exploit the high turgor pressure of phloem to achieve the high rate of ingestion that is typical of phloem ingestion by other insects.