CRDF quarterly report July 2016 Electrical Penetration Graph (EPG) is a powerful tool for monitoring the cryptic feeding behaviors of piercing-sucking insects that feed on plant xylem and phloem tissues. While the insect probes, ingests, and salivates within the plant, characteristic voltage waveforms are produced that, in conjunction with histological studies, allow researchers to determine feeding patterns associated with pathogen acquisition and inoculation. We have shown that EPG can be used for detecting resistance in trifoliate citrus genotypes. Waveform annotations of feeding behavior of Asian citrus psyllids on trifoliate and citrus cultivars showed reduced phloem feeding on trifoliate accessions compared to trifoliate/citrus hybrids and citrus cultivars. Phloem feeding is required for acquisition and transmission of CLas. The observed low incidence of phloem feeding on P. trifoliata and trifoliate hybrids suggests a mechanism to explain the observed tolerance of citrus genotypes in the field, despite demonstrated susceptibility to the pathogen by graft inoculation. Poncirus trifoliata may possess physical traits that confer resistance to transmission by interfering with the vector’s ability to attain the phloem. Psyllid feeding may be hindered by physical barriers to stylet passage conferred by fibrous rings of sclerenchyma cells associated with vascular tissue in P. trifoliata. The presence of sclerenchymatous cells associated with the phloem may be a valuable trait for citrus breeding to produce scions that reduce or prevent ACP feeding and reduce CLas transmission. To study the role of the fibrous rings on ACP feeding behavior on emerging and fully expanded leaves of susceptible citrus, we recorded xylem and phloem feeding by EPG on adaxial (upper) and abaxial (lower) surface of young and old leaves. ACPs were allowed to feed on upper or lower surfaces of young and old leaves of Valencia (Citrus sinensis) for 21 hours. We monitored ACP feeding behaviors (np-non probing, c-intercellular stylet penetration, d-phloem penetration, e1-phloem salivation, e2-phloem ingestion, g-xylem ingestion). The waveforms produced from leaf tissues were manually annotated based on prior studies. Leaves fed upon during the EPG runs were sectioned and examined by flourescence microscopy to visualize the stylet paths and to correlate stylet position with EPG wavefoms. EPG waveform annotations showed that ACP feeding behavior on younger and older leaves was modified by the presence of sclerenchymatous cells associated with the phloem. Significantly longer phloem feeding was observed on upper surface of young leaves compared with lower and upper surfaces of older leaves. Although fewer phloem penetrations (d-e1-e2) were observed on younger leaf surfaces, they resulted in longer, sustained bouts of phloem feeding. This could be attributed to the reduced presence of sclerenchymatous rings in younger leaves that facilitates access to phloem compared with older leaves. Histological correlations showed that the fibrous rings are better developed in lower surface of older leaves, which may have resulted in decreased phloem feeding by ACPs. These findings have a significant role in understanding the feeding behavior of ACPs on young and older leaves of susceptible cultivars, and how it affects the acquisitions and transmission of HLB pathogen. Publications related to this project: George, J, PS Robbins, RT Alessandro, LL Stelinski, SL Lapointe. 2016. Formic and acetic acids in degradation products of plant volatiles elicit olfactory and behavioral responses from an insect vector. Chem. Senses 41 (4): 325-338. Lapointe, SL, J George, DG Hall. 2016. Phagostimulants for the Asian citrus psyllid (Hemiptera: Liviidae). J. Chem. Ecol. (accepted 6/29/16).