Behavioral Plasticity in Probing by Diaphorina citri (Hemiptera, Liviidae): Ingestion from Phloem Versus Xylem is Influenced by Leaf Age and Surface

Abstract

Diaphorina citri is a major pest of citrus because it transmits Candidatus Liberibacter asiaticus, a phloem-limited bacterium that putatively causes Huanglongbing (HLB). The disease moves slowly through a tree, and the vector facilitates further within-tree movement via transmission of the pathogen. However, this only happens when D. citri stylets contact the phloem, to inoculate bacteria during phloem salivation and acquire bacteria during phloem sap ingestion. Behavioral changes in D. citri associated with different plant parts would affect how long it takes to reach phloem and how long the psyllids stays in phloem to ingest, thereby influencing the risk of disease spread. D. citri feeding was recorded on the abaxial and adaxial surfaces of mature and immature citrus leaves. Adults in the field can be found on these surfaces at all times of year. On abaxial surface of immature leaves, phloem salivation would occur after 11 h on average, but rarely as soon as 0.56 h. The corresponding values on mature leaves were 16 and 2.7. In general, psyllids spent more time ingesting phloem sap on immature leaves than on mature leaves. Psyllids on abaxial surfaces spent more time ingesting from phloem, though the strength of this effect was less than for immature versus mature leaves. In contrast, xylem ingestion increased on mature leaves compared with young. The biological differences that could produce this outcome are discussed. The results discussed herein are of relevance to further studies on the efficacy of an insecticide to act quickly enough to prevent pathogen transmission.

Keywords: EPG; Experimental design; discriminant analysis; electrical penetration graph; electropenetrography; feeding; insect; multivariate analysis.

Fig. 1

The sequence of leaves showing color differences between a mature leaf (#1), a fully expanded leaf (#2), immature leaves (#3-#16), and the youngest leaves at the apical meristem (#17). In this sequence, leaves #5-#12 would be suitable for testing. Note that the mature leaf was from the same plant as the other leaves, but was not part of the flush

Fig. 2

Behavioral kinetogram of Diaphorina citri feeding on immature versus mature leaves and the abaxial versus adaxial surface of those leaves. The durations (circle area) of behaviors with different colors are significantly different (Supplement Table). Arrows represent transitions, with arrow size proportional to frequency. Some arrows are broken lines because the proportional arrow size does not make the transition clear

Fig. 3

Plot of first two canonical variables derived from D. citri behavioral variables based on feeding on immature versus mature leaves and abaxial versus adaxial surface. This is a graphical representation of the range in behavior and the separation between the treatments

Fig. 4

Histological comparison of immature and mature Midsweet and Valencia cultivar leaf midveins. The exterior abaxial surface of the midvein is to the left in each image. Note tissue tearing in both mature variety images, near the fibrous ring. The fibrous ring makes the leaf very tough and difficult to section. P = phloem. X = xylem. a Immature Midsweet. 1. Oil gland. 2. D. citri salivary deposits. 3. Location between parenchyma and functional phloem, showing absence of fibrous ring. b Mature Midsweet. 1. D. ctiri salivary deposits. 2. Fibrous ring- 3-5 cells thick. c Immature Valencia. 1. Location between parenchyma and functional phloem, showing absence of fibrous ring. 2. D citri salivary deposits. d Mature Valencia. 1. Fibrous ring- 3-5 cells thick