|Ebert, Timothy - University Of Florida|
|Rogers, Michael - University Of Florida|
Submitted to: Journal of Insect Behavior
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/31/2018
Publication Date: 1/20/2018
Citation: Ebert, T.A., Backus, E.A., Rogers, M. 2018. Behavioral plasticity in feeding by Diaphorina citri (Hemiptera, Liviidae): ingestion from phloem versus xylem is influenced by leaf age and surface. Journal of Insect Behavior. 31:119-137.
Interpretive Summary: Citrus greening disease, or Huanglongbing (HLB), is a phloem-limited bacterial disease that arrived in Florida in 2005 when its vector, the Asian citrus psyllid, Diaphorina citri, was introduced into the state. In the 10 growing seasons between 2005 and 2014, cumulative citrus losses from greening caused citrus production to decline by 58% and number of acres by 37%. In the last five years, nearly $8 billion in industrial output and over 7,500 jobs have been lost statewide. At present, the only effective means of disease management is insecticidal control of the psyllid. Yet, killing the vector via insecticides is not effective to control the disease if the insect has time to acquire (pick up) or inoculate (inject) the bacterium from/into the plant before it dies. The best assay method to test the effectiveness of insecticides in preventing the insect’s mouthparts from reaching phloem is electropenetrography (EPG). D. citri feeding was EPG-recorded on the upper and lower surface of old and young citrus leaves. Psyllids on young (immature) leaves (either surface) spent more time taking up and swallowing (ingesting) fluid from phloem than did psyllids on old (mature) leaves. In contrast, fluid ingestion from xylem cells increased on old leaves (especially upper surfaces) compared with young. Thus, testing psyllid feeding on the lower surfaces of young leaves provides the greatest indication of an effective insecticide, i.e., one able to prevent acquisition or inoculation of the greening bacterium from/to the phloem. Use of EPG to test insecticide efficacy will greatly aid choice of the best chemical for psyllid control, potentially improving disease management.
Technical Abstract: Diaphorina citri is a major pest of citrus because it transmits the bacterium that causes Huanglongbing (HLB) (a.k.a. citrus greening). One approach to disease management is vector management using insecticides. However, knowledge of vector mortality alone is not sufficient if the vector has had time to contact the phloem and either acquire or inoculate HLB before it dies. Thus, behavioral assays are necessary to determine insecticidal effectiveness, and the only method of tracking the location of psyllid mouthparts in real time is via electropenetrography (EPG). D. citri feeding was EPG-recorded on the abaxial and adaxial surface of mature and immature citrus leaves. Adults in the field can be found on these surfaces at all times of year. While there was considerable variability, in general, psyllids on young leaves spent more time ingesting from phloem than psyllids on mature leaves. Psyllids on abaxial surfaces also spent more time ingesting from phloem than on adaxial surfaces, although the strength of this leaf surface effect was less than for leaf age.This pattern was the same for the length of time to first contact the phloem. The relationship for xylem ingestion followed the opposite trend, with xylem ingestion increased on mature leaves compared with immature. Again, leaf age had a stronger effect on psyllid behavior than did abaxial versus adaxial surface. Thus, testing psyllid feeding on the abaxial surfaces of young leaves provides the greatest indication of an effective insecticide, i.e., one able to prevent acquisition or inoculation of Huanglongbing from/to the psyllid. In addition, this work shows that Asian citrus psyllid is not strictly a phloem-feeder, but ingests from both phloem and xylem.