I have been hearing a lot about Asian citrus psyllid and citrus greening disease. Can you explain the relationship between them? What effect will they have on citrus grown in California and my back yard?

The Asian citrus psyllid (ACP), a tiny mottled brown insect about the size of an aphid has been found in every county in Southern California. An agricultural quarantine has been established to help keep the pest from spreading to other areas. Backyard citrus fruit, citrus trees and plant parts cannot be taken out of the quarantine area. For more information about the quarantine call the California Department of Agriculture Exotic Pest Hotline 1-800-491-1899.
The Asian citrus psyllid is a serious threat to citrus everywhere in California. It can carry and transmit (vector) the bacteria that cause a fatal citrus disease, huanglongbing (HLB), also known as “Citrus Greening”.  ACP takes the bacteria into its body when it feeds on a plant which has HLB. The disease is spread when a bacteria-carrying psyllid flies to a healthy plant and injects bacteria into it as it feeds. HLB can kill a citrus tree within 3-5 years, and there is no known cure.
HLB is currently established in several southeastern states and in part of Mexico. In March 2012 a citrus tree with HLB was found in a yard in Los Angeles County. The tree was removed, but this discovery means it now even more important to keep the ACP population low so the psyllid won’t find another infected tree and spread the disease.
ACP also damages citrus directly by feeding on new feather-flush leaf growth. It feeds on plant sap, twists and curls the young leaves and kills or burns back new shoots. Since psyllid larvae only feed on feather-flush growth, inspecting this foliage is the best way to detect the psyllid.
Slowly walk around each citrus tree in your yard and inspect the flush growth on a monthly basis, especially from spring to fall. Look for signs of psyllid feeding and damage on the flush growth, including twisted leaves, waxy deposits, clear sticky liquid (honeydew), and sooty mold. If you think ACP is present, use a magnifying lens to look for small yellow eggs, psyllid nymphs with their waxy tubules, and adult insects. If you think you have ACP on your citrus, call the California Department of Agriculture Exotic Pest Hotline, 1-800-491-1899. Personnel from CDFA will inspect trees for the presence of ACP, and send insect specimens to a laboratory for identification and testing for the presence of bacteria that cause HLB.
For more information, see UC Pest Note #74155 “Asian Citrus Psyllid and Huanglongbing” http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7455.html  Also see www.californiacitrusthreat.org .
By V. Lazaneo, Urban Horticulture Advisor, Emeritus, UC Cooperative Extension, October 2012