Last month I joined citrus farmers from California's central valley on a field trip to Southern California to see the Asian citrus psyllid firsthand. The Asian citrus psyllid (ACP) is an insect that spreads a disease that has the potential to kill every citrus tree in California and in every other citrus state; most of the trees in Florida are already infected and dying. The disease is called citrus greening or huanglongbing (HLB). Citrus greening is akin to Ebola for citrus trees, but it is more deadly (100% of infected trees die) and unlike Ebola, infected trees can spread the disease even when they do not show symptoms. Ebola is spread between humans via bodily fluids, but citrus greening is spread via Asian citrus psyllids. As Asian citrus psyllids spread throughout California, they will spread citrus greening whenever they feed on an uninfected tree after feeding on an infected tree. Citrus greening has already been discovered in Los Angeles in a citrus tree with an infected pummelo graft.
It is a good sign that since the first detection in Los Angeles, no more Asian citrus psyllids in California have tested positive for the bacteria that causes citrus greening; the reason may be that there has not been enough trapping and testing to find them, however. Scientists fear that there are more infected trees that have yet to be discovered. Just this month, Asian citrus psyllids were discovered in San Jose, Lodi, and Manteca in Northern California. These psyllid finds in Northern California were not anticipated and will dilute the resources of those fighting the psyllid and surveying for citrus greening. The neighborhood in San Jose where a breeding population of Asian citrus psyllids was found is considered high risk for greening-infected citrus trees.
The field trip took us to a citrus park in Redlands, California with a heavy infestation of Asian citrus psyllids. We were all given hand lenses for magnification since the psyllid is so small and hard to see with the naked eye. The biggest surprise for me on the field trip was how hard it was to spot the psyllids even in a heavily infested area.
Waxy Tubules
One of the telltale signs of an infestation of Asian citrus psyllids is the characteristic waxy tubules excreted by Asian citrus psyllid nymphs; a nymph is an immature stage of a psyllid. My expectation was that I would see these tubules everywhere, but aside from the tubules shown in this photo, I did not see any.
New Growth Attracts Psyllids
The adult female psyllids seek out new growth and lay their eggs on it. This piece of new citrus growth is heavily infested with Asian citrus psyllid nymphs. Although larger-than-life here with my thumb for scale, the infestation is difficult to see. The highly-magnified photo below shows psyllids and eggs in detail on this piece of growth.
These four Asian citrus psyllid nymphs were found on the above piece of growth. I was surprised to see that psyllid nymphs can be green.
Asian Citrus Psyllids are Hard to Spot
The below photo shows a psyllid-infested branch. Although magnified, the nymphs are hard to see. Can you spot them?
The below photo is the same one as above, but with the psyllids further magnified.
Sampling for Asian Citrus Psyllids
I also saw a demonstration of tap sampling, one of the methods used to sample Asian citrus psyllids. A white clipboard is sprayed with a mixture of water and dawn detergent. The clipboard is held below a branch and the branch is struck three times with a piece of PVC pipe. The number of winged adult psyllids on the clipboard is then counted.
Conclusion
The recent finds of Asian citrus psyllids in urban areas of San Jose, Lodi, and Manteca were all found not by homeowners, but by government traps set out to catch another pest, the Glassy-winged sharpshooter. Based upon what I learned on the field trip, this is not surprising as the psyllids can be very hard to see even in a heavily infested tree. As the state resources for surveying for ACP are highly stretched as it is, the best chance for slowing the spread of the Asian citrus psyllid and the deadly citrus greening disease is for homeowners in Northern California and in California's central valley to inspect their trees carefully and closely every month as recommended by California's Citrus Pest & Disease Prevention Program (CPDPP). If you think that you have found a psyllid, be sure to immediately call the CDFA hotline at 1-800-491-1899 to report it.
Hi Dan, and thanks for posting this information. In the photo with the caption “Asian Citrus Psyllids are Hard to Spot” showing the infected branch, is the damage on that branch caused by ACPs, or was the damage from something else? Is it the nymphs that cause the damage? Thanks!
Hi Lizzy,
I think that the infested branch in those photos was also attacked by citrus leafminer. My understanding is that ACPs can also cause some cosmetic damage, but it is not that significant. An entomologist told me that if ACPs did not spread a tree-killing disease then they would not be that concerned with them. In Florida there is currently a research project underway with the goal of replacing the current population of ACPs with ACPs genetically engineered so that they do not transmit HLB.
Thank you for the question!