By: Sean McNulty

The University of California at Riverside, located in the hinterlands of the Los Angeles metro area, is the site of the first citrus research station in California. Just two miles away, a backyard citrus tree has fallen ill from a deadly disease. The contagion has leapfrogged from house to house across southern California. Now, with its arrival next to Riverside’s campus, it’s reached the brain of the citrus industry on the west coast.

If UCR staff don’t beat the disease, called Huanglongbing, then there may be no more reason to have a citrus research center at their university. In Florida and China, orange growers have been decimated. Billions of dollars of fruit and trees have been lost. The vector of transmission is a tiny insect, and there’s no known vaccine, treatment, or cure.

What is it?

The Asian citrus psyllid, Diaphorina citri, is a tiny insect – at 4 millimeters in length, it’s a little more than half the width of an iPhone 5S. The psyllids bury their heads in the bark of citrus trees to suck the sap inside. To breed, they lay tiny clutches of eggs on the growing shoots of a tree. Each egg is only a third of a millimeter long – smaller than a grain of sand.

The psyllid carries the bacteria which causes citrus greening disease, or Huanglongbing – henceforth abbreviated as HLB. It withers the trees unlucky enough to be infected. Their veins turn yellow and the leaves go splotchy. Branches recede, twigs and leaves fall off, and the tree goes erratic – flowers blooming out of season, fruit going bitter and sour, and the whole plant wasting away. There is no known treatment, and all species of citrus are susceptible to it.

But how?

Huanglongbing came to Florida in 2005. Since then, it’s wiped out billions in value from the orchards of the gulf coast, and a firm beachhead has been established in California. Growers there, who preside over one of the richest citrus producing regions in the world, have been working feverishly to contain what outbreaks there are and to develop a treatment for the disease.

How bad is the damage? It’s hard to say. Estimates for the economic impact of HLB vary widely. It’s not just a matter of lost citrus, after all – the disease withers the trees that it touches and causes growers to remove or dramatically trim others in a bid to slow HLB’s spread.

Trees and fruit lost to the virus and preventative measures have cost farmers billions of dollars. One estimate from the University of Florida extrapolating from the 2007-2008 season estimated $9 billion; another from the same university estimated another $4.5 billion lost between 2008 and 2013. These figures aren’t just fruit and trees – they also account for economic intangibles like market elasticity.

Can We Remedy This?

Nonetheless, they represent a staggering blow to the citrus market. For the time being, there’s no silver bullet that will stop the psyllid and HLB. That doesn’t mean growers aren’t trying, however, to contain it.

The most spectacular method of psyllid control involves a parasitic wasp called Tamarixia radiata. These wasps, which weigh in at about the size of a flea, hail from Pakistan and are still tinier than the already tiny psyllids – they’re about three times smaller. Like the monster from Alien, these tiny creatures are parasites; they need a host in which to grow their young.

That host, as luck would have it, is the nymph form of the citrus psyllid. The wasps lay their eggs on the nymph; the larvae hatch two weeks later, attaching themselves to the pest and feeding on it. Eventually, the psyllid dies, and the nymph crawls inside to pupate into an adult wasp.

It’s a gruesome process, but it might be the key to stopping the spread of the psyllid and Huanglongbing bacteria by extension. Growers have found limited success in application of pesticides – a solution that is necessarily limited in scope by the distribution of citrus trees. In places like Southern California, most of those trees grow in the yards of ordinary citizens. They’re in the backyards of suburbs, not the orchards of growers, and homeowners are slow to spray synthetic poisons with unpronounceable names.

Genetic engineering is another possibility – creating trees that aren’t so tasty to the psyllids or that have an immune response to HLB bacteria. So far, plant geneticists haven’t been able to develop and deploy HLB resistant trees. It’s also worth bearing in mind that genetic engineering often kicks up controversy in the public sphere – supermarket shoppers, fearful of a Gattaca future, might steer clear of GMO oranges.

One study done in Puerto Rico, as reported by the Washington Post, found that psyllids don’t do well at altitude. Trees growing higher than 500 meters above sea level there see fewer psyllids – after 600 meters, the psyllids can’t hack it. The thinking goes that it might be possible to protect trees against the pests and the HLB that they carry by replicating the conditions of an orange tree growing in this zone.

How to do that in Florida, of course, is another question. The Floridian peninsula is punishingly flat – there are no orange groves there above 600 meters. The highest point in the state rises only 345 feet off the panhandle – a little more than a sixth of the height needed. There’s no higher ground for orange growers to retreat to so that they might try to escape the psyllid.

One option on the table is the use of pressure. Air pressure decreases steadily as you climb up from sea level. Put some of your citrus trees in a tent and pump a bit of the air out, the thinking goes, and you might be able to simulate the conditions that are found at 600 feet. Whether or not this strategy will work for deterring psyllids closer to sea level is still an open question.

 

Sources:

http://entnemdept.ufl.edu/creatures/beneficial/wasps/tamarixia_radiata.htm

http://latimesblogs.latimes.com/home_blog/2012/01/asian-citrus-psyllid-wasp.html

http://news.nationalgeographic.com/news/special-features/2014/06/140617-citrus-greening-wasp-california-biocontrol-psyllid-science/

https://www.washingtonpost.com/news/speaking-of-science/wp/2015/01/20/one-way-to-beat-a-bug-thats-destroying-floridas-citrus-get-them-high/?utm_term=.734d63c3344b

https://www.newtimesslo.com/sanluisobispo/eats-shoots-and-leaves-the-fight-continues-against-the-asian-citrus-psyllid/Content?oid=2944902

http://ageconsearch.umn.edu/bitstream/230093/2/Estimating%20CitrusFinal.pdf

https://conference.ifas.ufl.edu/citrus12/PDF%20Presentations/Fri-pdf/1045%20Spreen.pdf

https://conference.ifas.ufl.edu/citrus12/PDF%20Presentations/Fri-pdf/1045%20Spreen.pdf