Invasive pests and fungi are some of the deadliest threats to many trees. Because the trees under attack have not evolved mechanisms to combat the alien attackers, significant losses can occur very quickly.
Unfortunately, California’s citrus trees are currently under threat from just such a pest, known as the Asian citrus psyllid (Diaphorina citri). First documented in Florida in 1998, psyllids have significantly damaged the state’s citrus production. Recently, the state destroyed 250,000 acres of trees in a desperate attempt to regain control. (Valentine, 2014) Time will tell if their efforts were in vain.
Plenty of Precedent
Unfortunately, history amply demonstrates the damage exotic species can cause.
Around 1900, a pathogenic fungus of Japanese origin arrived on American shores. The fungus, known as chestnut blight (Cryphonectria parasitica), soon began killing American chestnut trees (Castanea dentata) at an alarming rate. In as little as 40 years, the trees essentially disappeared. While a few scattered individuals cling to life, scientists are still trying to solve the problem and save the species that formerly dominated eastern forests.
A more recent example comes in the form of an Asian beetle, called the emerald ash borer (Agrilus planipennis). During the larval stage, these beetles feed on the inner bark of the trees. This destroys the tree’s ability to transport water and nutrients up and down the trunk, ultimately killing the tree. First documented near Detroit, Michigan in 2002, the beetle has spread throughout most of the eastern United States, killing tens of millions of ash trees along the way.
The Bad News Bug
Psyllids damage citrus trees through their feeding behavior. Like the other members of the family Hemiptera, these eighth-of-an-inch-long insects are “true bugs,” who feed via piercing-sucking mouthparts. When feeding, they pierce the surfaces of leaves and then suck out the sap on which they feed. In addition to the trauma caused by their mouthparts, psyllids inject toxic saliva into the plant, which causes further harm.
These insects have hitchhiked around the world, colonizing the Middle East, Central America, South America, Mexico and several Caribbean Islands, along its way. Since colonizing Florida, Asian citrus psyllids have spread to Louisiana, Georgia and South Carolina as well.
While California’s psyllid problem is not yet as dire as Florida’s is, the destructive bugs have been documented in nine counties within the state. So far, psyllids have not yet colonized many of the most important commercial citrus groves, but without an aggressive response, the problem is sure to grow over time.
Even Worse News Bacteria
The invasive insects are only half of the problem. Psyllids in many parts of the world harbor bacteria that cause “Citrus Greening Disease,” also known as Huanglongbing (a Chinese term, which translates to “Yellow Dragon Disease”). The bacteria– known as Candidatus Liberibacter asiaticus* – infects the phloem of citrus trees. Among other problems, the bacteria cause twigs to die, trees to bloom off-season and a general decline in health and vigor. Additionally, the bacteria cause trees to produce green fruit, thus giving the disease its common name.
After years of nervously watching cases appear in Baja, Mexico, California researchers found what they feared. In March of 2012, samples from a single tree in a residential yard in Hacienda Heights tested positive for the problematic bacteria. Fortunately, this is the only documented case in the state, but it is unlikely to be the last.
Hope on the Horizon
A cure for Huanglongbing still eludes scientists, but they continue to attack the problem. Some trees have responded to treatments for a brief time, but they all succumb eventually. To help prevent the spread of the disease, the United States Department of Agriculture has imposed a quarantine, banning interstate commerce of citrus trees from the nine infected counties.
Nevertheless, it is ultimately preferable to control the insects that spread the disease. The insect itself causes damage, but it is also the vector for the debilitating bacteria.
Biological control efforts may offer some relief in the future, as several animals parasitize (and ultimately kill) psyllids in their native lands. Scientists have released two different ectoparasites of the psyllids in Florida: Tamarixia radiata and Diaphorencyrtus aligarhensis. Both insects have proven effective in reducing local psyllid populations, but further research is required to determine the viability of the solution in other locations. Over the past few years, scientists in California have released 75,000 Tamarixia radiata. (Lopez, 2013)
*Interesting note: The tern “Candidatus” refers to the fact that these bacteria have not yet been cultured in a laboratory.
Lopez, R. (2013, August 4). Citrus growers use predator wasp to fight disease threat. L.A. Times.
Valentine, K. (2014, November 6). ASIAN CITRUS PSYLLIDS FOUND IN GROVE NEAR EXETER. ABC Action News.