The disease that threatens to destroy Florida’s $10.7 billion citrus industry appears to have its own mechanism to promote its spread, making it harder to control. A recent study by five University of Florida researchers shows Asian citrus psyllids fly earlier in their life cycles, more frequently and farther when they are infected with the citrus greening bacterium.
Kirsten Pelz-Stelinski and the team of researchers at UF’s Institute of Food and Agricultural Sciences’ Citrus Research and Education Center in Lake Alfred say these conclusions have global implications for how the disease spreads and strategies to control it.
“To our knowledge, this is the first description of direct changes to insect behavior caused by a bacterial pathogen in an insect-plant-pathogen system,” said Pelz-Stelinski, whose work in the entomology and nematology department focuses on insect carriers of plant diseases. “These newly discovered behavior changes seem to increase dispersal of the insect – and thus the disease.”
Citrus greening bacterium first enters the tree via the psyllid, which sucks on leaf sap and leaves behind greening bacteria. The bacteria then move through the tree via the phloem – the veins of the tree. The disease starves the tree of nutrients, damages its roots and the tree produces fruits that are green and misshapen, unsuitable for sale as fresh fruit or, for the most part, juice.
Most infected trees eventually die and the disease has already affected millions of citrus trees in North America. It has recently been found twice in California.
Citrus greening was first detected in Florida in 2005. Between 2004 and 2011, the Florida commercial citrus acreage and the number of trees decreased by 28 percent, with greening one of the major reasons for this loss, along with development.
Early fruit drop, which can cause a loss of up to 30 percent of an orange crop, was another consequence of greening disease in Florida. The citrus industry, in Florida alone, has lost approximately 100,000 citrus acres and $3.6 billion in revenues since 2007, according to researchers with UF/IFAS.
Although current methods to control the spread of citrus greening are limited to the removal and destruction of infected trees and insecticide-based management of psyllid populations, UF/IFAS researchers are working to defeat it on a number of fronts, including trying to eradicate the psyllid, breeding citrus rootstock that shows better greening resistance, and testing treatments that could be used on trees.
Adult psyllids have a lifespan of about 40 days at 77 degrees Fahrenheit, reaching sexual maturity and maximum flight capacity four days after emerging from their nymphal stage. Pelz-Stelinski and her team used psyllids ranging in age from five to 15 days after emergence for their experiments in a controlled, quarantined lab.
Pelz-Stelinski says male adults are more attracted to female psyllids when she is carrying the greening bacterium. But the females are more attracted to plant odors than to males and prefer uninfested new vegetation over infested vegetation because it is more nutritional. In addition, females are repelled by high densities of other psyllid females.
Other researchers on the study are Xavier Martini, Mark Hoffmann , Monique R. Coy, Lukasz L. Stelinski – all with the UF/IFAS CREC’s Department of Entomology and Nematology. They are hoping to conduct further testing and mathematical modeling to explore how greening is spread.
Their study can be seen at http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0129373.