Image copyright SPL Image caption The tell-tale signs of infection

New research suggests that a bacterium, thought to be the cause of a deadly disease of citrus plants, aids its own spread by altering insect behaviour.

Asian psyllids - the insects behind the spread of citrus greening - were shown to fly further and more frequently after feeding on infected plants.

This increased mobility is thought to enhance the chances of the insect passing the bacterium on.

The research was published in the journal PLOS ONE.

Citrus greening or huanglongbing as it is also known has a dramatic effect on the appearance and health of citrus plants.

The University of Florida's Dr Kirsten Pelz Stelinski, who has been studying the disease, highlighted some of the consequences of the infection during a recent interview with the BBC's Science in Action.

"The leaves start to yellow and [grow] mottled in appearance, the branches begin to die back, the root system dies back and ultimately the tree declines and dies," she said.

"It's a global problem in terms of citrus production throughout the world. Just to give you an example, in Florida alone the industry is a $9bn industry per year.

"There have been thousands of jobs lost and billions of dollars lost, just because of the impact of the disease. We have 100% of trees infected in many parts of Florida, so it's not an insignificant problem.

"It's only becoming worse as the disease spreads to other citrus growing areas."

Citrus plants affected include oranges, grapefruit and lemons.

Insect transmission

The disease is thought to be caused by a group of related bacteria and the current study focused on one of the strains implicated: Candidatius Liberibacter asiaticus or CLas for short.

The bacterium replicates inside the citrus plants; in the sap that transports nutrients around the plant.

To get from plant to plant the bacterium has to rely on an insect carrier called the Asian psyllid.

Image copyright Science Photo Library Image caption The citrus greening disease is spread by the Asian psyllid bug

As the name suggests, the psyllid is native to Asia but is thought to have been spread to other parts of the world on shipments of citrus plants - an unforeseen consequence of global trade.

The insect feeds on the citrus plant by inserting a special tube, a proboscis, deep into a leaf and then sucking up some of the nutrient-rich sap.

If it feasts on an infected plant, it too can become infected with the CLas bacterium. When it moves to an uninfected plant to feed, the contagion can spread.

Chemical attraction

And Dr Pelz-Stelinki's research is highlighting how the bacterium has evolved some Machiavellian means to increase its chances of survival.

"We know from some of our previous research that psyllid insects are particularly attracted, at least initially, to plants that are infected with the pathogen," she said.

"When plants have the bacteria in them they produce a volatile called methyl salicylate and psyllids find this very attractive."

The chemical is produced when groups of psyllids feed and the bacteria make the plants produce more of the chemical making it irresistible to the ravenous psyllid, but this attraction is short-lived.

"When the bacteria's present the psyllids feed and the volatiles are released, but the psyllids will realise that this is maybe not the most suitable host plant, and we think that's because infected plants tend to have lower nutritional quality," Dr Pelz-Stelinki explained.

"So the psyllids will move from that infected plant to find a new host and when they move they take some of those bacteria, so the bacteria are promoting their own spread."

And the manipulation doesn't stop there.

Whilst infected psyllids are not thought to live as long, the evidence suggests that they have more offspring. And larger populations benefit the bacteria by providing more dispersal options.

Dr Pelz-Stelinski also said that their findings suggest that the bacterium affects other aspects of the insect's behaviour to increase its chances of transmission.

"The bacteria can impact psyllid behaviour by causing psyllids to move more frequently and to move, or fly for a longer duration.

"So it actually increases the propensity for movement… so in that sense it is also driving itself out into the environment more, by manipulating its vehicle."

The researchers hope that these insights will provide new possibilities for controlling citrus greening disease.