Photo courtesy of Fundecitrus
A recent article by the São Paulo Research Foundation explains how a potent compound might aid in controlling the Asian citrus psyllid, the vector of citrus greening disease.
In São Paulo, greening is managed in part by controlling the psyllid. In an attempt to manipulate psyllid behavior, a team of scientists discovered α-copaene, a molecule present in large quantities in copaiba oil. They found it to be quite effective at repelling the psyllid — 100 times more potent than the previously investigated substance, β-caryophyllene.
Fundecitrus chemist Rodrigo Facchini Magnani said a national committee was formed in 2009 to investigate greening. This committee was in constant contact with an international counterpart that monitored the disease worldwide.
“At some point, small producers in Vietnam observed and reported that when guava trees were interspersed with tangerine plants, there was a smaller insect population and a lower incidence of the disease on the tangerine plants,” Magnani said.
Guava trees could be emitting compounds that interfere with the psyllid’s approach to the plants. After investigation, a compound was found in large quantities in the guava tree’s aroma: β-caryophyllene. Researchers concluded that the compound did repel the insect and that this could be used as a strategy in the citrus plants themselves, causing them to increase the production of this compound. Citrus already produces the compound, but in smaller quantities.
“Initially, we inserted caryophyllene-producing genes into fast-growing Arabidopsis plants,” Magnani said. “Arabidopsis is a widely used model in molecular biology and genetic engineering and also naturally produces a very small amount of β-caryophyllene. After the experiment, the plant not only overexpressed the compound but also repelled the insect. However, when this gene is introduced, in addition to producing a greater amount of β-caryophyllene, two other molecules are also increased: α-copaene and α-humulene. So, we wondered what their role might be.”
Fundecitrus entomologist Haroldo Xavier Linhares Volpe said when the plant emits this scent, the insect receives not one molecule, but a mixture of these compounds in a ratio of 1 (α-copaene) to 100 (β-caryophyllene) to 10 (α-humulene). Volpe said researchers found that α-copaene repels the insect at a dose 100 times lower than that required for β-caryophyllene.
The researchers tested the repellent potential of copaiba oil diluted in a solvent (hexane) to understand if it also worked and had positive results.
The scientists developed and perfected a dynamic releaser (diffuser) to disseminate the compounds in an arena where the insect is confined, walking and making choices for or against the scents offered to it.
“We used a small glass lamp, closed at the top, with a cotton wick that draws the liquid from inside the vial and releases it into the arena used to evaluate the psyllid’s behavior,” said Walter Leal, a researcher at the University of California, Davis and a collaborator on the project. “With the help of this diffuser, we tested both copaiba oil and the molecules individually, as well as in mixtures of two or three molecules.”
Other host plants for the insect are curry and myrtle. “They are good for the psyllid to multiply, but it cannot acquire the bacteria from curry, and only 1% of the insects that feed on myrtle acquire the bacteria,” said Volpe. He noted the idea is to work with a scientifically widespread and commercially used technology called “push-pull and kill.”
“We want to work with the orange tree as a way to repel insects, attracting them to a bait plant, which could be curry,” Volpe said. “However, we’ll have to kill the insects in the curry, and we’re still studying how to make it lethal to them.”
Read the research report here.
Source: Fundecitrus
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