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Ray of hope for citrus farmers as Greening vector’s sexual pheromone identified
Feb. 23, 2018
A group of researchers linked to the Brazil National Institute of Science and Technology of Semiochemicals in Agriculture - one of the INCTs based in São Paulo State with support from São Paulo Research Foundation (FAPESP) and the National Council for Scientific & Technological Development (CNPq) - has identified and synthesized the sexual pheromone of the Asian citrus psyllid (Diaphorina citri), an insect that measures only two millimeters in length but has scarred the citrus growers of Brazil, China, and the United States.
The dreaded insect acts as a vector to the bacteria of greening, a disease that damages citrus plants. Greening disease, also known as huanglongbing (HLB), has a devastating effect on the citrus crop. The pheromone identified can be used as a bait to attract and kill the insect and impede mating.
The breakthrough, achieved by scientists of the Luiz de Queiroz Agricultural School of the University of São Paulo (Esalq-USP), the University of California at Davis, and the Defense Fund of Citrus (Fundecitrus), was published at the Scientific Reports.
“The synthesis of this insect’s sexual pheromone opens the perspective to monitor and control more efficiently its population and, this way, reduce the incidence of the disease,” José Roberto Postali Parra, professor at Esalq-USP and coordinator of the INCT semiochemicals in Agriculture, said.
In order to test whether the composition was efficient enough to attract insects in the fields of Araraquara on the countryside of São Paulo, a region that presents a high incidence of greening, different doses were used as baits on traps with yellow color.
The number of insects caught in traps baited with lignoceryl acetate did not differ from the number caught in control traps during the first weeks of the field tests, but significantly more males were caught with the sex pheromone-derived substance than in control traps at 35 and 42 days, after which the lure became ineffectual.
Analysis of volatile compounds detected small amounts of acetic acid in bait left in the field but not in newly set traps, suggesting that lignoceryl acetate, although chemically stable, might undergo slow degradation and that acetic acid might be the product of this degradation and act as an attractant for males.
To test this hypothesis, the authors of the study analyzed the volatile compounds released by the insects during peak mating activity. The results clearly confirmed the presence of acetic acid.
Electroantennography (EAG), a technique for measuring the output of insect antennae to the brain for a given odor, and analysis by olfactometry showed that males were attracted by acetic acid. The researchers also used field tests to prove that traps baited with acetic acid captured significantly more male insects that control traps without the substance.
Low application cost
Surprisingly, traps baited with acetic acid also captured significantly more females than control traps, possibly owing to physical stimuli emitted by captured males, the researchers believe.
"We now plan to estimate the number of traps needed per area in citrus groves and work out a correlation with the insects attracted by traps in order to verify the reduction in the incidence of the disease and map infected sites," Bento said.
The idea, he added, is to make sure the bait lasts at least 30 days in the field, attracting insects to sticky traps so that growers can obtain weekly samples of captures to quantify and evaluate the presence of the pest. "Our aim is to enable citrus growers to use this pheromone to control the disease at the lowest possible cost," he said.
In Brazil, where HLB first appeared in 2004, some 46.2 million citrus trees (or 26% of the existing stock) have since been eradicated. In São Paulo State, the country's largest producer of oranges, 17% of orange groves have been affected.
In Florida (USA), Brazil's main competitor in orange production, HLB first appeared in 2005 and currently affects 80%-90% of the state's citrus groves. As a result, growers are expected to produce 68.7 million boxes of oranges this year, down from 150 million in 2017 and 240 million 15 years ago.
The US Department of Agriculture (USDA) forecasts production of 27 million boxes of oranges by Florida in 2025.
"HLB is the most important disease affecting the world's citrus industry today. Infected citrus plants can't be cured and must be eradicated," Parra said.
Source: AgroNews
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