The Asian citrus psyllid (Diaphorina citri) is the bogeyman of the citrus industry. Its appearance in fields is a dark harbinger for farmers, for carried within this insect is the bacteria Candidatus Liberibacter, the cause of citrus greening disease.
“This is a worldwide problem,” said Distinguished Professor Walter Leal, Department of Molecular and Cellular Biology, who’s devoted his research to combating citrus greening.
In Leal’s home country of Brazil, the state of São Paulo has eradicated around one-fourth of its citrus trees since 2004 in an attempt to control citrus greening outbreaks. China’s been hit with the disease, as has the United States. In California—which produced 51 percent of Unites State’s citrus during the 2018-2019 season—about 1,100 cases of citrus greening have been reported in urban orchards.
“This is the most serious problem to citriculture throughout the world, because the psyllid is very effective in transmitting it from infected to non-infected plants,” said Leal. “And the bug is ‘invisible’ until the population is too high and too late to act.”
While infections haven’t yet hopped to California’s commercial orchards, Leal and his colleagues aren’t waiting. In a study published in Scientific Reports, they report that an acetic acid-based, slow-release trap is capable of capturing Asian citrus psyllids even when the insect populations are low. The team worked with the pest management company ChemTica Internacional to design the sticky, chemical lure trap.
“It’s very important to be able to detect these insects when they’re at low levels,” said Leal, who noted that current sticky traps (which don’t use chemical lures) aren’t very effective.
During tests, the acetic acid-based trap captured on average three times more male insects than the sticky traps, which are colored yellow to attract insects. What’s more, acetic acid is a simple compound and can be found in household vinegar.
“You can buy this thing by the ton, very cheap,” said Leal. “It’s very good for the farmers because the price becomes low.”
During the experiments, Leal and his colleagues tested the efficacy of three different chemical attractants on Asian citrus psyllids, including formic acid, proprionic acid and acetic acid. They tested the chemicals by themselves and in various combinations.
“What we do normally is when we have multiple attractants, we hope that attractants are going to synergize, so chemical A and B may be better together than A and B alone,” said Leal. “But unfortunately in this case, these compounds attract but they do no synergize with each other, so we pick the best one of them and we stick to that one.”
Acetic acid, a reputed sex pheromone of the Asian citrus psyllids, won the efficacy contest. ChemTica’s slow-release chemical traps were designed to last for two weeks, the usual amount of time sticky traps last.
A notable finding from the study was that acetic acid’s efficacy as an attractant was dependent on the dosage amount.
“That’s one thing that I want to emphasize,” said Leal. “This is a very simple compound that we encounter in vinegar, but if you put vinegar in the field, the insects are not going to be attracted to it because there’s a certain dose that’s required to be effective.”
Not too much, not too little
According to Leal, it’s very common for chemicals to act as attractants at certain doses and as repellents at higher doses.
“What seems to happen is that the insect olfactory systems get so overwhelmed with information, that they start to send the wrong information to the brain,” said Leal. “It upsets the olfactory systems in such a way that they no longer are able to respond the same.”
Finding the right dosage amount required patience and meticulousness. While previous research helped Leal and his colleagues make educated guesses about what amounts to start with, the experimental process was contingent on trial and error.
Leal hopes the publication of this research will help kick-start interest in acetic acid-based traps. The research was supported by the Fund for Citrus Protection.