A failed field test has led to a major discovery about the Asian citrus psyllid, the insect that spreads the devastating citrus greening disease. According to new research, the bacteria that cause citrus greening can interfere with the insect’s sense of smell, rendering some kinds of insect traps useless. The work is currently available as a preprint.

Citrus greening is caused by the bacteria Candidatus Liberibacter asiaticus (CLas), carried by the Asian citrus psyllid. Infected trees cannot be cured and produce small, bitter fruit. The disease has wrecked millions of acres of citrus groves worldwide and has now appeared in the U.S..

Like many insects, male psyllids use their sense of smell to locate females. One strategy to control the pest is to use traps baited with acetic acid to lure and kill males. Field tests of acetic acid traps in California citrus groves, which are free of the disease, showed promising results.

But when researchers from Fundecitrus (Fund for Citrus Protection), São Paulo, tested the same traps in Brazil, where citrus greening is present, they failed. The researchers collected psyllids from these citrus groves and found that many were infected with CLas and Wohlbachia bacteria.

Working with Professor Walter Leal at the UC Davis Department of Molecular and Cellular Biology, an expert in ‘chemical ecology’ or how insects use chemical signals, the researchers studied how the bacteria affected psyllids’ response to chemical lures.

Infected psyllids, they found, were much less responsive to acetic acid than uninfected insects, although electrical recordings from the insects’ antennae showed that infected males were generating stronger signals.

“To the best of our knowledge, this is the first report of a pathogen infection affecting a vector’s response to a sex attractant,” the authors wrote. The discovery raises both new avenues of research as well as challenges to controlling this devastating crop disease.

Leal is corresponding author on the preprint. Coauthors are: at Fundecitrus, Haroldo Volpe, Michele Carmo-Sousa, Rejane Luvizotto, Renato de Freitas, Victoria Esperança, Josiane Darolt, Abner Pegoraro, Nelson Arno Wulff and Marcelo Miranda; Diego Magalhães, Arodi Favaris and Jose Bento, University of São Paulo.