The newly discovered pear psylla pheromone is being tested in the field to find out how attractive it is to males. This mesh sticky trap has a septa of the chemical in the middle.
CHRISTELLE GUÉDOT, USDA
Scientists in Washington State have identified the sex pheromone of the pear psylla, opening up the possibility of monitoring the insect with traps. This is the first discovery of a pheromone in any psyllid insect, and it surprised some in the research world who doubted that a psylla sex attractant existed.
Pear psylla is a key pest of pears in the Pacific Northwest. Nymphs and adults feed on phloem from the tree, and honeydew produced by nymphs drips or runs onto fruit, causing dark blotches or streaks. Excessive feeding and injection of toxic saliva by psylla can defoliate trees. Other psyllid species are also serious crop pests, such as the citrus psylla and potato psylla.
Dr. Peter Landolt, research leader at the U.S. Department of Agriculture’s laboratory in Yakima, Washington, said researchers have worked for years to figure out first if the female pear psylla had a pheromone that attracted males, and then to identify it.
When Dr. Christelle Guédot joined the lab in 2005 as a postdoctoral entomologist, other researchers there had recently established that female pear psylla must have a sex attractant, but a pheromone had not been identified.
Guédot, who earned her doctorate at Utah State University, tried several methods to identify a pheromone. In one test, pear psylla adults were put in an air stream in the lab and the air was filtered and analyzed with gas chromatography, but nothing showed up, perhaps because the quantities of chemicals were too minute.
Guédot then had the idea of putting whole insects into a solvent and discovered that extracts from the cuticle of the females were attractive to males. When she compared chemicals extracted from males and females, most of the substances were similar, but she found one chemical that was much more prevalent on the female cuticle than on males and deduced that this must be the attractant.
Dr. Jocelyn Millar, a synthesis chemist at the University of California, Riverside, was able to produce the chemical of interest, methyl heptacosane, for further tests. In the lab, Guédot established that that males were attracted to the chemical, though females were not, and that the chemical was as attractive to males as a live female insect.
The chemical is now being field tested. Guédot and colleague Dr. Dave Horton put out sticky traps made of mesh with a septa of the chemical attached and proved that it was attractive to males but not females. They are testing different doses and various trap designs.
Pear psylla adults have two different forms, winterform and summerform. Some winterform adults leave pear trees to overwinter on other hosts, though many remain in the orchard. Winterform females pass the winter in a state of reproductive diapause and then lay eggs on pear trees in the spring. Summerform adults develop through several generations in the orchard.
Guédot looked at differences in the pheromone in diapausing and postdiapausing winterform females. Postdiapausing females—the psylla that return to the orchard in the spring ready to mate—have more of the chemical than diapausing females or males. She is also studying the pheromone of summerform females and has found that although they are in an orchard from May until September, they do not appear to be attractive to males throughout the season.
A patent for the chemical is pending. Horton said the discovery is significant in the research world as it’s possible that pheromones for other psyllid pests could be identified using the same method.
Guédot said that identifying the psylla pheromone was far more difficult than it would be for other types of insects, such as moths.
“These insects are not easy to work with,” Horton agreed.
That’s partly because they are small, but also because behavioral studies have to be done in just the right conditions with insects of just the right age. For example, olfactory tests seem to work best in the afternoons, for some unknown reason.
“It took a lot of pretty tedious behavioral work at the start to begin coming to the conclusion that there might be a volatile sex pheromone involved,” he said.
Horton has been working on pear psylla since he joined the ARS in Yakima as postdoctoral entomologist in the 1980s and had been convinced that pear psylla didn’t have a sex attractant. Scientists had noticed that male and female psylla tended to congregate, he said, but there were theories that vision or auditory signals were involved or that the males were attracted to shoots that had already been fed on by the females.
“We didn’t understand that very well, and nobody really pursued it real heavily until the mid-2000s. I would have bet a lot of money there’s no way these things had a sex pheromone.”
While the discovery opens up the possibility of trapping psylla more easily, Horton said many growers don’t monitor the pest because it shows up reliably on the trees as soon as the buds start to swell in the spring. Growers who do monitor can use the beating tray method.
He said growers seem interested in using the pheromone to disrupt the males as they return to the orchard after the winter to mate, but that’s not being actively explored, as the ARS researchers are focusing on the basic pheromone research. “We don’t know if the compound is volatile enough for that,” Horton said.