Good Fruit Grower / April-15th 2012 / Pheromones explored for psylla

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Pheromones explored for psylla

Male psylla are attracted to pheromone lures.

Reducing pear psylla populations in the spring is the key to keeping the pest in check later in the season, entomologists say, and a recently discovered pear psylla pheromone might play a role.

Currently, pear growers apply pesticides with oil in the delayed dormant season to target winterform adults as they return to orchards after spending the winter on other hosts. Growers also coat the trees with Surround (kaolin clay), which is somewhat repellent to the psylla and deters females from laying eggs. However, both oil and Surround need to be applied multiple times to be effective.

Dr. Dave Horton, entomologist with the U.S. Department of Agriculture in Yakima, believes that it might be possible to use the pear psylla pheromone to disrupt mating and delay egg laying by winterform females after they return to the orchard, as a supplement to the standard controls, although he cautions that this is all very hypothetical at the moment. He is exploring in the laboratory whether saturation of airspace with pheromone could affect the ability of males to rapidly find females and thus delay mating.

Delays in egg laying lead to more ­synchrony in egg hatch, which in turn simplifies control of the developing ­summerform generation, Horton said.

Horton and colleague Dr. Christelle Guédot began testing the pheromone in the field three years ago. The research shows that there’s a period in January and February when the females are not ­producing the pheromone during which males are attracted to traps with pheromone lures. Once the winterform females begin producing the pheromone in March, the traps with lures become less effective in attracting males. Horton is trying to improve the lure by testing different dosages of the pheromone and different types of traps.

Horton and Guédot have also studied the summerform pear psylla and found that the competitive effects of females are less. From June through August, traps with lures consistently attract more male psylla, regardless of the psylla density. Horton said he will explore this further by tests of different pheromone dosages and will explore whether saturation with the pheromone could affect the ability of the males to find females and thus delay ­mating and egg laying.

Unlike the pheromones of some other insects, the psylla pheromone appears only to work at close range, he said. The pheromone was isolated from the cuticle of the female insect and is not known at this time to be something she emits.

Horton said that a scientist in Japan has discovered a simple procedure to synthesize the pheromone, so if it does have commercial potential for controlling pear psylla, the new procedure might help keep costs down.

“I would suggest that if we could find a practical purpose for this, the best opportunity might be in disrupting winterform females as they’re returning to the orchard,” he said. “The females are not yet mated at that time of year. Growers want to push that egg laying back as far as possible, and if we can saturate the orchard with enough pheromone, there might be a way of slowing mating in late winter and spring as they’re returning to the orchard.”

Repellent

Horton is also testing a psyllid repellent that was discovered by scientists exploring why citrus trees planted near guava trees had fewer citrus psyllids. The compound dimethyl disulphide (DMSD) identified in volatiles emitted by the guava trees was found in laboratory tests to be highly repellent to citrus psyllid. Recent trials have shown that the potato psyllid is also repelled by the compound.

ISCA Technologies has manufactured a wax-based formulation called SPLAT to release DMDS. In tests in citrus, psyllids left plots that were treated with the repellent within three days. Horton said the DMDS disappeared within 28 days as it volatilized, but in pears, an application would only need to cover the period in late winter when the insects are returning to the orchard. Horton plans to test the response of both winterform and summerform psylla to the repellent on caged pear trees.