Olive psyllid, a serious pest of olives in Mediterranean locales, has been identified on landscape olive trees in Southern California and may pose a threat to the state's olive industry, according to a University of California, Riverside research entomologist.
Speaking at a recent central California olive day at the Lindcove Research and Education Center, Exeter, Marshall Johnson said the psyllid, a relative of aphids, mealybugs, leafhoppers and sharpshooters, has been confirmed on olive trees in shopping centers in San Diego and Orange counties.
About 1/16-inch long, its adults are noticed mainly by tufts of cottony waxy secretions that cover their bodies. Adults are pale green to tan in color mixed with light brown and tiny, black spots. They jump when disturbed.
“Psyllids are some of the most fecund insects I have worked with,” he said. “Females can each lay more than 1,000 eggs and become a pest in the spring when olive shoots start to appear.”
Infested olive trees show masses of wax and sticky honeydew produced by nymphs. The deposits cause flowers and young shoots to drop prematurely, and large populations removing sap can retard growth of young trees. They have five instars.
Optimum temperatures for development are between 68 and 77 degrees, and development is slowed at 81 degrees or greater. “This may be what could save us in the San Joaquin Valley,” Johnson said, adding that climatic data from throughout the state is being collected to plot dates when temperatures are between 81 and 90 degrees to learn what areas might be most affected by it.
The psyllid is well-known in the olive producing areas of the Mediterranean and in Iraq and Iran where losses attributed to it have reached 40 percent to 60 percent. It may also occur in Greece. It was reported in California in San Diego and Orange counties late last year.
Johnson said he toured several sites having ornamental olives trees with San Diego County agricultural commissioner's office entomologists in July, but found very low populations of the insect.
He plans to make additional tours beginning in September, particularly in April when the first generation is expected to appear.
“We need to learn all we can about this insect to anticipate its impact on California's commercial olive production,” Johnson said.
Signs of infestations are trees with suckers at the bottom of the trunk and waxy deposits on foliage. Although he has not seen a severe infestation, Johnson said photos of them show trees practically entirely white from the deposits.
Currently, speculation is that the insect spreads by the waxy material and individual psyllids readily sticking to pruning tools and workers' clothing.
Johnson said the waxy material can protect nymphs from insecticide sprays, so any treatments would have to be made on the first generation before the material accumulates. Insecticides are being evaluated for its control, and researchers at this point expect it can be easily controlled by sprays.
For the moment, his advice to SJV olive growers not wanting to use insecticides, based on his talks with visitors from Italy and Spain, is to prune out centers of trees to promote more air circulation and reduce populations of black scale, which supports the psyllids.
Natural enemies of olive psyllid include green lacewings and ladybird beetles, and these have been deployed in San Diego and Orange counties. Additional funding for studies is being sought from the California Department of Food and Agriculture and the olive industry.
Turning to a more immediate concern, the destructive olive fruit fly (OLF), Johnson warned that its populations in the San Joaquin Valley will increase with cooler temperatures after August and growers should have active programs in place to manage it.
“Don't be fooled by OLF,” he said. “Don't stop your spraying programs because of the hot weather. Ten years of climate data from central San Joaquin Valley counties show that after August 15, temperatures start to decline, and the OLF that survive the peak summer temperatures will start to increase their populations.”
Feeding by OLF larvae is the major cause of damage to olives and scarring damage results from adult females depositing eggs on the fruit surface. Larval feeding also provides entry points for microorganisms that lead to rot in fruit and reduced oil quality.
In other olive producing areas of the world, the damage from pests can cause as much as 80 percent of the oil value to be lost to inferior quality. In some varieties of table olives, OLF can destroy the entire crop.
Detected in California in 1998, it has higher populations in oil olives in cooler coastal counties, but also strikes table olives in warmer interior valleys.
Johnson said three successive days of 100-degree temperatures can stress OLF because in the heat it cannot fly far enough for sufficient food and water. By early September, periods of high temperatures are generally past, allowing the insect to resume its flights of up to a mile within a couple of hours.
He also reminded growers that high populations of black scale, whose honeydew is an important food source for OLF, should be controlled.
Meanwhile, Johnson and Victoria Yokoyama, a research entomologist with USDA-ARS at Parlier, are continuing evaluations of Psyttalia cf. concolor, a wasp-like insect that parasitizes the eggs of OLF. The research has been funded in part by the California Olive Committee.
While demonstrating their monitoring traps and other equipment in an olive grove at the Lindcove station, Yokoyama said biological control has the greatest potential for reducing high populations of OLF.
The Psyttalia she is using was collected in Kenya with quantities reared in Guatemala by USDA. She has been identifying California locations best-suited for its release in hopes of it becoming established as a parasitoid. Studies are at San Jose, Santa Barbara, San Diego, and Grapevine, south of Bakersfield.
Psyttalia, which lives for 20 days, she explained, seeks out an OLF maggot on an infested fruit and “stings” it to deposit an egg. When the fruit drops, an adult Psyttalia emerges from the parasitized maggot and seeks out another OLF victim.