In 2022, Jeannine Lowrimore Onstott, technical sales representative for Pacific Biocontrol, got a call from independent PCA Paul Precissi, asking if she had any mating disruption (MD) pheromones for lilac borer, Podosesia syringae, a clearwing moth that was infesting olive trees under his watch.
Does the Peachtree Borer Pheromone Work for Lilac Borer?
Pacific Biocontrol did not have a lilac borer pheromone, but they did have a pheromone called Isomate P for another clearwing moth, the greater peachtree borer, Synanthedon exitiosa. The pheromone is a synthetic version of what greater peachtree borer females release from their abdomen to attract male moths for mating. Usually, moth pheromones are species-specific, but Onstott decided to test Isomate P on the lilac borer anyway.
For monitoring, two delta traps with peachtree borer lures from Alpha Scents were hung in both the control block and the MD block on April 26, 2022. On April 29, Isomate P-containing twist-ties were deployed at the rate of 134 per acre — one for every fifth olive tree — in the MD block. The traps were checked the same day, and six more times through June 17.
The twist-ties released pheromone at a consistent rate for 170 days — way more than was needed to cover the mid-March to early-June flight time of the lilac borer. Each twist-tie released a higher concentration of pheromone than did the traps.
The pheromone-treatment “shut down” the traps for the duration of the flight due to the pheromone load from the MD twist-ties being stronger and more numerous than the pheromone lures in the traps.
Achieving “trap shut down” (few or zero moths caught) was the first measure that the pheromone was effectively disrupting mating. By the end of the test, only five moths had been caught in the MD block (all on April 29), whereas 415 were caught in the control group.
Since the male moths could not find the traps, presumably they also could not find the female moths to mate with them. The moths remained in the orchard until early June, but presumably most failed to reproduce and create the next year’s population.
Once the effectiveness was confirmed, Pacific Biocontrol applied for approval to use Isomate P to control the lilac borer. Even though growers were waiting to use it, and it was registered in other states for organic use, not to mention that pheromones have zero toxicity, the application sat on DPR’s (California Department of Pesticide Regulation’s) desk for 18 months before they finally approved it in 2025.
What Is the Lilac Borer?
Native to North America east of the Rockies, the lilac borer, or ash borer, was first noticed in California in the 1970s.
A clearwing moth that flies in the daytime, it resembles a large paper wasp — as a way of defending itself.
It should not be confused with the emerald ash borer, a beetle native to Asia that has decimated North American ash tree populations. The lilac borer does infest ash trees, though, and other trees in the same plant family, Oleaceae, including lilac, privet and olive.
What Is the Lilac Borer’s Life Cycle?
The adults emerge early to mid-March and immediately start mating. Each mated female lays about 400 eggs in crevices in the bark within 7-14 days of emerging.
The eggs hatch in another 14 days and then burrow into the wood, leaving sawdust protruding from the hole. The following March, the larvae pupate and the cycle starts over. The pupal case often is left hanging at the entrance hole.
Where Is the Lilac Borer Found?
No one knows why the lilac borer is now infesting olives, nor the exact range of where they are a problem. San Joaquin County extension specialist Jhalendra Rijal reported the borer in olive trees in the northern San Joaquin Valley in May of 2020. It has also been reported as far south as Modesto and as far north as Corning, 100 miles northwest of Sacramento — overall, in Stanislaus, San Joaquin, Yolo and Tehama counties.
Precissi says he would love to see most California olive growers put at least one trap in each olive orchard to get an idea of how widespread the borers are. (California is the only US state that produces olive oil on a large, commercial scale.)
Onstott concurs with him. The moths are very sensitive to the pheromone and “super easy to trap,” whereas “the sawdust and tunnels are easy to miss,” she notes.
Precissi finds the lilac borer to be more common in riparian areas and near ornamental olives. He says the borers can kill trees aged 1- 4 years and damage stressed trees, whereas mature, healthy trees seem to weather the infestation.
He mainly consults for oil olives, but people have called him about ornamental olives, and the borer was present in those, too.
Matt Morelli, a Nutrien PCA based in Woodland, finds the borer infestation to be most severe in the Picual cultivar of oil olives.
He recalls pulling out a block of 15 or 20 acres in an orchard west of Yolo after about 12th leaf. The moth was boring into the base of the tree even though it usually likes smaller diameter wood, girdling half the tree.
Morelli thinks Picual may be more susceptible because it has softer wood. For the most part, though, he agrees with Precissi.
“It’s not common to see a mature orchard go out like that — it usually affects young trees more,” Morelli says.
What Are the Signs & Symptoms of Lilac Borer?
The best way to detect lilac borer is to set up pheromone traps in the orchard, by March 1 if possible. One visible sign is 3 mm diameter holes in pruning cuts. Digging into such holes reveals the larvae. Adults can be seen from March through early June.
Some of the entry holes will have a pupal case hanging out, left behind by the emerging adult. The holes tend to have frass around them, too.
How to Prevent and Manage Lilac Borer
“It’s easier to prevent rather than battle your way out — especially since it is a borer, so hard to control,” says Onstott.
According to Precissi, prevention includes avoiding suckering or pruning soon before or during the lilac borer’s flight, because wounds tend to be the entry point. Olive knot infections, frost damage, wet feet — anything that weakens the tree makes it more enticing to the opportunistic lilac borer.
Management of the borer is three-fold, according to Randy Post of Agricultural Advisors. First, use the pheromone.
Onstott says the 2026 price for Isomate P is about $45/acre for 100 dispensers/acre and about $60/acre for 134 dispensers/acre, the rate used in the study.
To give an idea of the labor involved, the test run required six people 1.5 hours to cover 13 acres, hanging 134 dispensers/acre at five to six feet in the canopy.
Second, use insecticides. Not many are registered for oil olives, but the ones registered are effective in combination with the pheromone. Altichor or Danitol attacks the egg and larval stages. If sprayed on the tree, and the female lays her eggs on it, the eggs should not hatch.
Carbaryl, in turn, attacks the adult stage of the borer, and has the side benefit of controlling olive scale as well.
Third — according to Post — is to cut out dead limbs and burn them before March 1 when the overwintered generation starts emerging as adults. “It’s very labor intensive but it’s the life of your orchard,” he asserts.
Since olive trees do not age out at 20 years like almond trees, he sees it as a big hit to the grower if a tree dies or is impaired. “With a pest like this that is detrimental to the tree, you want to throw the kitchen sink at it to eradicate it, not manage it,” he proffers.
Biocontrol of Lilac Borer
So far, biocontrol is not being used in commercial olive orchards in California to combat the lilac borer, but some options may be developed in the future from the borers’ natural enemies. One such enemy is ichneumonid wasps that parasitize the borers within their tree galleries.
The University of Massachusetts recommends the fungus Beauveria bassiana, which is commercially available for organic use for borers in some states.
The Missouri Botanical Garden recommends spraying gallery entrances with Steinernema carpocapsae nematodes on ornamental trees infected with lilac borer.
Finally, woodpeckers are important predators of lilac borers.
Even without commercial biocontrol options, growers can prune before March and use pheromone lures, insecticides and sanitation to prevent this new pest from causing major disruption to California olive growing.
— By Nancy Power, Assistant Editor