A relatively new pest continues to wreak havoc on L.A.'s trees and threatens the avocado industry. Until now hope for control of the pest has been slim -but just recently that has changed.
The polyphagous shot hole borer (PSHB) is one of those new pests that makes entomologists and pest control advisors look bad. This small beetle from somewhere in southeast Asia (we think) bores into perfectly healthy trees (we're mostly sure) where it completes its life cycle. It infests more than 200 trees (but that number is growing). Over 30 trees are reproductive hosts in which the borer can reproduce and wreak havoc on other nearby trees (but we're not sure that number is going to stay the same) and many more area adversely effected by the beetle (another number that keeps on growing).
What makes this pest particularly irksome is that it goes after trees that are the heart and soul of what it is to be a Californian: Avocados, Coast Live Oaks, California Sycamores, and many other native California trees. Right now it's in Los Angeles, Orange, San Bernardino, Riverside, and now parts of San Diego counties, but there is nothing, save for its seeming preference for well watered trees, to keep it from spreading to the rest of the state. No pesticide has yet found to be really effective in controlling it -this fact due to the pest feeding only on the fungus that it spreads from tree to tree and not on the wood tissue that traditionally carries insecticides from the roots or trunk where they're applied to the feeding jaws of the target insect.
They are also incestuous, with the small number of sibling male beetles mating with their much more numerous sisters. This means that there is no mating cycle to disrupt because they mate in the moist, fungus filled chambers of the beetles galleries. Galleries that penetrate deeply into the heartwood of the tree so they mate there and not out in the open where a pesticide coupled with a mating pheromone could kill enough of the pests to control them. The best treatment so far has been a once every 6 week bark spray -a frequency of application and amount of pesticide that is far too expensive for most homeowners and growers.
Yes, this pest is depressing. Every time I've talked about, consulted about it, or lectured about it I feel like Debbie Downer on pessimistic steroids. But there is finally some good news. It appears that a pheromone-like compound is effective in attracting the beetle. P-menthenol, a chemical compound called a terpene that has been isolated from among other plants Yarrow, can attract the beetle from over 50 feet.
In p-menthenol could be used to attract the beetle to a concentration of pesticide or spores of a fungus that is pathenogenic to the beetle and allow that beetle to bring those substances back to the galleries from where it first emerged to chase the scent.