A tree gets planted in June. By September the southwest side of the trunk has gone papery and slightly sunken, a patch of bark that doesn't look like the rest. The following spring that patch has a small D-shaped hole in it, and the branch above it is thin.
People find the hole and conclude they have a beetle problem. What they have is a sunburn problem that a beetle noticed.
Getting that order right changes everything about what you should spend.
The insect doesn't go first
Flatheaded borers are a large group. At least 69 Chrysobothris species occur in California, and the Pacific flatheaded borer, Chrysobothris mali, infests at least 70 species of broadleaved trees on its own. So this isn't an exotic pest and it isn't rare.
What matters is how the female chooses where to lay. The University of California's guidance on flatheaded borers says adults are attracted to lay eggs on diseased or stressed trees and shrubs and on wounds on woody limbs and trunks. Mated females lay eggs in bark crevices and wounds, such as injury caused by pruning cuts, trunk stakes and ties, sunburn, or the graft union.
Sunburn is on that list, sitting between pruning cuts and stakes as one of the ordinary ways we hand the insect a place to start.
UC also notes that drought stress increases tree susceptibility to bark- and wood-boring insects. Two conditions, both of which the Coachella Valley supplies generously and both of which are things we control.
Why this is a desert problem more than a beetle problem
Bark burns here in a way it does not in most of the country.
A trunk facing southwest in La Quinta, Bermuda Dunes or Indio takes hours of direct afternoon sun at temperatures that would be a heatwave anywhere else and are just July here. Thin-barked young trees have no defense. Neither does bark that spent five years in shade and got exposed last month because a crew thinned the canopy.
That last one is the version we see most, and it's self-inflicted. A tree gets opened up hard in early summer, the interior branches and the trunk are suddenly in full sun, and by the end of the season there's damaged bark on the west side. The pruning didn't invite the beetle directly. It made the burn, and the burn invited the beetle.
The pattern is the same one behind sooty canker, which also gets in through sunburned bark. Two completely different organisms, one shared entry point. When two unrelated problems share a cause, the cause is worth more attention than either problem.
The cheapest fix in tree care
UC's recommendation for this is unglamorous and it works. Whitewash the trunks of heavily pruned, newly planted and young trees by applying white indoor latex paint diluted with an equal amount of water. Or wrap the trunk in heavy paper to prevent the sunburned bark that attracts the egg-laying females.
Half paint, half water, painted on the trunk. That's it. A gallon does a lot of trees and it costs less than a single service call.
It looks strange for a season if you're not used to seeing it, and in a manicured Indian Wells or Rancho Mirage front yard people resist it on appearance. It's still the right answer for a newly planted or freshly thinned tree through its first desert summers, and it beats replacing the tree in year four.
The other half of prevention is water. UC's phrasing is appropriate irrigation, which is not the same as more. A tree on a couple of emitters at the trunk has a root plate the size of a dinner table, and a root plate that small cannot support a canopy through a Palm Desert August no matter how often it gets watered. Wider, deeper, less often is the shape of it.
Prevention really is the whole strategy
UC states it directly: prevention is the most effective method of managing flatheaded borers. That sentence is doing a lot of work, because it's also an admission about the other direction.
Once larvae are under the bark, you are not reaching them with anything sprayed on the outside. The insect is feeding in the layer between bark and wood, and the affected tissue is already dead. There's no product that puts that back.
So a quote to spray a tree that already has borer damage deserves a hard question about what exactly the spray is expected to do. The useful work at that point is different: work out why the bark was damaged in the first place, fix that, and give the tree the conditions to wall the damage off. That's a diagnosis job, which is what an arborist is for, rather than a spraying job.
UC's first line of prevention is also the one nobody wants to hear after the fact: plant only species well adapted for the conditions at that location. A tree that is marginal in this climate spends its whole life stressed, and stressed is exactly what the female borer is looking for. Half of the borer calls we take in Cathedral City and Palm Springs are really species-choice calls from six years ago.
What to look for on your own trees
Walk the west and southwest side of the trunk in the fall, after the worst of the heat. You're looking for bark that has changed texture, gone papery, cracked or sunk slightly compared to the rest. That's the burn, and it's the thing to catch.
Small D-shaped exit holes mean adults have already left. Loose bark over a dead patch, or bark that sounds hollow when tapped, means the layer underneath is gone.
A young tree with a stake and tie that's been on for two years is worth checking at the tie itself. UC lists stakes and ties alongside sunburn as wound sources, and a tie left on too long cuts a wound in exactly the place nobody looks.
When a tree health assessment is worth it
Call if you've got damaged bark on a trunk and want to know whether the tree can wall it off, if a young or newly planted tree is losing ground each summer, or if a crew has recently thinned a tree hard and you want to know what to protect before next June.
That last one is the best call to make, because it's the only one where the answer is still cheap. We're on Highway 111 in La Quinta and cover the valley from Palm Springs to Coachella. The Coachella Valley office can send a tree doctor out, and on this particular problem the visit often ends with a recommendation that costs the price of a can of paint.