From the March/April 2022 issue of GLS TRACKS
By Jeff Johnson
Editor’s note Sept. 1, 2024: This article is being republished to accompany a new Conservation Notes article on tree-boring insects in southern California. The information in this article was current when it was published.
By Jeff Johnson
On a recent GLS hike, I talked to Linda Eremita about her current work with Davey Resource Group on the City of Los Angeles street tree inventory and I learned some interesting things about trees in southern California. Later, I got to talk to her and ask a lot of questions. Before working for Davey, Linda was at Tree People in Beverly Hills.
L.A. Street Tree Inventory
The inventory is recording all street trees in the City of Los Angeles. “Street trees” means the trees growing in the green strip between the sidewalk and the curb, or in cutouts in the sidewalk, or in green medians in the middle of the street. The City owns these trees and is responsible for maintaining them. To manage all these trees, the City needs to know what’s there, but there hasn’t been a street tree inventory since the 1990s.
You can read about the inventory at: https://www.streetsla.lacity.org/tree-inventory.
You can see information recorded in the inventory at: https://laparksca.treekeepersoftware.com/index.cfm.
The inventory locates each street tree or tree stump, and possible tree planting locations. It identifies each tree’s species, condition, size, height, and eco-benefits. The eco-benefits include carbon pulled from the air, stormwater capture and reduction, and cooling. Cooling reduces electricity used but cooling is also needed for people’s health. Linda explained: “With the increase in extreme heat days, people are getting ill from too many consecutive days of their bodies not cooling down.”
The inventory describes the grow space for each tree, such as a cutout in the sidewalk, or an improved parkway between curb and sidewalk, and describes the condition of the sidewalk, noting, for example, damage caused by tree roots.
Information collected for maintenance includes defects or problems with trees, such as cavities, visible rot or disease, or dead limbs. Problems are rated by priority for maintenance, including hazardous trees that are failing now and should be taken down right away. Linda said that inspectors have found hazardous trees, and she has had removed right away.
Street Tree Maintenance
The current pruning cycle for street trees in Los Angeles is now about 21 years. The City’s arborist would like it to be every 7 years, but there isn’t money in the budget for that.
Sometimes inspectors find street trees that have been damaged by neighbors’ amateur pruning. Linda gave an example: “Someone cuts all the branches off a tree because they like the way it sprouts back thickly. The tree is responding as if it’s dying, by sprouting those tiny branches that are all weakly attached to the tree. It’s illegal to do that to a tree, and the City will probably fine you if you do.”
Then, sometimes in affluent neighborhoods, inspectors find street trees in great shape because neighbors have been paying a professional arborist to come out and prune those trees.
Fruit Trees as Street Trees
Can fruit trees be street trees? Linda explained: “It used to be illegal for neighbors to plant fruit trees in street tree areas. Fruit would fall on the sidewalk, someone would slip and fall, then they’d sue. Now, neighbors can get a permit to plant a fruit tree in the parkway. Then that neighbor is responsible for maintaining that fruit tree after they plant it. It allows people who don’t have a private space to plant a tree in a public space. Though it is rare that anyone actually does get a permit for this.”
Linda has found that many of the fruit street trees that people plant in L.A. are indigenous to other parts of the world. People are planting trees that are a familiar food source from the land they came from.
One of the commonest of these introduced fruit trees is the ice cream bean tree, genus Inga. The “ice cream” in the name comes from the creamy vanilla-flavored edible pulp inside the big bean-like seed pods.
A telltale feature of ice cream bean trees is the wing-like leafy edges on the stems that hold the leaves. Linda says, “That tree was new to me when I started. Now, I see ice cream bean trees all over. It’s mainly from South America, but there are 300 species from around the world. We keep looking for better ways to distinguish the different species of genus Inga.”
The tree inventory aims to identify accurately each tree found, including these privately planted trees. After identifying thousands of trees, the inspectors recognize most of them, but sometimes there are still mystery trees that Linda has to consult an outside expert. To learn about local trees, Linda recommends A Californian’s Guide to the Trees Among Us, by Matt Ritter, a professor at Cal Poly San Luis Obispo.
Tree Pests
Among the tree problems that inspectors note in the inventory are several caused by insect pests, including Invasive Shot Hole Borer, Red Gum Lerp Psyllid, and Goldspotted Oak Borer. All these insects were introduced to southern California from other parts of the world. (http://www.iscc.ca.gov)
Invasive Shot Hole Borer (ISHB, Euwallacea whitfordiodendrus and kuroshio) is a group of ambrosia beetle species from Vietnam and Taiwan (https://ucanr.edu/sites/pshb/, https://ucanr.edu/sites/pshb/management/, http://www.iscc.ca.gov/docs/ISHB_Final_Report0909019.pdf). These beetles bore into trees, creating extensive tunnels (“galleries”) inside trees. They infect the trees with Fusarium fungus, which is what they eat. They don’t eat the tree tissue they chew through, they eat the fungus that they bring into the tree, which then grows inside the tree.
The beetles’ galleries inside a tree can be so extensive that the tree just falls apart. Linda recalled a class she took about ISHB. “When we were walking around where the beetles had taken hold, we all had to wear hard hats because you never knew if part of a tree was going to fall on your head.”
The Fusarium fungus spreads through the tree, clogging conductive tissue and preventing water from moving through the tree, which kills infected parts of the tree. A fungal infection in a tree may show up as a dead limb here or there. It may kill the whole tree in a few years.
The ISHB beetles came into California in wood pallets from southeast Asia brought into the Port of Los Angeles or Long Beach before 2003. ISHB first infected trees in Long Beach and South Gate, going up the 710. Now it’s found in most southern California counties.
Unlike some tree pest insects that attack only specific trees, ISHB infects and reproduces in many kinds of trees (https://ucanr.edu/sites/pshb/pest-overview/ishb-reproductive-hosts/). ISHB prefers riparian trees – trees that grow by water – including willows, cottonwoods, sycamores (native and non-native), and box elders. Locally, ISHB devastated Gardena Willows (https://gardenawillows.org/), a small, wooded wetland area in Gardena. Among commercial trees, ISHB attacks avocados.
How do you deal with ISHB and the Fusarium infection? Linda says “The best strategy at this point is to keep an eye out and prune off branches that have the borer. Follow agricultural agency protocols for dealing with the pest.
“A badly infested tree can be an amplifier, spreading the pest rapidly. If we see a tree like that, that tree needs to come out. There are times I’ve called for an emergency removal because the tree was so infested.”
The task is not done when an ISHB-infested branch or tree is removed since the beetles survive in the downed wood. To manage the downed wood, sources recommend: chipping the wood very small; solarizing the wood for a long time; sending the chipped wood to a professional composting facility (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=50171).
The ISHB management strategy that people are waiting and hoping for will involve some natural parasite that keeps the beetles in check in their native range in southeast Asia. Linda said that the scientists looking for an ISHB parasite “have found some insects in Taiwan and have been working with them with some success, but it’s not in general use yet.” (https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=31033).
Linda also noted that observers are “starting to see some ISHB resistance in our native sycamores and willows, resistance to the beetle and the fungus. It’s not clear what’s going on, but it is hopeful.”
Red Gum Lerp Psyllid (RGLP, Glycaspis brimblecombei) is a pest insect that attacks eucalyptus trees. Both eucalyptus and these pest psyllids came from Australia. The first eucalyptus trees were planted in California in the 19th century, but the psyllids were not found in California until 1998. Unlike the situation with ISHB, there is now a parasite on the loose in California that is effective against RGLP (http://ipm.ucanr.edu/PMG/PESTNOTES/pn7460.html).
The psyllids suck sap from eucalyptus leaves, secreting honeydew, making a mess and causing defoliation that weakens the trees and makes them susceptible to boring insects. A “lerp” is a small white protective shell that an immature psyllid builds to protect itself while it lives and develops on a eucalyptus leaf.
When the psyllids became a problem in California, scientists looked for a critter in Australia that attacks these psyllids. They found a tiny, tiny wasp, Psyllaephagus bliteus. “It’s like a horror movie,” says Linda. “The wasp sticks its ovipositor into the lerp and lays an egg inside the psyllid nymph in the lerp. Then the developing wasp larva eats the psyllid nymph. Eventually the wasp pupates, bores a hole in the lerp, and flies off.”
The wasps have become established in southern California and have had an effect controlling these psyllids. Linda described what she saw in Griffith Park: “When the psyllids were uncontrolled and at their worst, there were hundreds and hundreds of trees that were infested and had to be removed. Now there’s a kind of a balance. Now, when I see those lerps on eucalyptus leaves, if I see the round holes in the lerps, good. That means that parasitic wasp is around, doing the job, keeping thing in balance.”
Goldspotted Oak Borer (GSOB, Agrilus auroguttatus) is a pest beetle now in southern California that attacks only oaks, and mostly large oaks (https://ucanr.edu/sites/gsobinfo/). GSOB is native to Arizona and Mexico, and probably arrived in California in firewood in the 1990s or 2000s. Currently, it is a problem mostly in San Diego and Riverside Counties.
GSOB larvae feed on tissue in the tree, weakening the tree by disrupting the flow of water and nutrients. GSOB holes in trees are distinctively D-shaped, unlike the holes other boring insects make in trees, which helps with diagnosis (http://ipm.ucanr.edu/PDF/MISC/GSOB_field-identification-guide.pdf). Another sign of GSOB in infected trees can be woodpeckers pecking away the bark to get at the beetles underneath.
Scientist have not yet found an effective parasite or biological control for GSOB. Linda says, “The main strategy to control the Goldspotted Oak Borer now seems to be firewood management. It’s another example of why you shouldn’t move firewood.”
Firewood Management
This lesson—Don’t Move Firewood—seems to apply to everywhere that trees have diseases and pests. There’s even a website on the subject, www.dontmovefirewood.org, affiliated with The Nature Conservancy.
As Linda put it: “A tree dies from a disease. Someone cuts down the dead tree and moves the wood and, oh!, there’s the same disease in a new place.”
L.A. County Entomology and Plant Pathology Lab
The Los Angeles County Agricultural Commissioner office offers a free service to check on possible pests and diseases you may find (https://acwm.lacounty.gov/entomology-and-plant-pathology-laboratories/). You can send an insect or plant sample that seems to show a problem. They’ll tell you what it is and what to do about it. If your sample is something of no concern, they tell you that. The tree inventory has used this service and found it helpful.
Sites to Plant Trees
The L.A. tree inventory also records “vacant sites,” spots that would be good to plant street trees. What are some of the site conditions that make a difference, and that the City wants to know about, when you apply to plant a street tree? Here are some of Linda’s thoughts and recommendations.
“Is it 10 feet from a fire hydrant, 20 feet from a utility pole, 20 feet from an alley? Are there overhead utility wires?
“How wide is the grow space? You don’t want to plant a tree that’s going to be breaking up the sidewalk in ten years.
“What is the soil type? Some trees will not grow in clay or in sand.
“What other trees are nearby? You want to make sure there’s a good diversity of trees, with percentages of different species or genera nearby.
“Location matters if a street tree is going to survive and thrive. If an adjacent yard gets water, the tree roots will grow there, and that makes a difference. Tree roots may grow out two to four times farther than the canopy of the tree.
“If a tree is planted in a 4 x 4 cutout in the sidewalk, surrounded by buildings, it has to be a kind of tree that can take the heat. And then it needs attention for years before it’s established. You can’t just plant it, ignore it, and expect it to survive.”
Planting Partners
The City of L.A. has several planting partners working to plant the urban forest, including:
- TreePeople https://www.treepeople.org/
- North East Trees https://www.northeasttrees.org/
- Los Angeles Conservation Corps https://www.lacorps.org/
- Los Angeles Beautification Team https://hbteam.org/.
“The planting partners are a part of City Plants, which gets its funding from LA Dept. of Water and Power. L.A. City residents can get up to seven free trees for their yard. They can also request street trees. (https://www.cityplants.org)
“The planting partners target more of the underserved neighborhoods for getting trees planted, neighborhoods which also tend to be the hottest, so they will benefit from more street trees.
“A critical thing is that these planting partners are responsible to water and maintain the trees they plant for a few years, to make sure they get established. If a tree gets a good foothold and establishes a good root system, then it can survive. If an adjacent property owner is given instructions on how often to water a tree, and they do water the tree, it will help the tree get established and survive.”
Trees For a Hotter City
For people in the city, living with extreme heat will become more of a burden with climate change. Trees in the city can be effective to moderate the heat burden, but: “When we are looking at our urban forest and deciding what trees to plant that will survive with our changing climate, we have to look at places that are hotter than we are now. They say that in 2070, Eureka may have the climate Berkeley has now. Here in L.A., we need to be looking at species from San Diego or down in Baja, where it’s hotter. Trees cannot evolve as quickly as our climate is changing.”
When Linda worked for TreePeople above Coldwater Canyon, she noticed how extreme heat affected oak trees there. “The side of TreePeople that faces the Valley doesn’t get much coastal influence, so it’s hotter than the other side. In one period of several extremely hot days, we saw a lot of dieback in native Coast Live Oaks (Quercus agrifolia). The trees not affected by the heat were the Valley Oak (Quercus lobata).
“Valley Oak usually grows down in the valley bottoms and likes more water, but it’s evolved to be able to take more extreme heat. The Coast Live Oak, which is more coastal, doesn’t need as much water, but the problem for them that year was the heat.”
