Status report on Red Gum Lerp Psyllid and forest health
By the Forest Health Task Force
In 1906, the Atchison, Topeka and Santa Fe Railroad purchased an 8,650-acre tract of land called Rancho San Dieguito, in order to plant eucalyptus trees to use as railroad ties. By 1909, the railroad had planted approximately 3 million eucalyptus trees over much of what would later be known as Rancho Santa Fe. Subsequently, however, the railroad determined that eucalyptus wood was unsuitable for use as railroad ties and the timber growing experiment came to a halt, leaving the trees to grow into a dense eucalyptus forest.
Approximately 90 years later in 1998, a new pest emerged in Southern California: the Red Gum Lerp Psyllid (Psyllid). Psyllids are small insects that attack Red Gum Eucalyptus by sucking sap out of the leaves, which damages the tree and causes heavy leaf drop. Left unchecked over a few years, these psyllids can ultimately kill mature eucalyptus trees. This infestation and the subsequent observable damage to the eucalyptus forest resulted in the formation of the Forest Health Task Force (FHTF) in 2000.
Eucalyptus Forest Health
Because the forest in the Ranch consists primarily of one species – Red Gum Eucalyptus – it is more susceptible to damage from a single disease or insect infestation than a mixed evergreen or coniferous forest. Moreover, in this case, the eucalyptus trees were already stressed by attacks from a borer beetle and by several years of below normal rainfall. Over the last several years, these factors have led to a significant decline in the health of the forest in the Ranch.
Unfortunately, dead, dying or diseased trees may also present fire or safety concerns and represent an aesthetic blight for the community. But perhaps most importantly, the eucalyptus forest is the iconic symbol of Rancho Santa Fe. Along with Spanish Colonial Revival architecture, the forest is our link with the founding of the community. It was with this in mind that the FHTF set out to improve the health of the forest in the Ranch.
After some research and consultation with the University of California at Berkeley, the FHTF determined that the most promising treatment for the psyllid would be to release a small, stingless wasp. Like the psyllid, the wasp is native to Australia, where it serves as a natural biological control to keep the psyllid in check. The wasps were first released into the Ranch in 2001. At this point in time some nine years later, the wasps have been a success, albeit with one drawback.
Although the wasps have mitigated a significant amount of damage caused by the psyllid, their period of highest activity typically trails the emergence of the psyllid each year by two or three months. Depending on the particular year, this time lag has resulted in severe defoliation of the Red Gum Eucalyptus until the wasp becomes active as a biological control.
but it must be applied by a Certified Pest Control Applicator. Based only on field observations (in the absence of any definitive studies) the efficacy of this type of chemical control appears to be highly variable and is likely related to the ability of the individual tree to transporting the insecticide up into the branches and leaves.
Another treatment that has been suggested is a regimen of deep watering, sometimes in combination with the chemical control described above. But just as with the chemical control, the success of deep watering has also proven to be highly variable.
In sum, the wasps have proven to be the most effective type of control over a longer period time. But the other treatments may also be successful on a case-by-case basis, provided the infested tree is capable of transporting water, nutrients and insecticide up into its branches and leaves.
The following recommendations are divided into two categories; community-wide and tree or property specific.
On a community-wide basis, reforestation is the perhaps the most important recommendation. This simply involves replacing eucalyptus that have died or been removed. To assist with reforestation, the Association has arranged to contract grow approximately 2,000, one-gallon eucalyptus trees, which will be made available to members. Two types of eucalyptus trees will be available: Eucalyptus sideroxylon (Red Ironbark) and Eucalyptus citriodora (Lemon-scented Gum). The trees should be available in March of 2011.
In addition to planting new eucalyptus, it is also important to plant several different varieties of trees, so as to move away from a monoculture and diversify the forest. Doing so will substantially reduce the risk of damage to the forest from a single disease or insect infestation, as mentioned previously. Fortunately, as homes are remodeled or new homes are built here in the Ranch, different types of trees are planted as a part of the required landscaping. Not only do these new trees help offset some of the eucalyptus that have died or been removed, but they also add to the diversity of the forest and thereby improve overall forest health.
As a service to members and in conjunction with the FHTF, the Association developed a recommended tree planting list, which is available on both the FHTF (
) and Association’s (www.rsfassociation.org) websites. Trees on the list are grouped by height, with additional information on their drought tolerance, ignition resistance and whether they are evergreen or deciduous.
Several varieties of trees from the list have also been planted in an arboretum, along the hiking trail that runs next to the golf course, adjacent to San Elijo. The Association first established the arboretum in 2009, so that it would be easy for members to observe several different types of trees. Each tree in the arboretum is labeled with its botanical name and common name for easy reference.
When questions arise regarding an individual tree or property, perhaps the most important step would be to obtain a recommendation from a certified, consulting arborist before taking any specific action. In many cases, the arborist may recommend a “wait and see” course of action, with a follow-up visit a few weeks or months later. On the other hand, if the tree is healthy enough, they may recommend the chemical control treatment with an injectable insecticide. If however, the damage to the tree is irreversible or if the tree is clearly dead or poses a risk (e.g., from falling or breaking), the arborist may recommend removal.
Because the recommended course of action can vary so widely (even from tree to tree on the same property) depending on the diagnosis, it is always best to seek the advice of a certified arborist.
Looking towards the future, the FHTF will be focusing on three primary goals:
- To help define community standards for forest aesthetics
- To develop a forest management plan
- To convert the Red Gum Eucalyptus monoculture forest to a mixed eucalyptus/evergreen forest
The first goal, community standards for forest aesthetics, relates to how the community perceives the forest. Even in a healthy forest, there are always a few trees that are in a state of decline or decay. The questions for the Ranch are what level of decay is acceptable here and what is normal for this type of forest?
The next goal is to develop a management plan as a tool (based in part on the answer to the forest aesthetic question) to help maintain a healthy forest. Specific activities will likely include ongoing monitoring, removal of hazardous trees and thinning to improve tree health and reduce competition.
The third goal will be to replace the Red Gum Eucalyptus monoculture with a more diverse, mixed eucalyptus/evergreen forest.
Once implemented, these goals will help ensure the long term viability of the forest in the Ranch, while maintaining the eucalyptus as the iconic symbol of our community for future generations.