Updated: 11/22/2024 – Suggested Solutions by More Plant Experts
Sticky Sidewalks Downtown
San Carlos city crews are stepping up efforts to tackle the sticky sidewalks downtown from of a persistent infestation by aphids, as those along Laurel Street and San Carlos Avenue become increasingly sticky and challenging for pedestrians this time of the year. For several years now during fall, aphids, tiny insects that feed on tree sap, have been creating a “sticky situation” coating sidewalks with a residue from their excrement.
According to a recent interview with San Carlos Public Works Director Steven Machida, the intensity of aphid infestations varies year on year. Sometimes, there is a light infestation, while other times its heavier than usual. They had anticipated a lighter infestation this year; however, the infestation seems to have grown in the latter part of the season.
The Reason Behind This
Machida explained that the city’s downtown trees—planted decades ago—are now facing new challenges as they age. Note that the same trees have been in Laurel Street for many years, yet it’s only during the past several years that aphids have started to thrive on them.
According to Machida, as trees grow older, their roots tend to expand; and with limited space to expand in, their roots become cramped. This causes stress on the trees, and in response, the trees may release organic chemicals and sap which tend to attract more aphids. To settle some questions: the aphid infestation has little (or no relation at all) with the species of trees.
What The City Is Doing
The city has refrained from using topical insecticide sprays due to the dense downtown setting, where exposure could affect people, nearby parklets, and local businesses. In order to minimize exposure, they use a systemic approach: applying insecticide directly to the trees’ roots to prevent aphids from gathering.
“In the past, we have used clear water / soapy water treatment to clean the leaves, but because we’re in downtown, the parklets, and a lot of furniture, we haven’t done that as much as we would have wanted to” Machida replied. As with last year, the city has implemented a routine sidewalk washing (every Tuesday and Thursday) for each affected block along Laurel Street and San Carlos Avenue – a measure that will continue until all leaves have dropped and the aphid population decreases.
Various pest population studies suggest that warmer temperatures can enable aphids to reproduce more frequently, potentially leading to larger infestations. With a licensed arborist in their team, the Office of Public Works will continue to work out solutions with the City in line with the changing climate we live in.
Considering natural solutions, upcoming downtown renovations may also be an opportunity to allot more space for these aging trees to be restored to health. Adding more plant life may also attract natural aphid predators, such as ladybugs, lacewings, and other predator insects. The aphid infestation seems to be the result of an imbalance in Downtown’s natural ecosystem, and restoring that is a win-win situation for all.
Explanations From Plant Experts
Regarding the issue, we’ve been in touch with a couple expert resource persons and organizations on plants, particularly Steven Foster (Landscape Management Professional) and Christine D. (UC Master Gardeners Volunteer):
Interview with Steven Foster
Steven Foster, a horticulture degree-holder and a landscape management professional in the Bay Area overseeing tech campus accounts, offers interesting insights into managing aphid infestations in urban settings like San Carlos. He further explains the reasons behind the city’s actions and a few other things:
Can you please tell us more about the use of tanglefoot sticky bands, and how would you apply them in a situation such as that in San Carlos Ave and Laurel Street, where the trees are already of an old age and are surrounded by a lot of furnishings?
Tanglefoot is usually used on a banding material wrapped around a tree trunk, especially fruit trees, and capture or abate some rodents and crawling insects that like to attack foliage and bore into bark. It helps but is not the prettiest and could easily get stuck to by pedestrian clothing, curious dogs, etc. It may not be practical in a high-traffic downtown setting.
Typically you see this for use with ants when a tree is infested with scale bugs or aphids who produce honeydew excrement rich in sugars which the ants literally come in to protect them symbiotically from parasitoid and predatory insects to gain access to this substance…
It seems the city finds it difficult to wash trees nowadays maybe because of their size; and also because of the surrounding parklets, outdoor dining.. now that they’re planning to make it all a downtown pedestrian lane, getting rid of these aphids the best way possible may be a high priority. What suggestions would you give the city?
Yes, the city is correct in all of this. It is challenging to “wash” a tree, spray them down with water, horticultural oils, any liquid really in this setting we have created where they are overhanging parklets, dining, fixtures, etc. I am sure I am not stating anything they aren’t considering already, but relocation just isn’t a viable option for older hardscape trees that have outgrown their space, girdled their roots around metal grates, cement, pipes, etc. for decades and damage or shock would surely arise and economically doesn’t make sense.
New trees is the way to go with careful consideration for the balance of biodiversity and natural beneficial support for species while catering to a utilitarian application such as pedestrian traffic, dining, parking, etc. It is no simple task to consider a new species, and plant it wholesale across the avenue. This leads to vulnerability of diseases, pests, IPM issues in the future that could take them all out equally if not diversified in some way, or in this case, aphids infesting one and having an avenue of specimens to race through.
Root treatments, soil drenches with organic pesticides or systemics if possible, is the most effective if at this stage (later stages of infestation). It isn’t a fix overnight however and must be timed very carefully before populations explode. At this point (later stages), you are stuck with what you’ve got.
Aphids are weak flyers and don’t readily fly for the first couple of generations each season. If knocked off of a plant by water, they almost never return or crawl all the way back into it, but they give live births and are “born pregnant” so some nymphs will always be present.
Do you experience these type of infestations in tech campuses? If so, what do you usually do about it? If not, what could be the reason?
In commercial settings, I deal with these types of infestations almost daily or weekly. It almost always boils down to a cultural issue, meaning setting. These issues are either biotic or abiotic in nature meaning a pest or fungal infection on one side, or the abiotic nonliving causes like slope and grade issue, mechanical damage, irrigation issue, chemical, temperature, etc.
The golden rule is “Right plant, right place.” My crape myrtles that are infested each year like these trees almost always are planted in the shade, less water than they should be receiving, and this causes them some stress or weakness that leads to pest attacks.
Another thing that I always see is someone over-loving a shrub or a tree and fertilizing it at the incorrect time of year when it is already pushing nitrogen-rich growth like Spring. IF you overdo it, it pushes weak-rank, succulent green growth that is an instant magnet for thrips, aphids, scale, etc. No matter what, if it happens year over year, heavier and more consistently, something has to change and typically it means the tree itself has to be swapped or location considerations for its successor must change.
Lastly, another thing that can explode these insects in population is when one tree in the “wrong place” is infested because it is unhappy, possibly touching other trees nearby in a long line on an avenue, and then the insects are so successful and so close they can simply crawl right along to the next victim or float with the wind to close subjects.
Many municipalities may also use a “monitor trap” to detect early instars (life stages) of invasive or damaging insects by hanging a triangle trap or yellow sticky pad in a tree canopy and checking it weekly or monthly and counting the numbers of target pests to see a threshold correct for application of pesticides. The way systemics work is they essentially “poison” the phloem or juices of the tree or plant, and once chewed or ingested by the target pests, they die but this poses some risks in settings like flowering trees browsed by pollinators.
Another way, and in certain settings, you can help natural predators along is to release larvae of predatory insects or parasites such as tiny wasps, lacewings, ladybugs (less effective), or assassin bugs. My crape at home gets aphids, but it has recently seen a huge attraction naturally of ladybugs laying their tiny yellow eggs giving way to tons of larvae that look like small black caterpillars with orange patches on their back, and this is a huge win.
Your best HOPE is that natural predators can do their thing, but we need to invite them with urban forest and proper plantings and parks nearby. It cannot exist in an isolated island if no ecosystem exists.
Interview with Christine D.
Meanwhile, Christine, a volunteer from the UC Master Gardeners, affirms the actions of the city public works, and also mentions the importance of regular monitoring, the use of tanglefoot, and how to protect beneficial and predatory insects from pesticides:
- Regular monitoring to catch infestations early is important. Aphids tend to gravitate along the upwind edges of gardens and close to other infested plants. Monitoring should include looking for evidence of natural enemies and for off-color, bloated, flattened or fuzzy aphid corpses, which can mean natural predators are at work so the tree and may not require treatment.
- Ants, which feed on honeydew, protect aphids from predators and parasites. A band of sticky material such as Tanglefoot around the truck will prevent ants from climbing up, allowing natural predators to do their jobs.
- Both high heat and extreme cold suppress aphid populations, which could explain the recent surge of aphids in the cool weather following our recent heat waves. However coastal California’s mild winters allows aphids to proliferate year round.
- Avoiding application of insecticides (spray or synthetic, chemical or natural) to plants that are blooming or about to bloom will help reduce their harmful effects on pollinators and other beneficial insects.
Resources
- Phone Interview with Steven Machida, Director of the San Carlos Office of Public Works
- Email Interview with Steven Foster, Landscape Management Professional
- Email Interview with Christine D, volunteer from the UC Master Gardeners
- https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0106228
- https://en.wikipedia.org/wiki/Pest_insect_population_dynamics
- https://link.springer.com/chapter/10.1007/978-1-4613-9044-2_2
- https://link.springer.com/chapter/10.1007/978-90-481-8601-3_1
- https://www.smdailyjournal.com/news/local/san-carlos-addresses-sticky-situation-on-laurel-street/article_21c1ea88-6fb3-11ee-9b84-ff8d2796f0e9.html
- https://scotscoop.com/san-carlos-battles-aphid-infestation/
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