Aphids drip honeydew and lead to sooty mold on objects below, including cars. Photo: Clipart Library & Clipartmax, montage: laidbackgardener.blog
Question: I have a linden on the front of my house and it gives off a sticky product that covers the paving stones and my car. Afterwards, they turn an ugly black color! I suspect this is caused by an insect. What can I do to both save my tree and get avoid having to clean everything?
Answer: This is a common enough problem, but one for which there is no easy solution. I have the same situation at my house and have to park my car in the street all summer long to avoid damage. The black mold that forms can be so hard to remove that the only solution is to repaint the car!
And yes, the problem is an insect: an aphid called the linden aphid (Eucallipterus tiliae). Originally from Eurasia, it’s now found worldwide wherever linden trees (Tilia spp.), also called basswoods and lime trees, are grown. This tiny insect from 2 to 3 mm in length is greenish-white or pale orange with a lateral line of black spots, red eyes and long antennae. Adults may also have wings, depending on the time of year. That said, however, you’ll likely see nothing of its appearance, as the insect itself usually goes unnoticed, living as it does in the crown of a tree.
The egg, laid the previous fall in a crack in the bark of the linden, hatches just as the first leaves emerge and the nymph immediately moves to the underside of the leaf where it settles on a vein, which it punctures so as to absorb the sweet sap that is exuded. Since there are several generations a year, the linden aphid is present from spring to the end of summer and its population is increases over time. There can easily be over a million aphids in a large linden by summer’s end!
Your Car Suffers More Than the Tree
Interestingly, the aphid is not particularly harmful to the tree. Yes, the countless hordes drink sugary linden sap, but only a few buckets worth total. Since a healthy tree produces much more sap than it really needs, it can afford to lose a bit. As a result, it grows normally even when it’s heavily infested. Only rarely are a few leaves lost, even when the infestation is severe.
The real damage is elsewhere. The aphid “gives off” honeydew, a sticky sweet liquid. No one likes to say so, but honeydew is essentially aphid poo. The honeydew falls on and covers the objects below (cars, sidewalk, furniture, plants, etc.). Sometimes enough falls that there seems to be a fine drizzle under the tree, but few people suspect the tiny droplets are actually insect poop!
As the objects below are showered with honeydew, they become shiny and sticky … for a while. Then sooty mold sets in. This is a black fungus that discolors objects. It isn’t harmful to plants per se, as it doesn’t invade or directly damage the leaf, although if the leaves pf plants below are thoroughly covered with sooty mold, they’ll have a hard time carrying out photosynthesis. And sooty mold stains objects: cars, garden furniture, rocks, sidewalks, pavement, plant leaves, etc. On many surfaces, the stains can be difficult if not impossible to remove.
Usually linden aphid infestations are repeated yearly, but in some years, the aphids are not as abundant and less sooty mold is produced.
Do Ants Farm Linden Aphids?
You may have heard that ants are known to “farm” aphids, taking care of them and protecting them, moving them to new locations to start new colonies, then feeding on their honeydew. They even “milk” them, tenderly caressing them, to encourage them to produce more. However, they don’t seem to farm linden aphids. Maybe they don’t like the taste of their poo?
What to Do About Linden Aphids?
In general, this insect is not considered to be harmful enough to merit treatment, especially since there really are no effective treatments you could logically use.
The usual advice is simply not to place anything valuable under a linden. Better yet, simply don’t plant lindens where sticky honeydew and the resulting sooty mold would be a problem. Now, if only nurseries told buyers that before they sold them a linden tree!
Lindens were once widely planted as street trees and still line many city avenues, but informed municipalities no longer use them for that purpose because of the sticky black mold that coats sidewalks and roads and brings complaints from citizens.
You could theoretically treat young linden trees by spraying them with almost any insecticide starting in spring—insecticidal soap or pyrethrum, for example, would work fine—and carefully coating the underside of all the leaves, but as the trees grow, it becomes impossible to reach all of its parts with a spray.
As for systemic insecticides, those which penetrate the sap of the tree, making the whole thing toxic, yes, this treatment does work, but has been largely abandoned for ethical and ecological reasons.
First, honeybees regularly visit blooming linden trees and this has resulted in both bee deaths and toxic honey. Yes, treating lindens with systemic insecticides turned out to be one of the causes of colony collapse disorder!
The Wilsonville Bee Kill
On June 17, 2013, the largest native bee kill in the USA occurred in Wilsonville, Oregon, where some 50,000 bumblebees dropped dead onto a Target store parking lot. This occurred while colony collapse disorder was still big news and the incident was picked up by media worldwide. It was eventually traced back to treatment with a systemic neonicotinoid insecticide, dinotefuran, that had been sprayed onto the 55 linden trees that dotted the parking lot. The idea had been to protect cars from the “ravages” of linden aphid honeydew and subsequent sooty mold. No one, apparently, had considered that lindens are popular with bees!
As a result of the Wilsonville bee kill and other similar incidents, treating linden trees with systemic insecticides is now banned pretty much everywhere. And even where it isn’t, you won’t find many tree care companies willing to risk the ire of the public by using such a treatment!
Sometimes beneficial insects do come to the rescue of the gardener and reduce the population. These include ladybugs, lacewings and parasitic wasps. In some cities where lindens were planted as street trees, parasitic wasps (small insects that don’t sting humans) have been released to control aphids, but often with only modest success.
As you can see, there is no quick fix for the linden aphid. It’s up to you to adapt to the insect and, at the very least, move your car!
This was the main reason so many tulip trees were cut down in San Jose in the 1990s! Many people disliked the trees anyway, so this was justification for their removal, even though other remedies had not been tried. My colleague down south is desperately trying to take care of this problem with his tipu trees that he planted on Orange Drive in Los Angeles before neighbors cut down more of them. It is discouraging the pesticides that have worked well for some of my clients are not effective for his particular application.