I suspect most gardeners think of roses at the mention of aphids, those little insects of various colors, typically found clustered on soft shoot tips and flower buds, sucking sugary sap from the plant.
But trees get them too. This email, and a recent local house call made me think it’s time to put aphids on the table for discussion, so to speak:
Dear Mr. Pramuk,
My neighbours (sic) have a linden tree on their front lawn. Because of this tree we can't park our cars in front of our house. So on that note, I was wondering if you know or have any idea and can tell me about how long the aphids drop off the these messy trees. We are located in Southwestern Ontario, Canada if that helps.
Please advise as soon as possible. Thank you.
I look forward to your reply.
The local example is a large tricolor beech tree -- a fairly rare tree here -- with its canopy extending over the neighbors’ property. Sticky droplets of sap are making an intolerable mess on the patio.
Aphids are responsible in both cases.
Lindens are notorious for this problem. I have not noticed aphids as a common problem in lindens here in the Napa area, but then, we have relatively few lindens.
In the tricolor beech, which can be a wonderful tree here if well located, the problem is a particular species called the “wooly beech aphid” (Phyllaphis fagi). This aphis is tiny and green. It secretes a sticky, wooly material, supposedly self-protective, similar to the material produced by mealy bugs on houseplants.
A similar pest is common here in some of the native oaks: wooly oak aphids (Stegophylla quercicola). When numbers build up, sticky droplets of sap and drifting bits of the sticky wool become a major nuisance.
Another tree, notorious for aphid infestation is the Eastern tulip, or Eastern “tulip poplar” (Liriodendron tulipifera). Locally it is a common, large shade tree. There are two good-size examples at the west end of the old Franklin Street Post Office.
Locally, I have not seen aphid infestations to the severe extent common in this tree species in other parts of the Bay Area. Perhaps Napa has a stronger population of aphid predators or parasitoids.
The reference mentioned above lists about 14 aphid tree pest species including wooly apple aphid, Asian wooly hackberry aphid, birch aphid, bowl-legged fir aphid, California laurel aphid, crape myrtle aphid, green peach aphid, manzanita gall aphid, Norway maple aphid, oleander aphid, and poplar gall aphid. I’ve seen about half of those species in the course of my work.
Though aphids transmit viruses in many annual crops, “this is not usually a problem in landscape trees and shrubs”, the reference states. The problem is seasonal, cosmetic and nuisance related.
So, should we intervene? And if so, how?
There was a horrifying example of unintended consequences of spraying to control aphids in linden trees in Eugene, Ore. in spring last year. A tree care spray service sprayed imidicloprid, a “neonicitinoid” insecticide, on 17 lindens in full bloom, apparently unaware that bees are strongly attracted to linden flowers and the topical spray is very toxic to bees. The estimated loss was 5,000 bees.
Aside from the topical spray method that material and newer, closely related neonicitinoids may be applied as a soil drench, soil injection, trunk injection and basal bark spray (sprayed on the trunk). The chemical is systemically translocated through the tree.
Research and debate is ongoing as to their environmental effects and possible involvement in honeybee colony collapse. I hope to have and share news on this soon.
For now, I suggest exercising due caution when considering intervention on aphid infestations. Various mild and environmentally friendly pesticides are available as options. In many cases infestations resolve to insignificant levels by allowing natural predators and parasitoids to do the work.