Wasps in oak trees near sidewalk

Asked August 19, 2016, 10:12 AM EDT

Hi Folks,
I have removed several paper wasp nests in my yard but have just recently realized that two of the small oak trees nearest the sidewalk are infested with wasps. Whenever I walk by there are wasps all over the leaves but I don't often see them flying around nor have I seen a nest. They appear to just be milling around.

Is this common behavior for a particular species of wasp? and how much longer are they likely to be active? I am debating on having a professional exterminator come since I can't locate a nest but you you have suggestions or think their activity will diminish soon, I would wait.

Thank you,


Weld County Colorado

3 Responses

Thank you for contacting the Weld County Master Gardeners.

You are most likely dealing with gall wasps. They are the largest group of gall-making insect, producing a wide variety of galls, such as woolly or mossy galls, or woody and rounded galls. The gall wasp produces galls on oak trees and roses; affecting leaves, stems and twigs. Gall wasps lay their eggs on oak trees; the larvae develop inside the gall and emerge when mature. The galls are not very pretty to look at, but they are also not harmful to the tree. Recommended controls are healthy tree practices, because chemical controls aren’t effective. Interestingly, some species of gall wasps act to modify gall shapes. Gall wasps are “heavily parasitized” and targeted by other wasps. So you most likely are also seeing wasps that prey on gall wasps as well. I have included links to information about the gall wasp and other wasps, when they are active and how you can manage them.

CSU Extension Fact Sheet: #5.577 Insect and Mite Galls: http://extension.colostate.edu/topic-areas/insects/insect-and-mite-galls-5-577/

Plantalk Colorado: #1473 European Paper Wasp: http://www.ext.colostate.edu/ptlk/1473.html

CSU Extension Fact Sheet: Nuisance Wasps and Bees: http://extension.colostate.edu/topic-areas/insects/nuisance-wasps-and-bees-5-525/

Liz U
Weld County Colorado Master Gardener
525 North 15th Avenue
Exhibition Building, Island Grove Park
Greeley, CO 80631
(970) 304-6535

Hi Liz,
I just spoke with an exterminator and he recognized my issue after describing wasps milling about on the leaves of oak trees. His experience is that for reasons unknown to him, oaks on occasion create an over abundance of sap that comes from the leaves which wasp feed upon. His recommendation was an early morning spraying of an anti-bacterial soap in a hose sprayer to wash off some of the sap.

Any thoughts or feed back would be appreciated.

Thank You,


Evan, your exterminator may be correct. These are probably yellow jackets and they are after the sugar in the sap OR after the sugar in the honeydew (excretions) of aphids or scale insects. These wasps are not a problem for the tree, but they can get aggressive in their search for sugar.

Aphids in the tree this late in the season does not harm the tree, but can set up the plant for problems next spring, as eggs are laid near buds and in bark crevices. Scale insects are also sap feeders, but because they're small and immobile for most of their life, most people never notice them quietly pulling the life from the tree. Cousins to aphids, they have an armored shell that protects them from predators and people. If the tree is heavily infested with either, applications of dormant oil during late winter will smother the insect or eggs and kill it. A word of caution; make sure the oil is labeled for use on the plant. (There are many kinds of both aphids and scale - most are specific to tree species.)

Back to the yellow jackets. They nest in the ground or holes in rock walls, creating paper chambers, and the entrance is about an inch across. They keep aggressive guards at the entrance! Yellow jacket traps (the bright yellow hanging bottles filled with lure) work well - just remember, that the trap is filled with heptyl butyrate lure and can bring even more to the site. The paper nests that hang exposed belong to native or European paper wasps, that primarily feed on insects (not sugar) and are not attracted to the yellow jacket traps (neither are honeybees).They are less aggressive, so less likely to sting.

There are some horticultural soaps that can be used. We don't recommend using household products on plants - we just don't know enough about the different formulations of the different products.

Before you do anything, check the tree for insects on the bark (aphids, scale). Spraying a jet of plain water will knock off aphids, but not scale. If you find scale, they can be gently scrubbed off with a nylon scratcher, but that doesn't work well for large trees or heavy infestations.

Just FYI - Gall wasps are very tiny (more like a gnat), can't sting humans, and you probably won't see them this time of year. However, if some are emerging (not usually until October), you may see a some paper wasps parasitizing them (since they eat insects, not sugar).

This is a lot of info! These insects are going to be around through the fall. How to treat them depend on what they are, and the best way to identify them is to see what is attracting them to the tree. They are not harming the tree, but what's attracting them may.

Don't hesitate to contact us again with any questions!