Ridding Linden trees of wasps and yellow jackets.
I have a LARGE linden tree that is located in my pasture. We affectionately refer to it as the "bee tree", because it is just buzzing with all types of bees, wasps and yellow jackets. Typically the bees are not a problem. This summer, however, ... I can not even go to the barn or be out in the driveway area without being stung. I need to know what I can safely spray the tree with to get rid of the wasps and yellow jackets -- given that I have a heifer and chickens in the pasture. I have already been stung several times.
I am very sorry to hear about the wasps. How awful.
The wasps are attracted to aphids on the trees. The aphids are producing sticky sap as a by-product of there feeding.
Controlling aphids on lindens, however, can have side-effects. If you recall, in 2013 there were large bumble bee kills that resulted from injections of trees with pesticides to control aphids. These pesticides stayed in the tree for over a year and were expressed in the nectar and pollen of the flowers resulting in bee kills. These products were very effective at controlling aphids, but because of these negative effects, they were pulled from the Oregon market.
In future, make sure to get the sod away from the tree and make sure to water it well before aphids move in. Proper watering will reduce aphid loads. Now that you have problems, you might consider treating the tree with insecticidal soap, which is effective against aphids. Depending on how large the tree is, you may require a boom to reach all the leaves.
The only other thing I would add is to call a certified arborist. Many have experience controlling aphids on linden and may provide some other solutions. Also, long term, you may want to replace the linden with another tree that is less prone to aphid infestations. Lindens are about the most susceptible tree to aphids that we have here in Oregon.
Hmm. Are you sure the wasps are not being attracted to honey dew being shed from aphids? Wasps typically don't aggregate in the way you observe unless there is something to feed on. If the ground under the tree is sticky, this is because of aphids, and it is this sweet sticky fluid that is attracting the wasps, not the tree itself. If this is the case and the aphids are not controlled, you will just keep attracting more wasps.
Assuming this is not the case, I am not sure focusing on spraying the tree is the best approach to getting the wasps back. Instead, I might suggest focusing on getting baits up around the perimeter of your property. In future years, having baits up early and maintaining them can take the pressure off your property. If you are determined to spray the tree, that is a large area to spray and I would absolutely suggest consulting with a professional arborist as this would be a tricky application to a big tree. Again, if the wasps are being attracted because the tree is sticky (which means there are aphids) your best option will be insecticidal soap (or even plain water to wash away the accumulation of sugars). Good luck!
Oh, and one last thing - this free online extension publication from Idaho Extension is one of the best at explaining wasp control options: