When to apply Imidacloprid via soil injection for trees and shrubs
My lawn service has used soil injection of Imidacloprid in the late fall for all the trees and shrubs the last two years. Purpose being to control EAB in Ash trees as well as potential insect infestations such as aphids in other trees and shrubs. I have read that this chemical is best taken up when the tree is actively taking up nutrients, i.e. in the spring when foliage is emerging or fall before the trees begin to lose their leaves. Is this poor timing for the injection, or would it at least be better in the spring? Also, I am thinking of using a trunk injection of Emamectin benzoate in the spring for the ash and wondering if a spring treatment, either drench or spray for any other trees/shrubs having problems might also be better.
Adams County Colorado
In respect to the EAB, do you have a lot of Ash trees? Are they healthy? Well planted and established? If not, have you considered planting a replacement tree? Replacement and or treatment is a personal decision and can be influenced based on the economics and tree value.
Imidacloprid soil applications can be made in fall, from mid-October to mid-November. However, this timing is less efficient and studies have shown that higher rates must be applied in the fall than in spring to achieve similar levels of control.
Emamectin tree trunk injection is most effective mid to late spring after tree has leafed out.
"Emamectin benzoate consistently provides at least two years of EAB control with a single application, even in large and very large trees under intense pest pressure. It also provided a higher level of control than other products in side-by-side studies." Citation link provided below.
An informative document on EAB treatment is here:
There is a talk on EAB on YouTube that you may find informative: https://www.youtube.com/watch?v=E1vhOisuv2k&feature=youtu.be
You mention treating for other insects. Have you positively identified infestations or is the health of your shrubs/plants look to be suffering?
There are two ash trees with a combined girth of 29 - 30 inches. They are both mature and seem to be very healthy right now. One of them has shown some signs of lilac ash borer infestation but no sign of EAB as yet, although we are in a watch area of sorts in Brighton.
The other main issue these trees have dealt with in the past, as well as a mature Canadian plum, lilacs, elderberry, and apple/crabapple trees is aphids. I had some significant infestation in the garden this year in the strawberries and rhubarb that I worry could spread to the trees and shrubs without a prophylactic treatment. I was wondering regarding that problem whether you recommend a fall or spring soil injection, or no injection at all. Thank you.
In regards of the aphids jumping from the strawberries to the ash trees, most aphids can feed on only a few species of plants. So it may likely take a different type of aphid to infest the ash tree.
There are some aphids that over winter on plum trees that may snack on your rhubarb in the summer. There is a list of cross species aphids in the link below. If you can get a great picture of the aphid at first sign of them to us, we may be able to help identify which particular aphid(s) you are dealing with.
In regards to timing, soil injection for aphids the control is at most a couple of months, so fall is not practical. The soil must be warm enough of up take by the root system, so late spring is more effective. It does take a few weeks for soil injections to provide protection and it may only last for a few weeks to a couple of months. I hope this helps.