controlling/eliminating woolly aphids on apple tree
For the past 2 or 3 years I have been contending with woolly aphid infestation on my backyard apple tree. At the base the tree is greater than 12" in diameter, don't know it's age. Must be on aggressive root stock. I'm having to prune hard to keep the height manageable at less than 16'. There's voluminous growth every year but with thinning of the fruit sets it is a yearly dependable producer of good apples, Wolf River I think, ever since I have been in charge of this tree, 25+ years. In the past I have used the agent 'Sevin" to treat against codling moths and medfly with great success but woolly aphids or their various forms of reproduction are seemingly resistant to that spray. This year I have not used 'Sevin". Instead I have used Surround which is stated to not be effective against woolly aphids which I have found to be true. I have used neem oil solution spray and also a pure castile soap solution spray also 'Safer',an insecticidal soap solution, all without much success. My question is whether there is a more effective program for controlling this insect that you can suggest. Also if I do nothing, what damage could they do to the tree and to the quality of the fruit? I have not tried using aphid predators, lady beetles for example, but intuitively I doubt that would be a worth while experiment. So my other question is the possibility of some systemic form of aphid control that would be effective against this variety of aphid. Is there a systemic agent that could be applied to the soil over the root distribution that would eradicate the root form of this insect and an above-ground spray for limbs and foliage that can be absorbed into the internal fluids of the tree that would adversely effect the insects as they feed on the fluids of the tree?
One agent that I discovered in my search is imidacloprid which if used in solution with a wetting agent and applied as a soil saturation can give significant control against woolly aphids in some circumstances and for a significant length of time. But would it work with a tree as old as mine and is this agent available to a home orchardist and from which source is it available?
It sounds like you have battled this insect for some time since you have tried quite a few things. If these aphid populations are high they can stunt growth and cause problems for the tree, so depending upon how many you have you may want to try to manage this pest.
In terms of imidacloprid, there are formulations that are available for homeowners, however this is a product that is systemic and lasts long-term in the tree. Looking at the label of one available product I see that it has a preharvest interval of 21 days, so you may want to avoid using it this year, but certainly read the label carefully especially for this information regarding when you can harvest after treatment. It is also toxic to bees and should be applied after petal fall. Below is some information about homeowner imidacloprid products.
Another source of information is the PNW Insect Handbook which has other products listed which you can use - note that the homeowner products except for insecticidal soap are toxic to bees.
If you go with a non-soil drench insecticide, mixing it with a sticker-spreader or horticultural oil (do not apply during hot weather) to assist with penetration of the waxy coating of the aphid may help.
Below is more information, however note that in these publications the products listed are for commercial growers:
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