Applenut identified a woolly aphid infestation on the roots of an apple tree that had stunted in my orchard (in another topic). I learned that you have to plant trees with Woolly Aphid resistant rootstock and nothing else really helps when they infect the rootstock.
I took stock of my apple trees - I have 4 or 5 on MM106, MM111 and some on Geneva Rootstock that should be okay. I have 4 that are unknown rootstock, 2 from Starkbros that are likely not resistant plus one Geneva 935 that is not resistant.
How bad do Woolly Aphids hurt trees that have grown well? Should I dig up the trees on susceptible rootstock and start over?.
Looking at B.9, B.118 I see no mention of them being Woolly Aphid resistant so I assume they are not - anyone know for sure?
I am sure this maybe an over reaction but I really do not want to have low yielding trees for the next 15 years due to bad choices I made now.
For what it’s worth, I did a little research on the topic and wooly apple aphids don’t affect mature trees that much (though that probably also depends on the rootstock-B118 is probably better to shake it off than G935.)
But…there’s apparently things you can do to help keep WAA from getting established on your trees. The lifecycle has them on the roots in colder temps then going above ground in warmer temps. If you’re diligent about keeping them off your trees via the appropriate pesticide treatments, then they won’t be able to establish in large numbers on the roots. For greater stopping power, you can use an imidacloprid drench, which is systemic and is good against aphids. It has its downsides, which is that imidacloprid may kill pollinators but it will take care of the aphids, and it’s a once and done insecticide that only needs to be done once a year. It’s recommended it be done immediately after petal drop to minimize risk to pollinators. Use it once and apparently there will be no aphids that summer. (I used it this year and after applying it I occasionally saw a few aphids, but nothing near like they were before. As a bonus, it’s also effective against several other insects.)
Please note that the Malling rootstocks (and perhaps some of the other resistant ones, but I wasn’t able to confirm) are RESISTANT to WAA but it is still possible for them to get them. It is just there will not be so many and the effect won’t be as bad. The Malling rootstock will tolerate the small amounts that it will get.
In the same way, it seems like a good spray program will greatly minimize the effects of this pest, so you needn’t worry even if you have non-resistant rootstock as long as you don’t really let them get established.
If you’re a no-spray or minimal spray grower, though, and you’re dealing with a lot of WAA pressure, a resistant rootstock might be the way to go.
That’s just some info from the research I did. (I always try to research questions I don’t know the answer to, in case I have to deal with the same situation myself.) I hope that may help you somewhat in making your decision. Hopefully, though, someone who has dealt with this problem can give us a personal account.