This is vexing for me as well. These cerculio are no joke. We have a reservoir of wild plums and the idiotic Bradford pears all through the woods, and they are numerous. They don't move very far and I had them on a peach the first year in, with no other target trees around. These trees have excellent nutrition (thanks to Bryant Redhawk), mineral and compost tea foliar, the full meal deal. So I think I have done what I can to get the trees inherently resistant.
Some things I am testing- please be aware there is a good chance none will work.
First is to remove any mummies every week. Don't let them have another generation every summer. I have most susceptible plants (peach, plum, cherry, nectarine apricot) in an area that I can rake for mummies. This is annoying and labor intensive and means I can't have heavy cover with classy things like comfrey. This is based on the Holistic Orchard, he uses pea gravel and meticulous control. It is not really my style, which is why I only have one row of susceptible plants, and if they are too much trouble they get whacked.
I have one other tree with major aromatic stuff around the base. In my climate this is monarda, mint, nettle. I have had no success with onions at the tree bases. This one got hit with cerculio this year. Not too bad, maybe 25% loss so far.
Other control per Living Energy Farm is to have a mix of the above and run poultry in the area for bug consumption, and I have heard of pigs to eat the mummies. I am suspicious that will do much, one hatched bug is very good at destroying your crop and they are out at night when the poultry are not active. Pigs are a definite maybe, but I don't have them. Would be interested in others experiences for sure.
Sticky traps on the trunk may work, the cerculio generally climb the trunk, not fly in.
Next is kaolin (Surround), sprayed on the tree. From my understanding this must be applied a couple times, to the whole tree, and is so-so for protection.
Lastly, I have tried to plant cherries as a trap tree. Not sure if you can even grow pie cherries down there, but the principle is the same- The earliest host is the one you need to control. Get the bugs as they come out of hibernation and try to prevent the generational expansion. Throw maximal effort into control on that crop/tree and place them where they can draw the bugs out of the woods. I have only two pie cherries, and they are only for that purpose. I probably will never get cherries, and I don't care if I get the other stuff. Cherries generally are marginal here anyhow.
I have not tried chemical control (but I have neighbors that do and they have marginal success). I think the integrated approach is the best chance to beat the bugs.
Hope this helps.
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