A few things you might trial (none of which I have tried):
1. Add
Orange Guard, an OMRI listed insecticide / deterrent, water and d limonene from orange peels. Manufacturer says that it exhibits a repellency effect for 2 weeks following application and the initial knock down capability. There is a household insects formula and a more potent horticultural formula available.
2. The old method of moth balls at the base of the tree [to kill borers already in trunk and causing damage] beneath a mound of soil could possibly be detoxified by the use of Camphor crystals. Will it kill borers as paradichloro benzene does or only act as a deterrent? If the latter then the addition to number 3 may be more appropriate. What about other essential plant oils that may help with deterrence?
3. Use Surround or other clay slurry to coat trunk both as a deterrent and as a carrier / protectant for nematodes and/or neem or other insecticidal material. Does any one have experience using the BD tree paste in this manner; being effective for PTB, LPTB, or flat head apple borers?
4. Most of the old literature recommendations were for coating peach tree trunks with lime / sulfur slurry for winter sun heating protection but also for some control of PTB. Maybe adding it to clay could make it more effective for PTB. Use calcium oxide (unslaked lime) or calcium hydroxide (slaked-mixed with water calcium oxide) lime not calcium carbonate (unburnt ground lime
stone). Review old literature recommendations to avoid "bark" injury potential.
If you trial any of this please report your results!
Addendum added 2/22/14
Effect of roots on PTB infestationDoes anyone know of any research or have their own empirical evidence to back the premise that the rootstock effects the susceptability to PTB invasion? More to the point do seedlings or self rooted trees have less PTB damage? My limited observation in my backyard, 4 seedling trees; 3 not vigorous, (the oldest and least vigorous, still only 4" at base & over 15 years old), have never exhibited any signs of PTB attack, but there has been a bit of LPTB in older scaffold wood, probably because its not been renewed enough by pruning . If being a seedling helps to prevent PTB damage; then I would say that there is a strong case to utilize self rooting of cultivars, since it is as easy to accomplish with peaches as doing budding. Self rooted peaches demonstrated higher levels of Ca uptake than the same varieties that were grafted; this should equate to stronger cell walls and thus more disease resistance. This was from report by Gary Couvallin(sp?), U of GA in the late '80's, who was doing "meadow culture" of peaches. Meadow culture is growing trees as a row crop and replacing them about every 5 years; with tree(plant) densities in the thousands per acre, thus the need for easy mass propagation, i.e. self rooting. If you are interested I can relay the technique used for rooting cuttings; just contact me.
Dan Lefever
SE Penna, zone6b-7a
biorat@comcast.net 484-318-3789 mobile