Thanks for your enquiry. The general consensus from our Community of Practice is that it would be best to focus on bolstering natural enemies, in particular Aphelinus and earwigs. Part of this focus should consider what is causing woolly aphid to become so problematic. For example, are copper and sulphur-based sprays impacting populations of beneficials? Copper and sulphur may also impact mycorrhizal fungi, beneficial bacteria, and other soil biota and consequently influence WAA underground populations. Monitoring beneficial numbers using sticky cards or counting mummies could help to determine what is or isn’t happening.
Unfortunately, identifying an organic pesticide that is both suitable and registered for this use is not straight forward. There are several biotypes of WAA that have different growth rates and responses to tree cultivars, and there has been a trend away from resistant rootstocks. In addition, it is becoming increasing unclear whether controlling subterranean colonies effectively manages WAA in tree canopies (see Orpet et al 2019: https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0216424) and therefore even with an effective organic treatment, drenches may not yield the desired results.
See also pages 307-314 of the IPDM manual that focuses on WAA: https://extensionaus.com.au/ozapplepearipdm/ipdm-manual-for-apples-and-pears/
 
Here are some links to videos and articles on our webpage in regard to establishing earwigs in orchards:
https://extensionaus.com.au/ozapplepearipdm/earwig-control-of-waa/
https://extensionaus.com.au/ozapplepearipdm/earwigs-ugly-critters-we-should-learn-to-love/
https://extensionaus.com.au/ozapplepearipdm/woolly-aphid-parasitoids/
 
And some other possibly useful links:
http://treefruit.wsu.edu/article/monitoring-and-conserving-earwigs-in-washington-state-apple-orchards/
https://apal.org.au/wp-content/uploads/2019/08/Kevin-Dodds-presentation-FINAL.pdf