Well here are three sources backing up my claim, besides that I speak from experience:
- https://www.tipsbulletin.com/how-to-use-neem-oil-for-fungus-gnats/#does-neem-oil-kill-fungus-gnats
- https://plantcaretoday.com/neem-oil-fungus-gnats.html :
"Azadirachtin is a powerful systemic insecticide, killing grubs beneath the soil and being absorbed by the roots."
-
growdiaries.com/journal/neem-oil-organic-pesticide-for-cannabis :
"The neem tree has its own defence mechanism which protects it from being attacked by insects. The sap produced by the tree contains azadirachtin, which acts a natural insecticide and has shown to repel hundreds of different types of pests."
Cheers, mate. All the best with your grow!