Yes.
Yes.
Maybe
No, gnat infestations generally do not kill indoor apple tree saplings. They are just fungus gnats from the soil as a result of overwatering. Cutting back on watering amounts and times should make the fungus gnats die off.
They weaken plants by sucking nutrients from leaves. Their worm stage will destroy young plants and cuttings roots, and will kill the young plants.
Potting mix does not kill earthworms.
No.
Fungus does not kill pesticides, pesticides (fungicide to be exact) kill fungus.
It depends on what kind of bugs you are trying to kill. Soap and water works well on most soft bodied insects. For example spidermites, and fungus gnats.
Yes, bleach kills...well...anything. So bleach would definitely kill gnats. As a matter of fact, we have a gnat problem right now >.< and we are spraying bleach on them to kill them. Hope this helped.
Drying out the soil can help control gnats by disrupting their breeding environment, but it may not necessarily kill all of them. Gnats thrive in moist conditions, so reducing the moisture in the soil can discourage their presence.
Yes, disinfectants can effectively kill fungus.
Yes, acid can effectively kill fungus.