What blood sugar reading do diabetics need to go on to insulin?

No absolute value: Hi. That's an overly simplistic question. I must assume (hate to do that) that we're talking type 2 here. It depends on what other diabetes medications the patient is on. Often other non-insulin meds can be used before insulin. Nothing is more effective than insulin at bringing sugar down, but other drugs may be perfectly adequate without the risk of hypoglycemia that comes with insulin. Good luck