It depends COMPLETELY on the municipality, city, county, state that you live in. Someone who lives in Ohio cannot accurately answer a question about an issue such as this from someone in another state.
Check with THEM, not here. Its nothing more than a simple phone call to your governing organization where you live. You will get correct information there......usually.
Here, You need a pesticide license for something as small as round-up. (after all, weeds are pests) Irrigation contractors above a certain dollar amount need a license. You need a license to operate in the city here...and a separate one for other cities in my service area.
In reality just about every person on this site needs a license. You can't spray even a drop of a non-selective herbicide such as RoundUp without a license. I didn't have a license for years but got my applicators license last year when Top Choice became a RU product.
After going through about 400 pages of study material, and taking three tests with a total of 250 question I got my license. If you have a HS diploma you can probably handle the test.
I also got my Class A CDL license last year since I can gross over 26,000 lbs with two of my trailers.