“The farmer as well as the labourers IS hard at work.” OR “The farmer as well as the labourers ARE hard at work.”?

In the sentence “The farmer as well as the labourers is hard at work.”, the phrase “as well as” joins two noun phrases, i.e. “the farmer” and “the labourers”. But the two are not of equal importance in the sentence. “The farmer” is more important and is the subject of the sentence. The noun phrase that comes after “as well as” is considered an addition and not one of the subjects. This is also true of noun phrases that come after along with, in addition to, together with, and some other phrases.
Since “the farmer” in the sentence is singular, the singular verb “is” is used.
If the subject is plural, a plural verb is used: “The farmers as well as the labourer are hard at work.”