"There is" OR "There are" an unusual number of bees this summer?

Can you show me which one is the correct sentence?

"There is an unusual number of bees this summer."

or

"There are an unusual number of bees this summer."

I suppose that "There are" is more appropriate, but can you help me to chose the correct answer?

To me 'is' sounds right, but I'm wrong, according to Oxford dictionaries:

https://en.oxforddictionaries.com/explore/number-of-people-is-or-are/

Although the expression ‘a number’ is strictly singular, the phrase ‘a number of’' is used with plural nouns (as what grammarians call a determiner (or determiner)). The verb should therefore be plural:

A number of people are waiting for the bus.

This is not the case with ‘the number’, which is still singular:

The number of people here has increased since this morning.

There is/are an unusual number of bees this summer.

"Number" is a non-count quantification noun here, which is said to be 'number-transparent' for verb agreement purposes. This means that it is the number of the noun that is complement of the preposition "of" (called the 'oblique') that determines the number of the whole NP.

As it happens, in its number-transparent sense the noun "number" selects only plural obliques like your "bees", so the verb must be the plural "are".