Learn Spanish (Latin America): Cuando el escarabajo tenga alas, va a volar al árbol. - When the beetle has wings, it is going to fly to the tree.

This is the key rule for time-related phrases! When you use cuando (when) to talk about a future event that hasn't happened yet, Spanish requires the subjunctive. Since the beetle doesn't have wings yet, we must use tenga.

Would it be wrong to say Cuando el escarabajo tiene alas?

It wouldn't be grammatically wrong, but it would change the meaning! Using the indicative tiene tells the listener you are talking about a habit or a general fact. It would mean 'Whenever the beetle has wings' rather than referring to a specific, pending future event.

Could I use the future tense volará instead of va a volar?

Yes, you absolutely can! Both are correct. However, using ir a plus an infinitive (like va a volar) is extremely common in everyday Latin American Spanish to express what is going to happen, so it sounds very natural here.

Just a quick refresher: why is it al árbol instead of a el árbol?

Whenever the preposition a (to) is followed immediately by the masculine singular article el (the), they must combine into al. It is a mandatory rule in Spanish to make the pronunciation smoother.